|
DEPARTMENT
OF TRANSPORTATION
National
Highway Traffic Safety Administration
49
CFR Part 575
[Docket
No. NHTSA-2001- 9663; Notice 3]
RIN
2127-AI81
Consumer
Information;
New
Car Assessment Program;
Rollover
Resistance
AGENCY:
National Highway Traffic Safety Administration (NHTSA), DOT.
ACTION:
Final Policy Statement
SUMMARY:
The Transportation Recall Enhancement, Accountability, and Documentation
Act of 2000 requires NHTSA to develop a dynamic test on rollovers
by motor vehicles for the purposes of a consumer information program,
to carry out a program of conducting such tests, and, as these
tests are being developed, to conduct a rulemaking to determine
how best to disseminate test results to the public. This document
modifies NHTSA's rollover resistance ratings in its New Car Assessment
Program (NCAP) to include dynamic rollover tests after considering
comments to our previous document. The changes described in this
document will improve consumer information provided by NHTSA,
but will not place regulatory requirements on vehicle manufacturers.
DATES: NCAP
rollover resistance ratings in the 2004 model year will be determined
using the system established by this document.
Petitions:
Petitions for reconsideration must be received by [insert date
that is 45 days after date of publication in the Federal Register].
FOR FURTHER
INFORMATION CONTACT: For technical questions you may contact
Patrick Boyd, NVS-123, Office of Rulemaking, National Highway
Traffic Safety Administration, 400 Seventh Street, SW, Washington,
DC 20590 and Dr. Riley Garrott, NVS-312, NHTSA Vehicle Research
and Test Center, P.O. Box 37, East Liberty, OH 43319. Mr. Boyd
can be reached by phone at (202) 366-6346 or by facsimile at (202)
493-2739. Dr. Garrott can be reached by phone at (937) 666-4511
or by facsimile at (937) 666-3590.
SUPPLEMENTARY
INFORMATION:
-
Executive Summary
-
Safety Problem
-
Background
-
Existing NCAP Program and the TREAD Act
-
National Academy of Sciences Study
-
Notice of Proposed Rulemaking
-
Results of Dynamic Maneuver Tests of 25 Vehicles
-
J-Turn Maneuver
-
Fishhook Maneuver
-
Loading Conditions
-
Test Results
-
Rollover Risk Model
-
Comments to the Previous Notice
-
Combined or Separate Rollover Resistance Ratings
-
Crash Avoidance Technologies
-
The J-Turn and Fishhook Maneuvers
-
Tire Wear
-
Pavement Temperature
-
Surface Friction
-
Steering Reversal
-
Fifteen-Passenger Vans
-
Tip-up Criterion
-
Testing of Passenger Cars vs. Light Trucks
-
Testing with Electronic Stability Control Systems
-
Final Form for Rollover Resistance Ratings - Alternative I
-
Combined Ratings
-
Dynamic Testing
-
Demonstration Program
-
Cost Benefit Statement
-
Rulemaking Analyses and Notices
-
Executive Order 12866
-
Regulatory Flexibility Act
-
National Environmental Policy Act
-
Executive Order 13132 (Federalism)
-
Unfunded Mandates Act
-
Civil Justice Reform
-
Paperwork Reduction Act
-
Plain Language
Appendix
I. Fishhook Test Protocol
Appendix II. Development of Logistic
Regression Risk Model
<
I. Executive Summary
While the total number of highway fatalities has
remained relatively stable over the past decade, the number of
rollover deaths has risen substantially. According to NHTSA's
National Center for Statistics and Analysis, from 1991 to 2001
the number of passenger vehicle occupants killed in all motor
vehicle crashes increased 4 percent, while fatalities in rollover
crashes increased 10 percent. In the same decade, passenger car
occupant fatalities in rollovers declined 15 percent while rollover
fatalities in light trucks increased 43 percent. In 2001, 10,138
people died in rollover crashes, a figure that represents 32 percent
of occupant fatalities for the year.
In response to that trend, NHTSA has been evaluating
rollover testing since 1993. In 2001, NHTSA began publishing rollover
rating information for consumers, supplementing New Car Assessment
Program (NCAP) frontal crashworthiness ratings that began in 1979
and side impact ratings that began in 1997.
When Congress approved the "Transportation Recall,
Enhancement, Accountability and Documentation (TREAD) Act of November
2000", Section 12 directed the Secretary of Transportation to
"develop a dynamic test on rollovers by motor vehicles for a consumer
information program; and carry out a program conducting such tests.
As the Secretary develops a [rollover] test, the Secretary shall
conduct a rulemaking to determine how best to disseminate test
results to the public."
On July 3, 2001, NHTSA published a Request for
Comments notice (66 FR 35179) discussing a variety of dynamic
rollover tests that we had chosen to evaluate in our research
program and what we believed were their potential advantages and
disadvantages.
We published a Notice of Proposed Rulemaking on
October 7, 2002 (67 FR 62528) that proposed alternative ways of
using the dynamic maneuver test results in consumer information
on the rollover resistance of new vehicles.
Beginning with rollover ratings for the 2004 model
year, NHTSA will combine a vehicle's Static Stability Factor (SSF)
measurement with its performance in the so-called "Fishhook" maneuver.
The so-called "J-Turn" dynamic test maneuver discussed in previous
notices will be not be used by NHTSA for rating rollover resistance.
Our analysis has found that the J-Turn maneuver test does not
add any meaningful information to what is obtained from the fishhook
maneuver test alone (see Appendix II.B). The predicted rollover
rate will be translated into a five-star rating system that is
the same as the one now in use: One star is for a rollover rate
greater than 40 percent; two stars, between 30 and 39 percent;
three stars, between 20 and 29 percent; four stars, between 10
and 19 percent; and five stars for 10 percent or less.
This decision maximizes the vehicle information
used to make the rollover rate prediction and will allow us to
ensure that rollover NCAP information corresponds even more closely
to real-world rollovers. We have also decided to present our
rollover information as a single combined rollover rating that
most commenters agreed would be more understandable to consumers.
This document also includes a test procedure (Appendix
I) for conducting vehicle maneuver tests, and discusses testing
regimes that have been incorporated to minimize variability in
test data.
<
II. Safety Problem
Rollover crashes are complex events that reflect the interaction
of driver, road, vehicle, and environmental factors. We can describe
the relationship between these factors and the risk of rollover
using information from the agency's crash data programs. We limit
our discussion here to light vehicles, which consist of (1) passenger
cars and (2) multipurpose passenger vehicles and trucks under
4,536 kilograms (10,000 pounds) gross vehicle weight rating.[1]
According to the 2001 Fatality Analysis Reporting System (FARS),
10,138 people were killed as occupants in light vehicle rollover
crashes, which represent 32 percent of the occupants killed that
year in crashes. Of those, 8,407 were killed in single-vehicle
rollover crashes. Seventy-eight percent of the people who died
in single-vehicle rollover crashes were not using a seat belt,
and 64 percent were partially or completely ejected from the vehicle
(including 53 percent who were completely ejected). FARS shows
that 54 percent of light vehicle occupant fatalities in single-vehicle
crashes involved a rollover event.
Using data from the 1997-2001 National Automotive Sampling System
(NASS) Crashworthiness Data System (CDS), we estimate that 281,000
light vehicles were towed from a police-reported rollover crash
each year (on average), and that 30,000 occupants of these vehicles
were seriously injured or killed (defined as any fatality or an
injury with an Abbreviated Injury Scale (AIS) rating of at least
AIS 3).[2]
Of these 281,000 light vehicle rollover crashes, 225,000 were
single-vehicle crashes. (The NCAP rollover resistance ratings
estimate the risk of rollover if a vehicle is involved in a single-vehicle
crash.) Sixty-one percent of those people who suffered a serious
injury in single-vehicle towaway rollover crashes were not using
a seat belt, and 49 percent were partially or completely ejected
(including 40 percent who were completely ejected). Estimates
from NASS CDS indicate that 80 percent of towaway rollovers were
single-vehicle crashes, and that 83 percent (168,000) of the single-vehicle
rollover crashes occurred after the vehicle left the roadway.
An audit of 1992-96 NASS CDS data showed that about 95 percent
of rollovers in single-vehicle crashes were tripped by mechanisms
such as curbs, soft soil, pot holes, guard rails, and wheel rims
digging into the pavement, rather than by tire/road interface
friction as in the case of untripped rollover events.
According to the 1997-2001 NASS General Estimates System (GES)
data, 62,000 occupants annually received injuries rated as K
or A on the police KABCO injury scale in rollover crashes.
(The police KABCO scale calls A injuries "incapacitating,"
but their actual severity depends on local reporting practice.
An "incapacitating" injury may mean that the injury was visible
to the reporting officer or that the officer called for medical
assistance. A K injury is fatal.) The data indicate that
215,000 single-vehicle rollover crashes resulted in 49,000 K
or A injuries. Fifty percent of those with K or
A injury in single-vehicle rollover crashes were not using
a seat belt, and 24 percent were partially or completely ejected
from the vehicle (including 21 percent who were completely ejected).
Estimates from NASS GES indicate that 13 percent of light vehicles
in police-reported single-vehicle crashes rolled over. The estimated
risk of rollover differs by light vehicle type: 10 percent of
cars and 10 percent of vans in police-reported single-vehicle
crashes rolled over, compared to 18 percent of pickup trucks and
27 percent of SUVs. The percentages of all police-reported crashes
for each vehicle type that resulted in rollover were 1.7 percent
for cars, 2.0 percent for vans, 3.8 percent for pickup trucks
and 5.5 percent for SUVs as estimated by NASS GES.
<
III. Background
<
A. Existing NCAP Program and the TREAD Act
NHTSA's NCAP program has been publishing comparative consumer
information on frontal crashworthiness of new vehicles since 1979,
on side crashworthiness since 1997, and on rollover resistance
since January 2001 (66 FR 3388). This notice does not establish
a new consumer information program on rollover resistance ratings.
Rather, it refines our existing rollover resistance rating program
in accordance with the requirements of the TREAD Act and the recommendations
of the National Academy of Sciences.
The present NCAP rollover resistance ratings are based on the
Static Stability Factor (SSF) of a vehicle, which is the ratio
of one half its track width to its center of gravity (c.g.) height
(see www.nhtsa.dot.gov/hot/rollover/ for ratings and explanatory
information). After an evaluation of some driving maneuver tests
in 1997 and 1998, we chose to use SSF instead of any driving maneuvers
to characterize rollover resistance. As we explained in our notices
establishing rollover NCAP, we chose SSF as the basis of our ratings
because it represents the first order factors that determine vehicle
rollover resistance in the vast majority of rollovers which are
tripped by impacts with curbs, soft soil, pot holes, guard rails,
etc. or by wheel rims digging into the pavement. In contrast,
untripped rollovers are those in which tire/road interface friction
is the only external force acting on a vehicle that rolls over.
Driving maneuver tests directly represent on-road untripped rollover
crashes, but such crashes represent less than five percent of
rollover crashes[3].
At the time, we believed it was necessary to choose between SSF
and driving maneuver tests as the basis for rollover resistance
ratings. SSF was chosen because it had a number of advantages:
it is highly correlated with actual crash statistics; it can be
measured accurately and inexpensively and explained to consumers;
and changes in vehicle design to improve SSF are unlikely to degrade
other safety attributes. We also considered the fact that an
improvement in SSF represents an increase in rollover resistance
in both tripped and untripped circumstances while maneuver test
performance can be improved by reduced tire traction and certain
implementations of electronic stability control that we believe
are unlikely to improve resistance to tripped rollovers.
Congress funded NHTSA's rollover NCAP program, but directed the
agency to enhance the program. Section 12 of the "Transportation
Recall, Enhancement, Accountability and Documentation (TREAD)
Act of November 2000" directs the Secretary to "develop a dynamic
test on rollovers by motor vehicles for a consumer information
program; and carry out a program conducting such tests. As the
Secretary develops a [rollover] test, the Secretary shall conduct
a rulemaking to determine how best to disseminate test results
to the public." The rulemaking was to be carried out by November
1, 2002.
On July 3, 2001, NHTSA published a Request for Comments notice
(66 FR 35179) regarding our research plans to assess a number
of possible dynamic rollover tests. The notice discussed the
possible advantages and disadvantages of various approaches that
had been suggested by manufacturers, consumer groups, and NHTSA's
prior research. The driving maneuver tests to be evaluated fit
into two broad categories: closed-loop maneuvers in which all
test vehicles attempt to follow the same path; and open-loop maneuvers
in which all test vehicles are given equivalent steering inputs.
The principal theme of the comments was a sharp division of opinion
about whether the dynamic rollover test should be a closed loop
maneuver test like the ISO 3388 double lane change that emphasizes
the handling properties of vehicles or whether it should be an
open loop maneuver like a J-Turn or Fishhook that are limit maneuvers
in which vulnerable vehicles would actually tip up. Ford recommended
a different type of closed loop lane change maneuver in which
a path-following robot or a mathematical correction method would
be used to evaluate all vehicles on the same set of paths at the
same lateral acceleration. It used a measurement of partial wheel
unloading without tip-up at 0.7g lateral acceleration as a performance
criterion in contrast to the other closed loop maneuver tests
that used maximum speed through the maneuver as the performance
criterion. Another unique comment was a recommendation from Suzuki
to use a sled test developed by Exponent Inc. to simulate tripped
rollovers.
The subsequent test program (using four SUVs in various load
conditions and with and without electronic stability control enabled
on two of the SUVs) showed that open-loop maneuver tests using
an automated steering controller could be performed with better
repeatability of results than the other maneuver tests. The J-Turn
maneuver and the Fishhook maneuver (with steering reversal at
maximum vehicle roll angle) were found to be the most objective
tests of the susceptibility of vehicles to maneuver-induced on-road
rollover. Except for the Ford test, the closed loop tests were
found not to measure rollover resistance. Instead, the tests
of maximum speed through a double lane change responded to vehicle
agility. None of the test vehicles tipped up during runs in which
they maintained the prescribed path even when loaded with roof
ballast to experimentally reduce their rollover resistance. The
speed scores of the test vehicles in the closed loop maneuvers
were found to be unrelated to their resistance to tip-up in the
open-loop maneuvers that actually caused tip-up. The test vehicle
that was clearly the poorest performer in the maneuvers that caused
tip-ups achieved the best score (highest speed) in the ISO 3388
and CU short course double lane change, and one vehicle improved
its score in the ISO 3388 test when roof ballast was added to
reduce its rollover resistance.
Due to the non-limit test conditions and the averaging necessary
for stable wheel force measurements, the wheel unloading measured
in the Ford test appeared to be more quasi-static (as in driving
in a circle at a steady speed or placing the vehicle on a centrifuge)
than dynamic. Sled tests were not evaluated because we believed
that SSF already provided a good indicator of resistance to tripped
rollover.
<
B. National Academy of Sciences Study
During the time NHTSA was evaluating dynamic maneuver tests in
response the TREAD Act, the National Academy of Sciences (NAS)
was conducting a study of the our SSF-based rollover resistance
ratings and was directed to make recommendations regarding driving
maneuver tests. We expected the NAS recommendations to have a
strong influence on TREAD-mandated changes to NCAP rollover resistance
ratings.
When NHTSA proposed the present SSF rollover resistance ratings
in June 2000 (65 FR 34998), vehicle manufacturers generally opposed
it because they believed that SSF as a measure of rollover resistance
is too simple since it does not include the effects of suspension
deflections, tire traction and electronic stability control (ESC).
In addition, the vehicle manufacturers argued that the influence
of vehicle factors on rollover risk is too slight to warrant consumer
information ratings for rollover resistance. In the conference
report of the FY2001 DOT Appropriations Act, Congress permitted
NHTSA to move forward with its rollover rating program, but directed
the agency to fund a National Academy of Sciences (NAS) study
on vehicle rollover ratings. The study topics were "whether the
static stability factor is a scientifically valid measurement
that presents practical, useful information to the public including
a comparison of the static stability factor test versus a test
with rollover metrics based on dynamic driving conditions that
may induce rollover events." The National Academy's report was
completed and made available at the end of February 2002.
The NAS study found that SSF is a scientifically valid measure
of rollover resistance for which the underlying physics and real-world
crash data are consistent with the conclusion that an increase
in SSF reduces the likelihood of rollover. It also found that
dynamic tests should complement static measures, such as SSF,
rather than replace them in consumer information on rollover resistance.
The dynamic tests the NAS recommended would be driving maneuvers
used to assess "transient vehicle behavior leading to rollover."
The NAS study also made recommendations concerning the statistical
analysis of rollover risk and the representation of ratings.
It recommended that we use logistic regression rather than linear
regression for analysis of the relationship between rollover risk
and SSF, and it recommended that we consider a higher-resolution
representation of the relationship between rollover risk and SSF
than is provided by the current five-star rating system.
We published a Notice of Proposed Rulemaking on October 7, 2002
(67 FR 62528) that proposed alternative ways of using the dynamic
maneuver test results in consumer information on the rollover
resistance of new vehicles. We chose the J-Turn and Fishhook
maneuver (with roll rate feedback) as the dynamic maneuver tests
because they were the type of limit maneuver tests that could
directly lead to rollover as recommended by the NAS. We also
proposed to use a logistic regression analysis to determine the
relationship between vehicle properties and rollover risk, as
recommended by the NAS. The resulting rollover resistance ratings
were proposed to be part of NHTSA's New Car Assessment Program
(NCAP). Also, we proposed two methods for presenting rollover
resistance ratings for consumer information.
<
IV. Notice of Proposed Rulemaking
The TREAD Act calls for a rulemaking to determine how best to
disseminate rollover test results to the public, and our Notice
of Proposed Rulemaking (NPRM) of October 7, 2002 (67 FR 62528)
proposed two alternatives for using the dynamic test results in
consumer information on the rollover resistance of new vehicles.
In this case the term "rulemaking" refers more to the process
than to the product. This document does not amend the code
of Federal Register, but establishes NHTSA's policy on consumer
information regarding the rollover resistance program. As
mentioned above, this program places no requirements on vehicle
manufacturers, only some on NHTSA.
While the TREAD Act calls for a rulemaking to determine how best
to disseminate the rollover test results, the development of the
dynamic rollover test is simply the responsibility of the Secretary.
Based on NHTSA's recent research to evaluate rollover test maneuvers,
the National Academy of Sciences' study of rollover ratings, comments
to the July 3, 2000 notice, extensive consultations with experts
from the vehicle industry, consumer groups and academia, and NHTSA's
previous research in 1997-8, the agency chose the J-Turn and the
Fishhook maneuvers as dynamic rollover tests. They are the limit
maneuver tests that NHTSA found to have the highest levels of
objectivity, repeatability and discriminatory capability. The
notice announced that vehicles would be tested in two load conditions
using the J-Turn at up to 60 mph and the Fishhook maneuver at
up to 50 mph. Both maneuvers would be conducted with an automated
steering controller, and the reverse steer of the Fishhook maneuver
would be timed to coincide with the maximum roll angle to create
an objective "worst case" for all vehicles regardless of differences
in resonant roll frequency. Figures 1 and 2 illustrate the open-loop
steering wheel motions characterizing these maneuvers. The light
load condition would be the weight of the test driver and instruments,
approximating a vehicle with a driver and one front seat passenger.
The notice announced that the heavy load condition would add additional
175 lb manikins in all rear seat positions.
The National Academy of Sciences recommended that dynamic maneuver
tests be used to supplement rather than replace Static Stability
Factor in consumer information on rollover resistance. NHTSA
proposed two alternatives for consumer information ratings on
vehicle rollover resistance that included both dynamic maneuver
test results and Static Stability Factor. The first alternative
was to include the dynamic test results as vehicle variables along
with SSF in a statistical model of rollover risk that would combine
their predictive power. This is conceptually similar to the present
ratings in which a statistical model is used to distinguish between
the effects of vehicle variables and demographic and road use
variables recorded for state crash data on a large number of single-vehicle
crashes. The National Academy of Sciences recommended using a
logistic regression model for this purpose. Such a model would
be used to predict the rollover rate in single-vehicle crashes
for a vehicle considering both its dynamic maneuver test performance
and its Static Stability Factor for an average driver population
(as a common basis of comparison).
Under the first alternative, the "star rating" of a vehicle would
be based on its rollover rate in single-vehicle crashes predicted
by a statistical model. The format would be the same as for the
present rollover ratings (for example, one star for a predicted
rollover rate in single-vehicle crashes greater than 40 percent
and five stars for a predicted rollover rate less than 10 percent).
The present rollover ratings are based on a linear regression
model using state crash reports of 241,000 single-vehicle crashes
of 100 make/model vehicles. We proposed to replace the current
rollover risk model with one that uses the performance of the
vehicle in dynamic maneuver tests as well as its SSF to predict
rollover risk. The performance of a vehicle in dynamic maneuver
tests would be simply whether it tipped up or not in each of the
four maneuver/load combinations.
In order to compute this logistic model for rollover risk, it
is necessary to have the dynamic maneuver test results as well
as SSF for a number of vehicles with rollover rates established
by state crash reports of single-vehicle crashes. We had the
SSF measurements and established rollover rates for the 100 make/model
vehicles upon which we based the static rating system but not
their dynamic maneuver test results. Thus, we asked for comment
on the suitability of a rating method that combines static and
dynamic vehicle properties in a single rating and on the validity
of logistic regression analysis for the risk model that combines
the properties in a way that is predictive of real-world crash
experience.
The NPRM notice announced that we were going to perform the dynamic
maneuver tests on about 25 of the 100 make/model vehicles for
which we had SSF measurements and substantial state crash data.
Time and budget constraints would not permit testing all 100 vehicles.
With these dynamic maneuver test results and our existing crash
and SSF information we would be able to compute the new risk model
using a standard statistical package of computer programs (SAS)
for logistic regression analysis. This document notice presents
the dynamic maneuver test results for 24 of the 100 vehicles,
chosen to span the SSF range and to represent high production
vehicles of each type (passenger car, van, pickup truck and sport
utility vehicle (SUV)). An additional SUV with a lower SSF than
found among the 100 vehicles was also included. The resulting
risk model is presented in this document notice.
The second alternative we proposed was to have separate ratings
for Static Stability Factor and for dynamic maneuver test performance.
Dynamic maneuver tests directly represent on-road untripped rollovers.
Under this alternative, the dynamic maneuver test performance
would be used to rate resistance to untripped rollovers in a qualitative
scale. Barring unforeseen results of the dynamic maneuver tests
of the 25 vehicle group, the obvious qualitative scale would be:
A for no tip-ups, B for tip-up in one maneuver, C for tip-ups
in two maneuvers, D for tip-ups in three maneuvers and E for tip-ups
in all four maneuvers/load combinations.
A statistical risk model is not possible for untripped rollover
crashes, because they appear to be relatively rare events and
they cannot be reliably identified in state crash reports. For
this alternative, the current Static Stability Factor based system
would be used to rate resistance to tripped rollovers (since we
believe most of the rollovers reported in the state crash reports
are tripped). Again we asked for comments on the usefulness and
validity of the concept in the NPRM notice, but we could not offer
examples of actual vehicle ratings because the tests had not yet
been conducted.
<
V. Results of Dynamic Maneuver Tests of 25 Vehicles
This section presents an overview of the test maneuvers and the
results for 25 vehicles that were used to develop the logistic
regression risk model. A more extensive account of the test program
is contained in the Phase VI and VII Report that has been placed
in Docket NHTSA-2001-9663. A detailed description of how we will
perform the maneuver tests for NCAP ratings is contained in Appendix
I.
The NHTSA J-Turn and Fishhook (with roll rate
feedback) maneuver tests were performed for 25 vehicles representing
four vehicle types including passenger cars, vans, pickup trucks
and SUVs. We chose mainly high production vehicles that spanned
a wide range of SSF values, using vehicles NHTSA already owned
where possible. Except for four 2001 model year vehicles NHTSA
purchased new, the vehicle suspensions were rebuilt with new springs
and shock absorbers, and other parts as required for all the other
vehicles included in the test program.
<
A. J-Turn Maneuver
The NHTSA J-Turn maneuver represents an avoidance maneuver in
which a vehicle is steered away from an obstacle using a single
input. The maneuver is similar to the J-Turn used during NHTSA's
1997-98 rollover research program and is a common maneuver in
test programs conducted by vehicle manufacturers and others.
Often the J-Turn is conducted with a fixed steering input (handwheel
angle) for all test vehicles. In its 1997-98 testing, NHTSA used
a fixed handwheel angle of 330 degrees. In the testing that preceded
the NPRM notice, we developed an objective method of specifying
equivalent handwheel angles for J-Turn tests of various vehicles,
taking into account their differences in steering ratio, wheelbase
and linear range understeer properties. (See NHTSA's Phase IV
report docketed with the NPRM notice as item 38 in Docket No.
NHTSA 2001-9663). Under this method, one first measures the
handwheel angle that would produce a steady-state lateral acceleration
of 0.3 g at 50 mph on a level paved surface for a particular vehicle.
In brief, the 0.3 g value was chosen because the steering angle
variability associated with this lateral acceleration is quite
low and there is no possibility that stability control intervention
could confound the test results. Since the magnitude of the handwheel
position at 0.3 g is small, it must be multiplied by a scalar
to have a high maneuver severity. In the case of the J-Turn,
the handwheel angle at 0.3 g was multiplied by eight. When this
scalar is multiplied by the average handwheel angle at 0.3 g (observed
during NHTSA's 1997-98 rollover research program), the result
is approximately 330 degrees. Figure 1 illustrates the J-Turn
maneuver in terms of the automated steering inputs commanded by
the programmable steering machine. The rate of the handwheel
turning is 1000 degrees per second.
To begin the maneuver, the vehicle was driven in a straight line
at a speed slightly greater than the desired entrance speed.
The driver released the throttle, coasted to the target speed,
and then triggered the commanded handwheel input. The nominal
maneuver entrance speeds used in the J-Turn maneuver ranged from
35 to 60 mph, increased in 5 mph increments until a termination
condition was achieved. Termination conditions were simultaneous
two inch or greater lift of a vehicle's inside tires (two-wheel
lift) or completion of a test performed at the maximum maneuver
entrance speed without two-wheel lift. If two-wheel lift was
observed, a downward iteration of vehicle speed was used in 1
mph increments until such lift was no longer detected. Once the
lowest speed for which two-wheel lift could be detected was isolated,
two additional tests were performed at that speed to monitor two-wheel
lift repeatability.
<
B. Fishhook Maneuver
The second maneuver test, the fishhook maneuver, uses steering
inputs that approximate the steering a driver acting in panic
might use in an effort to regain lane position after dropping
two wheels off the roadway onto the shoulder. In the NPRM notice,
we described it as a road edge recovery maneuver. As pointed
out by some commenters, it is performed on a smooth pavement rather
than at a road edge drop-off, but its rapid steering input followed
by an over-correction is representative of a general loss of control
situation. The original version of this test was developed by
Toyota, and variations of it were suggested by Nissan and Honda.
NHTSA has experimented with several versions since 1997, and the
present test includes roll rate feedback in order to time the
counter-steer to coincide with the maximum roll angle of each
vehicle in response to the first steer.
Figure 2 describes the Fishhook maneuver in terms of the automated
steering inputs commanded by the programmable steering machine
and illustrates the roll rate feedback. The initial steering
magnitude and countersteer magnitudes are symmetric, and are calculated
by multiplying the handwheel angle that would produce a steady
state lateral acceleration of 0.3 g at 50 mph on level pavement
by 6.5. The average steering input is equivalent to the 270 degree
handwheel angle used in earlier forms of the maneuver but, as
in the case of the J-Turn , the procedure above is an objective
way of compensating for differences in steering gear ratio, wheelbase
and understeer properties between vehicles. The fishhook maneuver
dwell times (the time between completion of the initial steering
ramp and the initiation of the countersteer) are defined by the
roll motion of the vehicle being evaluated, and can vary on a
test-to-test basis. This is made possible by having the steering
machine monitor roll rate (roll velocity). If an initial steer
is to the left, the steering reversal following completion of
the first handwheel ramp occurs when the roll rate of the vehicle
first equals or goes below 1.5 degrees per second. If an initial
steer is to the right, the steering reversal following completion
of the first handwheel ramp occurs when the roll rate of the vehicle
first equals or exceeds -1.5 degrees per second. The handwheel
rates of the initial steer and countersteer ramps are 720 degrees
per second.
To begin the maneuver, the vehicle was driven in a straight line
at a speed slightly greater than the desired entrance speed.
The driver released the throttle, coasted to the target speed,
and then triggered the commanded handwheel input described in
Figure 2. The nominal maneuver entrance speeds used in the fishhook
maneuver ranged from 35 to 50 mph, increased in 5 mph increments
until a termination condition was achieved. Termination conditions
included simultaneous two inch or greater lift of a vehicle's
inside tires (two-wheel lift) or completion of a test performed
at the maximum maneuver entrance speed without two-wheel lift.
If two-wheel lift was observed, a downward iteration of vehicle
speed was used in 1 mph increments until such lift was no longer
detected. Once the lowest speed for which two-wheel lift could
be detected was isolated, two additional tests were performed
at that speed to check two-wheel lift repeatability.
<
C. Loading Conditions
The vehicles were tested in each maneuver in two load conditions
in order to create four levels of stringency in the suite of maneuver
tests. The light load was the test driver plus instrumentation
in the front passenger seat, which represented two occupants.
A heavier load was used to create a higher level of stringency
for each test. In our NPRM, we announced that the heavy load
would include 175 lb anthropomorphic forms (water dummies) in
all rear seat positions. During the test of the 25 vehicles,
it became obvious that heavy load tests were being run at very
unequal load conditions especially between vans and other vehicles
(two water dummies in some vehicles but six water dummies in others).
While very heavy passenger loads can certainly reduce rollover
resistance and potentially cause special problems, crashes at
those loads are too few to greatly influence the overall rollover
rate of vehicles. Over 94% of van rollovers in our 293,000 crash
database occurred with five or fewer occupants, and over 99% of
rollovers of other vehicles occurred with five or fewer occupants.
The average passenger loads of vehicles in our crash database
was less than two: 1.81 for vans; 1.54 for SUVs; 1.48 for cars;
and 1.35 for pickup trucks. In order to use the maneuver tests
to predict real-world rollover rates, it seemed inappropriate
to test the vehicles under widely differing loads that did not
correspond to the real-world crash statistics. Therefore, the
tests used to develop a statistical model of rollover risk were
changed to a uniform heavy load condition of three water dummies
(representing a 5-occupant loading) for all vehicles capable of
carrying at least five occupants. Some vehicles were loaded
with only two water dummies because they were designed for four
occupants. For pickup trucks, water dummies were loaded in the
bed at approximately the same height as a passenger in the front
seat.
To avoid disruption, the tests were completed under the original
loading plan. Then we conducted tests at a 5-occupant heavy load
only for those vehicles in which loading differences might influence
tip-up. If the vehicle had completed the maneuver without tip-up
with more than three water dummies in the rear it was not necessary
to retest at a lighter load. Likewise, if the vehicle tipped
up in the light load (no water dummies) condition, it was not
necessary to retest with three water dummies in the rear. We
have never observed a vehicle for which a greater passenger load
improved performance in a tip-up test.
<
D. Test Results
The test results in Table 1 reflect the performance either measured
or imputed as described for a heavy load condition representing
5 occupants except for the Ford Explorer 2DR, the Chevrolet Tracker
and Metro that were designed for only four occupants, and the
Honda CRV, Honda Civic and Chevrolet Cavalier that could not be
loaded to the 5 occupant level without exceeding a gross axle
weight rating because of the additional weight of the outriggers.
Note that Table 1 includes some results collected during tests
performed with alternative steering angles. Although the steering
angles used during these tests were still based on the handwheel
angle that would produce a steady-state lateral acceleration of
0.3 g at 50 mph on a level paved surface, the scalars used to
calculate the steering angles were smaller. These tests
Table 1.
Dynamic Maneuver Test Results (the check mark indicates tip-up
observed)
Veh.
Group
Number |
Model
Range / Make / Model |
Nominal
Static
Stability Factor |
Fishhook
Light (FL)
(2 occ.) |
Fishhook
Heavy (FH)
(5 occ.) |
J-Turn
Light (JL)
(2 occ.) |
J-Turn
Heavy (JH)
(5 occ.) |
| -- |
'92 -
'00 Mitsubishi Montero 4WD |
0.95 |
 |
|
-- |
|
| 47 |
'95 -
'03 Chevrolet Blazer 2WD |
1.02 |
|
|
-- |
|
| 43 |
'95 -
'01 Ford Explorer 2dr 2WD |
1.06 |
-- |
-- |
-- |
-- |
| 44 |
'95 -
'01 Ford Explorer 4dr 4WD |
1.06 |
-- |
|
-- |
-- |
| 66 |
'96 -
'00 Toyota 4Runner 4WD |
1.06 |
-- |
|
-- |
-- |
| 89 |
'93 -
'97 Ford Ranger p/u 4WD |
1.07 |
|
|
|
|
| 58 |
'88 -
'97 Jeep Cherokee 4WD |
1.08 |
-- |
-- |
-- |
-- |
| 59 |
'95 -
'02 Acura SLX / Isuzu Trooper 4WD |
1.09 |
|
|
|
|
| 70 |
'88 -
'98 Ford Aerostar 2WD |
1.10 |
|
|
|
|
| 74 |
'88 -
'02 Chevrolet Astro 2WD |
1.12 |
-- |
|
-- |
-- |
| 53 |
'89 -
'98 Chevrolet/Geo Tracker 4WD |
1.13 |
-- |
|
-- |
-- |
| 91 |
'88 -
'98 Chevrolet K1500 p/u 4WD |
1.14 |
-- |
-- |
-- |
-- |
| 88 |
'93 -
'97 Ford Ranger p/u 2WD |
1.17 |
-- |
|
-- |
|
| 85 |
'97 -
'02 Ford F-150 p/u 2WD |
1.18 |
-- |
-- |
-- |
-- |
| 54 |
'97 -
'01 Honda CR-V 4WD |
1.19 |
|
|
-- |
|
| 83 |
'88 -
'96 Ford F-150 p/u 2WD |
1.19 |
-- |
-- |
-- |
-- |
| 67 |
'88 -
'95 Dodge Caravan / Plymouth Voyager 2WD |
1.21 |
-- |
-- |
-- |
-- |
| 90 |
'88 -
'98 Chevrolet C1500 p/u 2WD |
1.22 |
-- |
-- |
-- |
-- |
| 68 |
'96 -
'00 Dodge Caravan / Plymouth Voyager 2WD |
1.23 |
-- |
-- |
-- |
-- |
| 73 |
'95 -
'98 Ford Windstar 2WD |
1.24 |
-- |
-- |
-- |
-- |
| 22 |
'95 -
'01 Chevrolet / Geo Metro |
1.29 |
-- |
-- |
-- |
-- |
| 19 |
'88 -
'94 Chevrolet Cavalier |
1.32 |
-- |
-- |
-- |
-- |
| 18 |
'91 -
'96 Chevrolet Caprice |
1.40 |
-- |
-- |
-- |
-- |
| 7 |
'88 -
'95 Ford Taurus |
1.45 |
-- |
-- |
-- |
-- |
| 26 |
'92 -
'95 Honda Civic |
1.48 |
-- |
-- |
-- |
-- |
| |
|
|
|
|
|
|
Total
Tip-ups |
|
|
6 |
11 |
3 |
7 |
were performed
because, for some vehicles, the methods used to calculate the
steering inputs used in the J-Turn and/or Fishhook maneuvers can
produce "excessive" steering—steering angles so great that maneuver
severity is actually reduced (i.e., the lateral force capability
of the tires is exceeded). As an example, consider the Ford Ranger
4WD and Aerostar. These vehicles required a reduction of the J-Turn
steering scalar from 8.0 to 7.0 (Ranger 4WD) or 6.0 (Aerostar)
before J-Turn steering was able to produce two-wheel lift.
During some
Fishhook tests, excessive steering caused some vehicles to reach
their maximum roll angle response to the initial steering input
before it had been fully completed (this is essentially equivalent
to a "negative" T1 in Figure 2). Since dwell time
duration can have a significant effect on how the Fishhook maneuver's
ability to produce two-wheel lift, we believe that excessive steering
may stifle the most severe timing of the counter steer for some
vehicles. In an attempt to better insure high maneuver severity,
a number of vehicles that did not produce two-wheel lift with
steering inputs calculated with the 6.5 multiplier were also tested
with lesser steering angles by reducing the multiplier to 5.5.
This change reduced the likelihood of excessive steering, and
increased the dwell times observed during the respective maneuvers.
In the case of the Ford Ranger 4x2, Fishhook maneuvers with steering
inputs based on the reduced multiplier were able to produce two-wheel
lift. Such lift was not observed when the original steering was
used (i.e., when a multiplier of 6.5 was used). We have modified
the Fishhook test procedure to include tests at the steering angle
determined by the 5.5 multiplier for vehicles that do not tip
up using the original steering angle determination.
Each test
vehicle in Table 1 represented a generation of vehicles whose
model year range is given. Twenty-four of the vehicles were taken
from 100 vehicle groups whose 1994-98 crash statistics in six
states were the basis of the present SSF based rollover resistance
ratings. The vehicle group numbers used to identify these vehicles
in the prior notices (65 FR 34998 and 66 FR 3388) are given for
convenience. The nominal SSFs used to describe the vehicle groups
in the prior statistical studies are given. While there were
some variations between the SSFs of the individual test vehicles
and the nominal vehicle group SSF values, the nominal SSFs were
retained for the present statistical analyses because they represent
vehicles produced over a wide range of years in many cases and
provide a simple comparison between the risk model presented in
this notice and that discussed in the previous notices.
The check
marks under the various test maneuver names indicate which vehicles
tipped up during the tests. Eleven of the twenty-five vehicles
tipped up in the Fishhook maneuver conducted in the heavy condition.
The heavy condition represented a five-occupant load for all vehicles
except the six mentioned above that were limited to a four-occupant
load by the vehicle seating positions and GVWR. All eleven were
among the sixteen test vehicles with SSFs less than 1.20. None
of the vehicles with higher SSFs tipped up in any test maneuver.
The fishhook test under the heavy load clearly had the greatest
potential to cause tip-up. The groups of vehicles that tipped
up in other tests were subsets of the larger group of eleven that
tipped up in the fishhook heavy test. There were seven vehicles
in the group that tipped up in the J-Turn heavy test, six of which
also tipped up in the Fishhook light test. The J-Turn light test
had the least potential to tip up vehicles. Only three vehicles
tipped up, all of which had tipped up in every other test.
<
VI. Rollover Risk Model
In its study of our rating system for rollover
resistance (Transportation Research Board Special Report 265),
the National Academy of Sciences (NAS) recommended that we use
logistic regression rather than linear regression for analysis
of the relationship between rollover risk and SSF. Logistic regression
has the advantage that it operates on every crash data point directly
rather than requiring that the crash data be aggregated by vehicle
and state into a smaller number of data points. For example,
we now have state data reports of about 293,000 single-vehicle
crashes of the hundred vehicle make/models (together with their
corporate cousins) whose single-vehicle crashes we have been tracking
in six states. The logistic regression analysis of this data
would have a sample size of 293,000, producing a narrow confidence
interval on the repeatability of the relationship between SSF
and rollover rate. In contrast, the linear regression analysis
operates on the rollover rate of the hundred vehicle make/models
in each of the six states. It produces a maximum sample size
of only 600 (100 vehicles times six states) minus the number of
samples for which fewer than 25 crashes were available for determining
the rollover rate (a data quality control practice). Confidence
limits computed for a data sample size of 600 will be much greater
than those based on a sample size of 293,000. On average, each
sample in the linear regression analysis was computed from over
400 crash report samples. However, ordinary techniques to compute
the confidence intervals of linear regression results do not take
into account the actual sample size represented by aggregated
data. The statistical model created to combine SSF and dynamic
test information in the prediction of rollover risk was computed
by means of logistic regression as recommended by the NAS. Logistic
regression is well suited to the correlation with crash data of
vehicle properties that include both continuous variables like
SSF and binary variables like tip-up or no tip-up in maneuver
tests.
We had previously considered
logistic regression during the development of the SSF based rating
system (66 FR 3388, January 12, 2001, p.3393), but found that
it consistently under-predicted the actual rollover rate at the
low end of the SSF range where the rollover rates are high. The
NAS study acknowledged this situation and gave the example of
another analysis technique (non-parametric) that made higher rollover
rate predictions at the low end of the SSF scale. In the NPRM,
we discussed our plan to first examine ways to improve the fit
of the logistic regression model to the actual rollover rates
in the simpler model with SSF as the only vehicle attribute before
expanding the logistic regression model to predict rollover rates
using maneuver test results and SSF as vehicle attributes. In
this way, the addition of maneuver test results is more likely
to have an effect that reflects the additional information they
represent on rollover causation.
Appendix II discusses the details of seeking a
mathematical transformation of SSF to improve the accuracy of
logistic regression models. We found that logistic regression
on the transformation "Log(SSF - 0.9)" rather than on SSF directly
computed a risk model whose predictions of rollovers per single-vehicle
crash more closely matched the relationship between vehicle SSF
and actual rollover rates observed in state crash data. We sought
to optimize the accuracy of the predictions in the SSF range between
1.0 and 1.25 that includes the vehicles with the highest rollover
rates, even at the expense of accuracy in predicting the low rollover
rates at high end of the SSF scale. The risk model that resulted
from this exercise is equivalent to the SSF-based rating system
used for 2001-2003 NCAP rollover resistance ratings except that
it was computed using logistic regression rather than linear regression
as the statistical technique. Figure 3 compares the logistic
regression model and linear regression model formerly used for
NCAP ratings. The linear regression model is not in the form
of a straight line because it also operated on a transformation
of SSF (Log(SSF) in this case). The logistic regression model
is the more accurate at lower half of the SSF range, and the linear
regression model is the more accurate at the upper half of the
SSF range. The two curves are quite similar.
A good logistic regression risk model using SSF
only was the starting point for models using dynamic variables
together with SSF. The dynamic maneuver test results (tip-up
or no tip-up in each maneuver/load combination in Table 1) were
used as four binary dynamic variables in the logistic regression
analysis. The dynamic variables were entered in addition to SSF
to describe the vehicle. The same driver and road variables from
state crash reports discussed above were used. The state crash
report data for twenty four of the vehicles used in the logistic
regression analysis with dynamic maneuver test variables was a
subset of the database of 293,000 single-vehicle crashes described
above. One extra vehicle was added for the maneuver tests that
was not among the 100 vehicle groups we had studied previously,
but state crash report data from the same years and states was
obtained for it. However, the database with SSF and dynamic maneuver
test was much smaller than the 293,000 sample size available for
the logistic regression model with SSF only. Its sample size
was 96,000 single-vehicle crashes of 25 vehicles including 20,000
rollovers. Appendix II contains a more detailed discussion.
First, we tried each dynamic variable separately
in conjunction with SSF. The models using variables for performance
in the Fishhook heavy and J-Turn heavy maneuvers predicted a greater
rollover risk for those vehicles that tipped up in the maneuver
test. However, the models using variables for performance in
the Fishhook light and J-Turn light maneuvers predicted a greater
rollover risk for vehicles that did not tip up.
We do not believe vehicles that tip up in the
least severe maneuvers are actually safer than those that do not
tip up. A more rational interpretation is that the numbers of
vehicle tipping up in these maneuvers were too few to establish
a definitive correlation. Only three vehicles tipped up in the
J-Turn light maneuver, and six vehicles tipped up in the Fishhook
light maneuver. Only one more vehicle tipped up in the J-Turn
heavy maneuver than in the Fishhook light, and the prediction
of the model with J-Turn heavy was consistent with expectations
that tip-up in the test predicts greater rollover risk. However,
the extra vehicle in the J-Turn heavy tip-up group was the Ford
Ranger 2 WD with a very large sample size of over 8,000 single-vehicle
crashes (nearly 10 percent of the entire data base).
Next we computed a logistic regression model combining
SSF with the dynamic variables for both maneuvers, Fishhook heavy
and J-Turn heavy, that were observed to have a directionally correct
result when entered into the model individually. The variable
for J-Turn heavy was rejected by the logistic regression program
as not statistically significant in the presence of the Fishhook
heavy variable. In other words, the predictions based on tip-up
in the Fishhook heavy maneuver do not change whether or not the
vehicle also tips up in the J-Turn heavy maneuver.
Figure 4 shows the final model that uses Fishhook
heavy as the only necessary dynamic variable. This model has
a risk prediction for vehicles that tip up in the dynamic maneuver
tests based on the greatest number of vehicles possible in our
25 vehicle data base. All 11 vehicles that tipped up in any maneuver
are represented on the tip-up curve, and the 14 vehicles without
tip-up are represented on the other curve. The risk curve in
Figure 4 representing vehicles that tipped up in the Fishhook
heavy maneuver is very similar to the logistic regression model
based on SSF only in Figure 3 (that was based on the rollover
rates of 100 vehicles). This result is logical because the SSF
only model was optimized for best fit in the 1.00 to 1.25 SSF
range that included all vehicles tipping up in dynamic maneuver
tests. Also, the fact that the risk curve of the logistic regression
model in Figure 3 that was based on the SSF of 100 vehicles closely
matches the risk curve in Figure 4 that was based on 11 vehicles
that tipped up in the dynamic tests suggests that the curve in
Figure 4 is robust. However, the small difference in Figure
4 between the risk curve for vehicles that tip up in the dynamic
test and the risk curve for those that do not tip up suggests
that the predictive power of tip-up in the dynamic test may not
be great.
Our testing and logistic regression analysis was
sufficient to assign a greater rollover risk to vehicles that
tipped up in the most severe maneuver than to those that did not
tip up at all. However, the extra risk was small, and we were
not able to distinguish a rollover risk difference between vehicles
that tipped up in the less severe Fishhook maneuver with a two
occupant load from those that tipped up only with a five occupant
load. In general, vehicles that tip up in the Fishhook maneuver
with a two occupant load also tip up at a slower entry speed in
the Fishhook maneuver with a five occupant load than those that
do not. Therefore, our data does not allow us to distinguish
rollover risk differences between vehicles on the basis of maneuver
entry speed for tip-up. The objective of using different load
conditions and different maneuvers instead of different speeds
in a single maneuver to provide a range of test severity was to
reduce the sensitivity of the result to extraneous factors such
as tire wear.
It is noteworthy that the final rollover risk
model required results from only the fishhook maneuver. This
is an advantage from the standpoint of minimizing the practical
problems of the effects of tire wear during a test series and
of deviations from uniformity of surface friction at a test facility.
The fishhook maneuver produces less wear on the test tires and
requires only about 2 or 3 lane widths of uniform test surface
versus 10 or more lane widths for the J-Turn maneuver. The commenters
also considered it more representative of a real driving situation
than the J-Turn.
<
VII. Comments to the NPRM Notice and Agency Response
We received 39 comments to the NPRM notice from
vehicle manufacturers, equipment suppliers, test labs, public
interest groups, the National Transportation Safety Board, the
Insurance Institute for Highway Safety, attorneys, and members
of the public. Mainly, the comments addressed whether the static
and dynamic measurements should be used for separate ratings of
rollover resistance or for a combined rating based on a risk model.
The nature of the dynamic maneuver tests, testing of 15-passenger
vans, and several practical testing issues such as the extraneous
effects of tire wear, surface condition and ambient temperature
were also addressed. The notice also introduced the related subject
of handling ratings that was not part of the TREAD Act requirements.
We received a number of valuable comments on handling tests, and
we are still soliciting information. However, the subject of
this notice is confined to the TREAD Act requirements for dynamic
rollover ratings.
<
A. Combined or Separate Rollover Resistance Ratings
The main question posed in the NPRM notice was whether the rollover
resistance ratings should reflect the combined statistical power
of SSF and dynamic tests for predicting rollover risk or whether
ratings of rollover risk using SSF alone should continue, supplemented
with a qualitative comparison of dynamic test performance. The
notice gave alternative A as a risk model determined by logistic
regression analysis of state crash reports of single-vehicle crashes
for about 25 vehicles with known SSF and dynamic test results.
That process led to the risk model described in Section VI, however
the mathematical calculation of the model could not be performed
until the completion of a lengthy dynamic test program. Alternative
B in the notice was a continuation of rollover risk prediction
using SSF-only plus qualitative separate dynamic scores of A,
B, C, D, or E signifying the number of maneuvers in which the
vehicle tripped up without a risk interpretation.
Commenters representing TRW Automotive, National Automobile Dealers
Association (NADA), General Motors (GM), Alliance of Automobile
Manufacturers (Alliance), Association of International Automobile
Manufacturers (AIAM), Insurance Institute for Highway Safety (IIHS),
Bosch, Consumers Union, Advocates for Highway and Auto Safety
(Advocates), Toyota, Continental-Teves and Public Citizen remarked
directly on the question of combined versus separate use of SSF
and dynamic maneuver tests in rollover resistance ratings. Except
for Continental-Teves and Bosch, the commenters were in favor
of ratings that combined the SSF and dynamic maneuver tests in
a single rating. Consumers Union specifically supported the logit
risk model operating on a moderate risk scenario (in which rollover
rates vary in the approximate range of 0.075 to 0.55 across the
range of vehicles) as a way of combining the SSF and dynamic maneuver
tests. It commented that using the risk model it described was
consistent with the recommendations of the NAS study. We believe
the risk model we have developed is consistent with recommendation
of NAS and Consumers Union. It is the logit model with the risk
scenario (of demographic and road condition variables) that represents
the average crash conditions of 293,000 actual single-vehicle
crashes. It produces predicted rollover rates in the range of
0.09 to 0.50 for vehicles ranging from tip-up to no tip-up in
maneuvers and from 1.0 to 1.55 in SSF.
The other commenters in favor of combined ratings were primarily
concerned that separate ratings would be too confusing to serve
as consumer information. They believed a combined rating was
the only viable option, but they did not comment specifically
on the means used by NHTSA to develop the combined risk model.
IIHS and the Alliance (along with Carr Engineering) suggested
that another comment period following the notice containing the
actual model (as opposed to the example given in the NPRM notice)
would be necessary. GM suggested that the risk model be developed
through a collaborative effort along the lines of the Motor Vehicle
Safety Research Advisory Committee, and the Alliance suggested
a working-level dialog between NHTSA and the auto industry to
develop the risk model. TRW supported a single rating that would
be computed on the basis of the SSF only model with a predetermined
number of stars added or subtracted for dynamic maneuver performance
(determined without a statistical relationship to risk). Advocates
expressed wariness that the combined rating could be misleading
to consumers unless it corresponded to real-world rollover rates.
Public Citizen preferred the combined rating developed from a
risk model. It was concerned that consumers would focus more
attention on the dynamic maneuvers in separate ratings although
the tests represent an event (on-road untripped rollover) that
occurs in less than 5 percent of actual rollover crashes.
Continental-Teves and Bosch prefer separate ratings for SSF and
dynamic maneuver tests. Continental-Teves stated that "the relative
effects of SSF and dynamic performance are not well understood,
and may not be the same for every vehicle or every driver." Bosch
stated that "static and dynamic ratings should be separate, as
they are both equally important with regards to indicating stability
and safety of the vehicle." Bosch further explained that " a
combined rating may not adequately show the influence of such
systems [Electronic Stability Control and Rollover Mitigation]
which in turn would not encourage manufacturers to add systems
to vehicles that increase overall vehicle safety in potential
rollover as well as many other situations."
<
B. Crash Avoidance Technologies
Some of the stated expectations of the commenters about rollover
resistance ratings are unrealistic. The rollover resistance
ratings predict the likelihood of a single-vehicle crash becoming
a rollover. They do not predict the likelihood of the vehicle
becoming involved in a single-vehicle crash. Similarly, the frontal
and side NCAP crashworthiness ratings do not predict the likelihood
of the vehicle striking an object head-on or being struck from
the side. The Alliance comment anticipates the dilemma. While
conceding that SSF is strongly correlated with a tripped rollover
once the vehicle is already off-road, it states that " the likelihood
of being involved in a single-vehicle crash in the first place
- particularly one involving off-road excursion - is influenced
much more by demographic and environmental influences than is
the scenario examined for SSF purposes." The scenario used in
the combined risk model is the same scenario used in the SSF model,
namely the average demographic and environmental variables reported
by the states for the entire 293,000 single-vehicle crash data
base we have collected. We think this is the best scenario to
characterize single-vehicle crashes.
The Alliance is concerned that our model "may fail to account
for potentially beneficial technologies for avoiding single-vehicle
and rollover crashes, such as electronic stability control and
variable ride high suspension systems." Its concern is unnecessary
for variable ride-height suspension systems, which will be tested
in the highway rather than off-road height for both SSF and dynamic
maneuver tests, and the technology will certainly improve the
rating of vehicles so equipped.
However, the Alliance is right that the model does not predict
the risk of a single-vehicle crash. NHTSA has been very clear
in public notices, consumer information and web site presentations
that neither the SSF risk model nor the proposed combined SSF
and dynamic maneuver risk model predict the risk of having a single-vehicle
crash. From the standpoint of rollover resistance, single-vehicle
crashes are a measure of exposure. The prediction is of the risk
of a rollover resulting from the exposure of the vehicle to a
single-vehicle crash. The risk of rollover in the event of a
single-vehicle crash is strongly influenced by vehicle properties,
but the vehicle properties of modern vehicles have far less influence
in comparison to demographic and environmental factors regarding
the risk of a single-vehicle crash in the first place. However,
electronic yaw stability control may provide a real-world reduction
in single-vehicle crashes.
We have been optimistic about the potential of electronic yaw
stability control to reduce single-vehicle crashes. NHTSA's
consumer information identifies its availability as standard or
optional equipment on individual vehicles and explains how it
operates to help a driver maintain control in extreme circumstances.
One of the reasons we are exploring the possibility of NCAP handling
ratings is to describe the effect of yaw stability control on
handling predictability. However, the technology has not been
in widespread use long enough to produce much crash evidence for
the evaluation of its real-world effectiveness in preventing single-vehicle
crashes. Our previous attempts at evaluating its effectiveness
were thwarted by insufficient data.
Part of the motivation for the NAS study of NHTSA's SSF-based
rollover resistance ratings was the Alliance's concern that yaw
stability control was not being considered. In its public oral
presentation to the NAS study committee in May 2001, NHTSA said
it did not expect yaw stability control to have a large effect
on the risk of rollover given a single-vehicle crash. In its
view, the large majority of rollovers were the result of various
types of tripping, and SSF represented the most important vehicle
attributes in those circumstances. NHTSA believes that the greatest
potential effect of yaw stability control was in reducing single-vehicle
crashes in the first place. Therefore, we suggested to the committee
that rather than trying to predict rollovers per single-vehicle
crash with dynamic maneuver tests, we should keep SSF for that
purpose and adjust the comparative risk for vehicles with yaw
stability control by the effect of yaw stability control to reduce
exposure to single-vehicle crashes. However, establishing the
effectiveness of yaw stability control would require data not
available for at least two or three more years. Neither the NAS
committee nor the Alliance, which was active in providing the
committee information, expressed interest in this suggestion.
But the present comments indicate that finding a way to include
the crash avoidance potential of yaw stability control is a principal
concern of the Alliance and several suppliers of these systems.
IIHS's comment also shows an expectation of more than what is
possible for a rollover resistance rating. It discusses a comparison
of the 1997 Jeep Grand Cherokee and 1997 Toyota 4Runner made in
one its reports. In that report, the Toyota had four times the
number of fatal rollovers per 100,000 registered vehicles as the
Jeep, but they had very similar SSFs. They also had very similar
rollover rates in terms of rollovers per single-vehicle crash
that were consistent with their SSFs. IIHS expects a good dynamic
rating to show a large difference between the Grand Cherokee and
the 4Runner. That will not be possible because differences in
dynamic maneuver test performance predict only small differences
in rollover rate, and, in fact, there is not a large difference
in rollover rate between these vehicles in terms of rollovers
per single-vehicle crash in our six state crash data base. The
difference is in the definition of rollover rate. A rollover
rate in terms of fatal rollovers per 100,000 vehicles depends
on the rate of single-vehicle crashes per 100,000 vehicles and
on the occurrence of a fatality in the rollover as well as on
the rate of rollover per single-vehicle crash. The first two
of these factors depend primarily on demographic and environmental
influences and can mask actual differences or similarities between
vehicles as in this case. Neither vehicle had yaw stability control,
which would have created a plausible vehicle-related difference
in single-vehicle crash rate. The difference in fatality rate
could involve crashworthiness features, or particularly in the
case of rollover, it could merely reflect the seat belt wearing
habits of a risk taking demographic that also experienced a higher
rate of single-vehicle crashes. The rate of rollovers per single-vehicle
crash is much less sensitive to demographic influences than is
the rate of fatal rollovers per 100,000 vehicles.
Carr Engineering and Suzuki commented that the agency was not
following the recommendations of the NAS study by performing J-Turn
and Fishhook maneuver tests. They believe that the NAS recommended
handling tests to assess loss of control potential rather than
limit maneuvers to assess the resistance of the vehicle to actual
on-road tip-up. We agree that the language of the NAS study report
is somewhat ambiguous. That is why we included in our NPRM notice
the clarification the NAS study panel gave us during the presentation
of the report to NHTSA in response to our direct questions about
J-Turn and Fishhook tests versus handling tests. The NAS study
committee clarified that it envisioned dynamic maneuver tests
as limit maneuvers where loss of control and actual on-road vehicle
tip-up can be expected for vulnerable vehicles. The NAS study
panel stated it was not in a position to recommend a specific
test because that would require study of discriminatory capability,
repeatability and other properties, but J-Turns and Fishhooks
were of the type of tests it had in mind. Two outside experts
in vehicle dynamics and testing reviewed our test plan before
the Phase VI test of the 25 vehicles. One had been a member of
the NAS study committee. Once again, we were assured that our
tests were consistent with the NAS recommendations.
We believe that both our test selection and our analysis method
of developing a rollover risk model to combine SSF and dynamic
test results are entirely consistent with the recommendations
of the NAS study and therefore appropriate to satisfy the requirements
of the TREAD Act. We agree that it is important to inform consumers
of the effectiveness of yaw stability control in reducing single-vehicle
crashes, and we will determine its effectiveness from crash report
data as sufficient data becomes available.
<
C. The J-Turn and Fishhook Maneuvers
There were a number of comments regarding the J-Turn and Fishhook
test protocols from the Alliance, GM, Toyota, Honda, Nissan, Renfroe
Engineering, Carr Engineering, Mechanical Systems Analysis Inc,
and Automotive Testing Inc. In addition, Ford made a detailed
presentation elaborating on some of the subjects introduced in
the Alliance comment. The Ford presentation material was placed
in Docket NHTSA-2001-9663.
A number of the commenters objected to the J-Turn maneuver because
they thought it was not representative of real driving, involved
too fast a steering movement, or was redundant. Since its results
were not used in the risk model, we agree that it is redundant.
As a result, we are no longer planning to use it in the NCAP testing
program.
Except for Suzuki, Carr Engineering and Ford, those who commented
on the maneuver tests supported the Fishhook maneuver. Carr Engineering
and Advocates objected to calling the Fishhook maneuver a road
edge recovery test as we had done in the NPRM notice. While the
Fishhook maneuver includes steering commands like a crash involving
road edge recovery, it is performed on a smooth uniform surface
instead of one with vertical drop-offs and friction coefficients
differences that exist at road edges. To accommodate these concerns,
we will refer to the maneuver as the Fishhook.
<
D. Tire Wear
The effect of tire wear on test results and the tire changing
protocol was addressed by several commenters. Tire shoulder wear
during limit maneuver tests is much more severe than in ordinary
driving and has the effect of increasing the lateral acceleration
capability of the vehicle. After a number of tests, the tire
wear causes the vehicle to tip up more easily, and there is concern
that a vehicle with test-worn tires does not represent a typical
street driven vehicle. In the 25 vehicle test, new tires were
used for each maneuver (FH, FL, JH, JL) which limited the tires
to no more than 6 runs in each direction (4 for Fishhooks) before
detecting tip-up if it occurred.
Ford gave an example using a Ford Ranger 4WD that was apparently
known to tip up at 53 mph with worn tires in a J-Turn test. The
vehicle was equipped with new tires and tested repeatedly at 53
mph. It did not tip up during the first three runs, but during
the fourth run a large increase in lateral acceleration and sideslip
angle occurred and the vehicle tipped up. It continued this behavior
for two subsequent runs, and the tires exhibited a large amount
of shoulder wear after only six runs. We have noticed similar
tire wear effects, but not in so few runs. The J-Turn tests are
of much longer duration than Fishhook tests and produce more wear
per run. Also tests run at lower speeds approaching tip-up speed
produce less wear than tests performed at a higher speed just
below the tip-up speed. Ford's example of a worst case in which
the tire wear of just three runs changed vehicle behavior from
no tip to tip-up is an effective illustration of the tire wear
problem.
We believe this problem is much less acute for Fishhook tests.
We performed a similar experiment using a 2001 Ford Explorer 4
door 4WD that we knew would tip up at 40 mph on worn tires in
a Fishhook maneuver. We performed 18 test runs without tip-up
and then experienced a 20 degree tip-up against the outriggers
on the nineteenth run. We performed three more runs and experienced
two more tip-ups. Renfroe Engineering also commented about tire
wear effects citing an UMTRI study in which lateral tire forces
remained steady for about 10 runs and then increased to a maximum
force at about 20 runs.
Ford suggested a tire change protocol to limit tire wear. We
intend to test a number of vehicles in the summer of 2003. During
these tests we will use the tire change protocol of Appendix I
because we believe this appropriately limits the effect of tire
wear. However, we intend to confirm tip-ups using new (broken
in but not worn) tires when appropriate to make sure that the
vehicle scores have not been affected by tire wear. We will consider
the results of this exercise in deciding whether any changes in
the tire change protocol are necessary.
<
E. Pavement Temperature
The Alliance and Toyota commented on the potential effect of
pavement temperature on Fishhook maneuver results. Toyota has
observed increases in pavement friction as an apparent consequence
of increases in pavement temperature. It also supplied a computer
simulation of Fishhook tests that showed a large decrease in the
speed at tip-up with increases in surface friction. Taken together,
Toyota's information predicts a decrease in tip-up speed in a
Fishhook maneuver of over 15 mph for a 70 degree F increase in
pavement temperature. While the risk model for ratings does
not depend on tip-up speed, the temperature effects predicted
by Toyota would prevent most of the vehicles that tipped up in
a summer test from having tip-up in a winter test. NHTSA ran
a number of tests to evaluate the temperature sensitivity of J-Turn
and Fishhook tests (NHTSA Technical Report "Testing to Determine
the Effects of Ambient Temperature on Dynamic Rollover Testing",
docketed with this notice). We tested the 2001 Toyota 4Runner
4WD (with and without yaw stability control enabled) and the 2001
Chevrolet Blazer 2WD on the same test track during cold, moderate
and hot ambient temperature. The difference between cold and
hot ambient temperature was about 60 degrees F. We do not have
pavement temperatures, but there is no reason to believes that
the range of pavement temperature is less than the range of ambient
temperature. The whole test procedure including the determination
of handwheel angles based on the 0.3g steady state curve was repeated
at each temperature. The results are given in Table 2. Every
test that failed to cause tip-up in cold weather also failed to
cause tip-up in hot weather, and the two tests that caused tip-up
in hot weather also caused tip-up in cold weather. Thus, the
temperature effect predicted by the commenters did not occur.
The tip-up speeds for the Blazer in the right and left Fishhooks
repeated to within 1 mph despite differences in ambient temperature
of 60 degrees F, seasonal differences in pavement surface, and
the use of three different sets of tires. The only temperature
Table 2.
Results from NHTSA J-Turn and Fishhook Tests at Various Ambient
Temperature Conditions.
| Test
Vehicle and Configuration |
Test
Maneuver |
Test
Condition |
Ambient
Temperature
(°F) |
Commanded
Handwheel Angle (degrees) |
Initial
Steer Left |
Initial
Steer Right |
Wheel
Lift, Front/Rear
(inches) |
Maneuver
Entrance Speed (mph) |
Wheel
Lift, Front/Rear
(inches) |
Maneuver
Entrance Speed (mph) |
| Front |
Rear |
Front |
Rear |
Toyota
4Runner,
VSC disabled |
NHTSA
J-Turn1 |
Cold |
30 |
345 |
0 |
0 |
62.1 |
0 |
0 |
61.7 |
| Moderate |
79 |
354 |
0 |
0 |
60.4 |
0 |
0 |
60.0 |
| Hot |
87 |
358 |
0 |
0 |
61.8 |
0 |
0 |
60.3 |
| Fishhook2 |
Cold |
32 |
280 |
1 |
0 |
51.1 |
0 |
1 |
51.7 |
| Moderate |
74
- 73 |
287 |
0 |
0 |
48.0 |
0 |
0 |
48.5 |
| Hot |
89 |
290 |
1 |
0 |
51.4 |
0 |
0 |
50.8 |
Toyota
4Runner,
VSC enabled |
NHTSA
J-Turn1 |
Cold |
28 |
345 |
0 |
0 |
61.8 |
0 |
0 |
62.4 |
| Moderate |
75 |
354 |
0 |
0 |
59.4 |
0 |
0 |
58.2 |
| Hot |
90 |
358 |
0 |
0 |
61.9 |
0 |
0 |
61.6 |
| Fishhook2 |
Cold |
31 |
280 |
0 |
0 |
51.3 |
0 |
0 |
51.7 |
| Moderate |
72 |
287 |
0 |
0 |
48.8 |
0 |
0 |
50.1 |
| Hot |
90 |
290 |
0 |
0 |
50.7 |
0 |
0 |
51.3 |
| Chevrolet
Blazer |
NHTSA
J-Turn1,3 |
Cold |
29 |
381 |
5
- 8 |
5
- 8 |
58.0 |
5
- 8 |
5
- 8 |
54.8 |
| Moderate |
83 |
401 |
0 |
0 |
60.9 |
0 |
0 |
62.2 |
| Hot |
86 |
392 |
0 |
0 |
60.3 |
0 |
0 |
59.4 |
| Fishhook2,3 |
Cold |
30 |
309 |
5
- 8 |
5
- 8 |
40.2 |
2
- 3 |
2
- 3 |
39.1 |
| Moderate |
74 |
326 |
3
- 4 |
3
- 4 |
40.3 |
4
- 5 |
4
- 5 |
40.1 |
| Hot |
90 |
319 |
2
- 3 |
2
- 3 |
39.4 |
2
- 3 |
2
- 3 |
38.8 |
1NHTSA
J-Turn maximum nominal entrance speed was 60 mph
2Fishhook maximum nominal entrance speed was 50 mph
3Two-wheel lift ³2 inches was observed during tests highlighted
in bold
effect observed
was that the Blazer tipped up in the J-Turn in cold weather but
did not in the moderate and hot weather tests. This is the opposite
of the temperature effect predicted by the commenters and occurred
during a maneuver we no longer intend to use. We do not think
it is necessary to set tight surface temperature limits on the
test protocol as suggested by the commenters.
<
F. Surface Friction
A practical problem for the repeatability of any
limit maneuver test is the possibility that the surface friction
properties of the test track will change. Ford commented that
computer simulations of several of its SUVs showed that a change
in surface coefficient of 0.05 would change the tip-up speed in
a fishhook test by as much as 12 mph in one example (6 mph and
4 mph respectively for two other example vehicles). It also commented
that a seasonal variation in surface coefficient of 0.05 could
be typical of test tracks, and that its own test track exhibited
a long-term trend of an increase in coefficient of 0.02 per year
(which would change the tip-up speed of the first example vehicle
by 8 mph in Ford's simulation). Ford's simulations are even
more pessimistic than Toyota's regarding the possibility of repeatable
Fishhook tip-up speeds given normal variations in surface properties
and temperatures. However, we have not observed these large variations
in tip-up speed in actual tests. The very close repeatability
of tip-up speed for the Blazer in Table 2 extended over likely
seasonal changes in the pavement as well as changes in ambient
temperature.
Additionally, NHTSA performed a study using the
same 4Runner and Blazer mentioned above for J-Turn and Fishhook
tests at Daimler Chrysler's Arizona Proving Grounds (APG) and
General Motors Desert Proving Grounds (DPG) as well as TRC of
Ohio, where our maneuver test development has been conducted (NHTSA
Technical Report "Testing to Determine the Effects of Surface
Variability on Dynamic Rollover Testing", docketed with this notice).
Table 3 shows the peak and slide braking coefficients (multiplied
by 100) measured at these facilities.
Table 3.
Friction Numbers for all Test Facilities
| Test
Facility |
Peak
Braking Coefficient |
Skid
Number |
| Dry |
Wet |
Dry |
Wet |
| TRC |
94
- 96 |
69
- 83 |
81
- 84 |
47
- 54 |
| DPG |
86
- 93 |
74
- 77 |
83
- 85 |
60
- 64 |
| APG |
90
- 93 |
75
- 80 |
81
- 84 |
56
- 59 |
Table 4 shows
the results of the maneuver tests. As in Table 2, the vehicles
were loaded with the equivalent of a 2-occupant load, like the
light load condition of the 25 vehicle test. The 4Runner did
not tip up at TRC and it did not tip up at the other facilities.
The Blazer did not tip up in the J-Turn at TRC, but it did at
the other facilities. We do not think that this is a result of
the surface coefficient of friction (due to the similarities of
the ranges) but rather due to the greater degree of vertical irregularities
and pavement cracks at DPG and APG than at TRC. Tip-up is often
triggered by vertical oscillations of the vehicle suspension during
high cornering forces in maneuver tests. DPG had the most vertical
surface irregularities that caused the Blazer to tip up most easily.
The Blazer tipped up in the Fishhook at TRC, and it also tipped
up in the Fishhook at the other facilities. Again, the tip-up
speeds were lower at APG and DPG, which would be expected due
to the greater surface irregularities.
Table 4.
Results from NHTSA J-Turn and Fishhook tests
| Test
Vehicle and Configuration |
Test
Maneuver |
Test
Facility |
Commanded
Handwheel Angle, deg |
Initial
Steer Left |
Initial
Steer Right |
Moderate
or Major Lift
Yes / No |
Maneuver
Entrance Speed, mph |
Moderate
or Major Lift
Yes / No |
Maneuver
Entrance Speed, mph |
Toyota
4Runner,
VSC enabled |
NHTSA
J-Turn1 |
TRC |
354 |
No |
58.21 |
No |
59.29 |
| DPG |
402 |
No |
61.56 |
No |
61.21 |
| APG |
362 |
No |
61.68 |
No |
62.11 |
| Fishhook2 |
TRC |
287 |
No |
48.75 |
No |
50.13 |
| DPG |
327 |
No |
53.05 |
No |
50.94 |
| APG |
294 |
No |
52.63 |
No |
51.44 |
Toyota
4Runner,
VSC disabled |
NHTSA
J-Turn1 |
TRC |
354 |
No |
60.4 |
No |
60.00 |
| DPG |
402 |
No |
60.97 |
No |
61.63 |
| APG |
362 |
No |
62.38 |
No |
62.27 |
| Fishhook2 |
TRC |
287 |
No |
49.84 |
No |
49.79 |
| DPG |
327 |
No |
52.20 |
No |
51.93 |
| APG |
294 |
No |
51.04 |
No |
51.14 |
| Chevrolet
Blazer |
NHTSA
J-Turn1 |
TRC |
401 |
No |
60.90 |
No |
62.27 |
| DPG |
382 |
Yes |
49.80 |
Yes |
44.90 |
| APG |
395 |
Yes |
57.36 |
Yes |
58.68 |
| Fishhook2 |
TRC |
326 |
Yes |
40.32 |
Yes |
40.09 |
| DPG |
311 |
Yes |
37.80 |
Yes |
38.01 |
| APG |
321 |
Yes |
35.52 |
Yes |
38.54 |
1NHTSA
J-Turn maximum nominal entrance speed is 60 mph 2Fishhook
maximum nominal entrance speed is 50 mph
We recognize
the potential difficulties caused by changes in surface friction
coefficient, and we have tried to minimize them. We have observed
the Fishhook maneuver to be less sensitive to surface conditions
than the J-Turn, and we have used changes in vehicle load condition
rather than changes in tip-up speed to signify degrees of test
severity in a way least likely to be influenced by surface coefficient.
None of the changes of pavement and temperature in our test experience
has caused a change in the Fishhook result (tip-up or no tip-up)
for a vehicle. We believe the comments based on computer simulation
overstate the sensitivity observed in our actual tests.
<
G. Steering Reversal
Honda commented that using a roll rate measurement
within 1.5 degrees/sec of a zero crossing as shown in Figure 2
to trigger the reverse steering in a fishhook maneuver occasionally
leads to an unusually long dwell time (T1) for certain
vehicles at certain load conditions. It suggested setting a default
value for dwell time to force a reverse steering action if the
absolute value of the vehicle roll rate stayed too long at a value
that was very low but not low enough to trigger reversal. It
explained that tests in which excessive dwell times occurred would
be less severe and possibly not cause a tip-up that would have
occurred with a shorter dwell.
Automotive Testing Inc. commented at length on
the same phenomenon. It observed that the low but steady roll
rate above 1.5 degrees/sec that can delay the triggering of steering
reversal is a result of tire deflections continuing the roll motion
of the whole vehicle after the point of maximum roll of the suspension
system. It believes that a default trigger negates the design
of the maneuver to let the vehicle motions select the steering
response, but describes some ways of using filtering of the roll
rate signal to cause the steering to trigger earlier in these
cases. But it acknowledges that letting the vehicle react to
the actual roll motion of the whole vehicle rather than to a roll
signal distorted by signal processing may be preferable.
At this point we are preserving the consistent
application of the fishhook steering algorithm. We do not believe
that commenters have presented us a substantive reason to depart
from this application. If the vehicle tips up despite a long
dwell time, there is no change in test result. If the vehicle
does not tip, it will be retested with a reduced steering angle
according to the current procedure, which may change the roll
frequency harmonics and dwell time. We will observe the steering
reversal dwell times during the first group of tests and, if necessary,
reconsider the commenter's observations on this issue.
<
H. Fifteen-Passenger Vans
The National Transportation Safety Board, Public Citizen and
others commented on the rollover issues surrounding fifteen-passenger
vans. NHTSA agrees that it is important to investigate the commenters'
concerns about the rollover susceptibility of fifteen-passenger
vans. To do this, we will conduct an evaluation of fifteen-passenger
vans' rollover susceptibility at different loading conditions
and evaluate available electronic stability control systems on
these vehicles.
<
I. Tip-up Criterion
Mechanical Systems Analysis, Inc. and several other commenters
suggested that the tip-up criterion of 2 inches simultaneous wheel
lift is too conservative. It recommended a criterion of 20 degrees
body roll instead because suspension bouncing on test surface
irregularities could influence performance under our criterion.
Other similar recommendations were given for body roll angles
between 15 and 20 degrees. The 2 inch wheel lift criterion is
met at about 11 degrees of body roll on average.
NHTSA's tests were performed on a very smooth test area at TRC
of Ohio. The tip-up criterion maximized driver safety and minimized
tire wear by allowing us to increase speed in 5 mph increments
with a reasonable expectation of avoiding sudden violent tip-ups
that could 'pole-vault' the vehicle on its outriggers. However,
we observed tip-ups at lower than expected speeds during tests
at other facilities (DPG and APG as described above) that were
probably influenced by surface irregularity as described by the
commenter. We believe that our tip-up criterion is appropriate
for an excellent facility like TRC, but we agree that the criterion
should be revisited if NCAP tests were to take place at a facility
with a more irregular surface.
<
J. Testing of Passenger Cars v. Light Trucks
Consumers Union and IIHS recommended that we not test passenger
cars in order to devote all the available time and resources for
maneuver tests to light trucks. We agree that it is very unlikely
that passenger cars will tip up in the maneuver test. We have
tested passenger cars at the low end of the SSF range for passenger
cars without observing any tip-ups. It seems reasonable to rate
passenger cars using the "no tip-up" curve of the risk model along
with SSF measurements. However, we prefer to track whether this
continues to be true. Hence, we will continue to test a few passenger
cars each year at the low end of the SSF range to reinforce the
"no tip-up" assumption. Therefore, two passenger cars are listed
in Table 5.
<
K. Testing with Stability Control Systems
Toyota suggested that NHTSA should selectively choose vehicles
with optional equipment that assists the driver in controlling
the vehicle such as electronic yaw stability control, while in
a previous comment Honda suggested the opposite policy. Honda
believed that even a vehicle with standard stability control should
be tested with it turned off if the vehicle has an "off" switch.
It has been NHTSA's policy for rollover resistance ratings that
we test vehicles most representative of those sold. Also, we
are interested in the potential safety benefits of electronic
yaw stability control and have alerted consumers to its purpose
and availability on individual models in our present consumer
information. Therefore, when it is standard equipment or optional
equipment found on the majority of vehicles of a particular model,
we will test with stability control turned on and report that
the test vehicle was so equipped. Also, if the market penetration
of a stability control option is too low for NHTSA to choose it
for inclusion on our test vehicle, we will consider optional NCAP
tests at the manufacturer's expense.
<
VIII. Final Form for Rollover Resistance Ratings - Alternative
I
<
A. Combined Ratings
NHTSA will use the statistical model shown in Figure 4 to combine
the vehicle's SSF measurement and its performance in the Fishhook
maneuver with 5-occupant loading as a prediction of its rollover
rate per single-vehicle crash. The predicted rollover rate will
be translated into a star rating in the same way used in the present
rollover resistance ratings: one star for a rollover rate greater
than 40 percent; two stars, greater than 30 percent; three stars,
greater than 20 percent; four stars, greater than 10 percent;
five stars, less than or equal to 10 percent.
The decision to combine the static (SSF) and the dynamic (maneuver
test) vehicle measurements in a single rollover resistance rating
is consistent with the view of most commenters that separate ratings
would be confusing to consumers. It is also the best way of achieving
NHTSA's goal of presenting risk-based ratings because it maximizes
the vehicle information used to make the prediction of the rate
of rollovers per single-vehicle crash. Those who favored separate
static and dynamic ratings expressed concern that the influence
of electronic stability control would be small in the combined
rating. It is true that electronic stability control will not
have a great influence on rollover resistance ratings because
the dynamic test result has less predictive power than the static
measurement on rollover rate and the effect of electronic (yaw)
stability control on the dynamic test is also modest. We believe
that the potential benefit of electronic stability control lies
in helping drivers to stay on the road and away from tripping
devices rather than providing much increase in rollover resistance,
especially regarding tripped rollovers. Rather than reduce the
rate of rollovers in single-vehicle crashes, electronic stability
control may reduce the number of single-vehicle crashes in the
first place. However, its effectiveness in reducing single-vehicle
crashes remains to be demonstrated by crash statistics.
For the present time, we will retain the use of five stars to
express rollover resistance ratings. Focus groups consistently
find that presentation understandable. However, the NAS and a
number of commenters were in favor of presentations that are able
to show smaller differences between vehicles, contrast the range
of ratings between types of vehicles and show the relative position
of a vehicle's rating among other vehicles of the same type.
NHTSA is performing additional consumer research to determine
the best approach to providing consumers with more detailed information
to supplement the star ratings. Several presentation methods
are being tested, and we will consider those test results and
propose appropriate changes to how we present rollover information
to consumers.
<
B. Dynamic Testing
The Fishhook maneuver test will be conducted according to the
procedure in Appendix I, and we will discontinue the J-Turn maneuver
test. This decision is a consequence of the logistic regression
analysis of the crash data, SSF and results of the J-Turn and
Fishhook tests at two load conditions for 25 vehicles. From a
statistical point of view, the J-Turn test results were redundant
in the presence of the Fishhook test results. The J-Turn test
also seems to be more sensitive to irregularities in pavement
surface and friction and changes in ambient temperature than the
Fishhook test. It also causes more concern about tire wear effects
than the Fishhook, and it was criticized by some commenters as
less representative of 'real-world' driving situations.
We have decided to change the heavy load condition from an anthropomorphic
dummy (water dummy) in every rear seating position (along with
the test driver and instruments of approximately a passenger weight
in the front) to a standard load representing five occupants in
all vehicles capable of at least that loading. During the test
of the 25 vehicles, it became obvious that heavy load tests were
being run at very unequal conditions especially between vans and
other vehicles (two water dummies in some vehicles but six water
dummies in others). While very heavy passenger loads can certainly
reduce rollover resistance and potentially cause special problems,
crashes at those loads are too few to greatly influence the overall
rollover rate of vehicles. Over 94% of van rollovers in our 293,000
crash database occurred with five or fewer occupants, and over
99% of rollovers of other vehicles occurred with five or fewer
occupants. The average passenger load of vehicles in our crash
database was less than two: 1.81 for vans; 1.54 for SUVs; 1.48
for cars; and 1.35 for pickup trucks. In order to use the maneuver
tests to predict real-world rollover rates rather than investigate
possible poor performance at high occupancy levels, it is not
useful to test the vehicles under widely differing loadings while
there is much less loading variation represented in the crash
statistics. Consequently, the maneuver test data used in the
logistic regression analysis involving the 25 dynamic test vehicles
in the heavy load condition represented performance with a 5-occupant
loading (obtained using three water dummies in the rear seating
positions) for all vehicles capable of carrying at least that
load.
The use of dynamic maneuver tests creates the need for a policy
regarding tire de-beading. The tests are conducted using the
tire pressure recommended by the vehicle manufacturer and labeled
on the vehicle. We have experienced a number of instances in
which the tire bead became unseated from the rim, resulting in
total air loss and rim contact with the paved surface. This causes
damage to the test facility and the possibility of a rollover
of the test vehicle. For at least a year, we have been using
inner tubes in all tires placed on rollover test vehicles. This
action reduces the instances of total de-beading, but does not
eliminate them entirely. In some instances, a tire with a tube
that is not pinched during the process can experience a partial
de-bead in which the rim makes contact with the pavement surface
and then the tire becomes remounted on the rim by the pressure
of the tube. It has been NHTSA's experience on the test track
that if a maneuver results in rim contact without destroying the
tube, the next run at a higher speed will destroy the tube and
cause a complete de-beading of the tire and hard contact of the
rim with risk to the driver, test surface and vehicle.
In the case of rim contact without total de-beading, it is a
near certainty that total de-beading would have occurred without
the tube, and total de-beading despite the tube is highly likely
at the next speed increment. Thus, we consider rim contact to
indicate de-beading, and it will be NHTSA's policy to terminate
the test if rim contact with the pavement is observed even if
the tube prevents total de-beading.
The vehicle did not actually tip up in the maneuver if the test
is terminated as a result of rim contact indicating tire de-beading.
However, debeading is a bad outcome for the test because tire
de-beading is associated with on-road tripped rollovers that actually
outnumber on-road untripped rollovers. Therefore, it would be
improper to ignore tire debeading and predict the vehicle's rollover
rate as if it had completed the test without tip-up or de-beading.
The only alternative in the case of rim contact is to simply not
compute a rollover resistance rating of the vehicle because the
test was not completed. It will be reported that the dynamic
test could not be completed because of tire debeading, but the
SSF measurement will be retained in the detailed consumer information.
<
C. Demonstration Program
In April 2003, NHTSA's VRTC began the Demonstration Test program
at TRC of Ohio using the test protocol of Appendix I for Fishhook
maneuver tests of 18 new vehicles. Table 5 lists the vehicles
in this group. We will verify tip-ups using new tires as explained
in our answer to Ford's comments in Section VII. Unless we discover
serious procedural problems, these vehicles will be given 2004
NCAP rollover resistance ratings according to the system established
in this final notice.
Table 5.
Vehicles included in Demonstration Test
| |
Make |
Model |
BodyStyle |
| 1 |
Chevrolet |
Silverado
4x2 |
PU ext.
cab |
| 2 |
Chevrolet |
Silverado
4x4 |
PU ext.
cab |
| 3 |
Chevrolet |
Trailblazer
4x2 |
4-dr
Utility |
| 4 |
Chevrolet |
Trailblazer
4x4 |
4-dr
Utility |
| 5 |
Ford |
Explorer
4x2 |
4-dr
Utility |
| 6 |
Ford |
Explorer
4x4 |
4-dr
Utility |
| 7 |
Ford |
Explorer
SportTrac 4x2 |
4-dr
Utility |
| 8 |
Ford |
Explorer
SportTrac 4x4 |
4-dr
Utility |
| 9 |
Ford |
Focus |
4-dr
wagon |
| 10 |
Jeep |
Liberty
4x2 |
4-dr
Utility |
| 11 |
Jeep
|
Liberty
4x4 |
4-dr
Utility |
| 12 |
Subaru |
Outback
(4x4) |
4-dr
wagon |
| 13 |
Toyota |
Echo |
4-dr
sedan |
| 14 |
Toyota |
4Runner
4x2 |
4-dr
Utility |
| 15 |
Toyota |
4Runner
4x4 |
4-dr
Utility |
| 16 |
Toyota |
Tacoma
4x2 |
PU ExCab |
| 17 |
Toyota |
Tacoma
4x4 |
PU ExCab |
| 18 |
Volvo |
XC90
(4x4) |
4-dr
Utility |
<
X. Assessment of Costs and Benefits
Since this is a consumer information
program, no Regulatory Evaluation was developed for this notice.
Adding the dynamic maneuver tests to the Rollover NCAP will not
require vehicle manufacturers to take any action. The costs are
Federal Government costs for developing the test protocol and
rating system, conducting the tests, and disseminating the information.
The benefits are information to consumers. Consumers want additional
information. It is impossible for us to quantify the effect on
consumer behavior or on manufacturer behavior.
<
XI. Rulemaking Analyses and Notices
<
A. Executive Order 12866
Executive Order 12866, "Regulatory Planning and Review" (58 FR
51735, October 4, 1993), provides for making determinations whether
a regulatory action is "significant" and therefore subject to
Office of Management and Budget (OMB) review and to the requirements
of the Executive Order. The Order defines a "significant regulatory
action" as one that is likely to result in a rule that may:
(1) Have an annual effect on the economy of $100
million or more or adversely affect in a material way the economy,
a sector of the economy, productivity, competition, jobs, the
environment, public health or safety, or State, local, or Tribal
governments or communities;
(2) Create a serious inconsistency or otherwise interfere
with an action taken or planned by another agency;
(3) Materially alter the budgetary impact of entitlements,
grants, user fees, or loan programs or the rights and obligations
of recipients thereof; or
(4) Raise novel legal or policy issues arising out
of legal mandates, the President's priorities, or the principles
set forth in the Executive Order.
NHTSA has considered the impact of this action under Executive
Order 12866 and the Department of Transportation's regulatory
policies and procedures. This action has been determined to be
economically not significant. However, because it is a subject
of Congressional interest, this rulemaking document was reviewed
by the Office of Management and Budget under Executive Order 12866,
"Regulatory Planning and Review."
<
B. Regulatory Flexibility Act
The Regulatory Flexibility Act of 1980 (5 U.S.C. § 601 et
seq.) requires agencies to evaluate the potential
effects of their proposed and final rules on small business, small
organizations and small governmental jurisdictions. I hereby
certify that the amendment will not have a significant economic
impact on a substantial number of small entities. The proposed
action does not impose regulatory requirements on any manufacturer
or other party.
<
C. National Environmental Policy Act
NHTSA has analyzed this proposal for the purposes of the National
Environmental Policy Act. The agency has determined that implementation
of this action will not have any significant impact on the quality
of the human environment.
<
D. Executive Order 13132 (Federalism)
The agency has analyzed this rulemaking in accordance with the
principles and criteria contained in Executive Order 13132 and
has determined that it does not have sufficient federal implications
to warrant consultation with State and local officials or the
preparation of a federalism summary impact statement. The action
will not have any substantial impact on the States, or on the
current Federal-State relationship, or on the current distribution
of power and responsibilities among the various local officials.
<
E. Unfunded Mandates Act
The Unfunded Mandates Reform Act of 1995 requires agencies to
prepare a written assessment of the costs, benefits and other
effects of proposed or final rules that include a Federal mandate
likely to result in the expenditure by State, local or tribal
governments, in the aggregate, or by the private sector, of more
than $100 million annually (adjusted annually for inflation with
base year of 1995). Adjusting this amount by the implicit gross
domestic product price deflator for the year 2002 results in $113
million (110.66/98.11 = 1.13). The assessment may be included
in conjunction with other assessments, as it is here.
The action does not impose regulatory requirements on any manufacturer
or other party.
<
F. Civil Justice Reform
This action will not have any retroactive effect. Under 49 U.S.C.
21403, whenever a Federal motor vehicle safety standard is in
effect, a State may not adopt or maintain a safety standard applicable
to the same aspect of performance which is not identical to the
Federal standard, except to the extent that the state requirement
imposes a higher level of performance and applies only to vehicles
procured for the State's use. 49 U.S.C. 21461 sets forth a procedure
for judicial review of final rules establishing, amending or revoking
Federal motor vehicle safety standards. That section does not
require submission of a petition for reconsideration or other
administrative proceedings before parties may file suit in court.
<
G. Paperwork Reduction Act
This document does not contain "collections of information,"
as that term is defined in 5 CFR Part 1320 Controlling Paperwork
Burdens on the Public.
<
H. Plain Language
Executive Order 12866 requires each agency to write all rules
in plain language. This action will not result in regulatory language.
Issued on:
_________________________
Jeffrey W. Runge, M.D.
Administrator
BILLING CODE: 4910‑59P
Figure 3: Logistic regression risk model
using SSF only and
Linear regression risk model for 2001-2003 NCAP
Rollover Resistance
Figure 4: Final dynamic model using Fishhook
maneuver with heavy load (FH) as the only necessary dynamic variable
<
Appendix I. Fishhook Maneuver Test Procedure
1.0 INTRODUCTION
1.1 General
This document describes the test procedure used by the National
Highway Traffic Safety Administration's (NHTSA) New Car Assessment
Program (NCAP) to evaluate light vehicle dynamic rollover propensity.
The procedure is comprised of one characterization maneuver and
one rollover resistance maneuver.
1.2 Rollover Resistance Requirements of the TREAD Act
Section 12 of the "Transportation Recall, Enhancement, Accountability
and Documentation (TREAD) Act of November 2000" reflects the desire
of Congress to supplement SSF [Static Stability Factor] with a
dynamic stability test using vehicle maneuvers. Congress directed
NHTSA to "develop a dynamic test on rollovers by motor vehicles
for a consumer information program; and carry out a program conducting
such tests." NHTSA's NCAP Light Vehicle Dynamic Rollover Propensity
Test Procedure described in this document was developed as part
of NHTSA's effort to fulfill the requirements of the TREAD Act.
1.3 Recent NHTSA Light Vehicle Dynamic Rollover Propensity
Research
During the spring through fall of 2001 NHTSA performed an extensive
assessment of many test track maneuvers potentially capable of
quantifying on-road, untripped rollover propensity. In brief,
five vehicle characterization and nine dynamic rollover propensity
maneuvers were studied. Each maneuver was either discarded or
retained for subsequent program phases. The 2001 research project
is documented in [1].
During the spring through fall of 2002 NHTSA performed a comprehensive
evaluation of rollover resistance for a broad spectrum of twenty-six
light vehicles. The test vehicles were evaluated with one Characterization
maneuver and two Rollover Resistance maneuvers. Up to two load
configurations per vehicle were used. The 2002 research project
is documented in [2].
2.0 TEST EQUIPMENT
2.1 Vehicle Load Configurations
NHTSA's dynamic rollover propensity test procedure uses one of
two loading configurations: Nominal or Multi-Passenger. A description
of each configuration is provided below.
Both vehicle load configurations include instrumentation, a steering
machine, and outriggers. Test vehicle bumper assemblies are removed
for outrigger installation. The reduction in vehicle weight due
to the removal of the bumpers is offset by the additional weight
of the outriggers and their mounting system. The outrigger system
typically outweighs the bumper assemblies.
2.1.1 Nominal Load Configuration
The Nominal Load Configuration consists of the driver, instrumentation,
steering machine, outriggers, and full tank of fuel. Weight and
location specifications for the data acquisition system and steering
machine are presented in Table I.1 and Figure I.1.
Table I.1.
Equipment Location and Weight.
| Equipment |
Location |
Weight,
typical
(lbs) |
| Data
Acquisition System |
Front
passenger seat |
58 |
| Steering
Machine |
Handwheel |
31 |
| Steering
Machine Electronics Box |
Passenger
row foot well behind the front passenger seat. If vehicle
does not have a rear passenger row foot well, the Electronics
Box should be placed in the front passenger seat foot well. |
39 |
Non-pickup
truck vehicles with only front designated seating positions use
the Nominal Load Configuration.
2.1.2
Multi-Passenger Configuration
The Multi-Passenger
Configuration includes all elements of the Nominal Load Configuration
plus ballast in the form of water dummies. Water dummies are
installed as follows:
For vehicles
with three or more designated rear seating positions, three 175
lb water dummies are used. The water dummies shall be positioned
on the rear seats (second seating row) closest to driver and front
passenger seats (first seating row). If there are only two seating
positions in the second seating row, the third water dummy shall
be placed in the center of the third seating row, provided it
is a designated seating position. Refer to Figure I.2.
For vehicles
with two designated rear seating positions, two 175 lb water dummies
shall be positioned in the rear seats. Refer to Figure I.3.
For pickups
with only front designated seating positions, three 175 lb water
dummies will be used. The water dummies shall be positioned behind
the cab in a manner that emulates a second seating row. If it
is not possible to fit three water dummies directly behind the
cab, the third water dummy shall be placed in the center of a
simulated third seating row. Refer to Figure I.4.
For pickups
with two seating rows, three 175 lb water dummies will be used.
If the second seating row includes three designated seating positions,
each water dummy shall be placed in these positions. If the second
seating row includes two designated seating positions, two 175
lb water dummies shall be positioned in the second seating row
of the cab, and the third water dummy shall be positioned behind
the cab in a manner that emulates the center seating position
of a third seating row. Refer to Figure I.5.
For all vehicles,
if the Multi-Passenger Configuration results in the vehicle exceeding
its Gross Vehicle Weight Rating (GVWR) and/or rear Gross Axle
Weight Rating (GAWR), the weight of each dummy will be equally
reduced until the GVWR and/or rear GAWR are no longer exceeded.
The weight of the water dummies shall not be reduced if only the
front GAWR is exceeded and the front axle weight does not exceed
the front GAWR by more that 50 pounds, i.e., if the Multi-Passenger
Configuration results in the vehicle exceeding its front GAWR,
and its GVWR and/or rear GAWR, the weight of each dummy will be
equally reduced until the GVWR and rear GAWR are no longer exceeded
and the front GAWR is not exceeded by more that 50 pounds.
For non-pickup
truck vehicles with only front designated seating positions, the
Multi-Passenger Configuration is omitted from the test matrix.
2.2 Safety
Outriggers
Safety outriggers
are installed on all test vehicles during all test maneuvers.
NHTSA uses outriggers machined from 6Al-4V titanium. NHTSA's
"short" outriggers are used for vehicles with baseline weights
under 3,500 pounds in a baseline condition (as delivered); "standard"
outriggers are used for vehicles with baseline weights from 3,500
and 7,000 pounds; and "long" outriggers are used for vehicles
with baseline weights from 7,001 to 10,000 pounds. Information
on NHTSA's titanium outrigger system is documented in [3].
2.3 Tires
All tires
must be new, and of the same make, model, size, and DOT specification
of those installed on vehicles when purchased new. Tire inflation
pressures are to be in accordance with the recommendations indicated
on each vehicle's identification placard.
2.3.1
Tire Mounting Technique
When mounting
tires to the rims used for testing, no tire mounting lubricant
should be used. Lubricant is not used due to uncertainty surrounding
the occurrences of tire debeading observed during NHTSA's rollover
research. To eliminate the possibility of tire lubricant contributing
to this phenomenon, it should not be used. Because no lubricant
is used, care must be taken to confirm that the tire is fully
seated on the wheel rim at the completion of the mounting procedure.
2.3.2
Frequency of Tire Changes
To minimize
the effects of tire wear on vehicle response and rollover propensity,
rollover research requires frequent tire changes. For each loading
condition, the following guidelines must be followed:
- One set
of tires is to be used for each Slowly Increasing Steer test
series. Each series is comprised of left and right steer tests.
- Up to
two tire sets are to be used for the Fishhook maneuver test
series. The actual number of tire sets used is dependent on
the response of each vehicle. The tire change protocol is presented
in the Fishhook maneuver test procedure (Section 3.2). Note:
A tire change between the completion of the Slowly Increasing
Steer maneuver and initiation of Fishhook testing is not required
provided the abbreviated Slowly Increasing Steer procedure described
in Section 3.1.2 is used. If the abbreviated procedure is not
used (i.e., the maneuver is performed such that maximum lateral
acceleration is achieved), a tire change between the completion
of the Slowly Increasing Steer maneuver and initiation of Fishhook
testing is required, as tire wear associated with these
tests may potentially confound Fishhook test outcome.
2.3.3
Use of Inner Tubes
Fishhook
maneuvers have been shown to produce debeading of the outside
front and rear tires. The occurrence of debeads can result
in significant damage to the test surface. NHTSA research
has concluded the easiest, most cost effective way to minimize
debeading is the use of inner tubes designed for radial tires.
Inner tubes must be installed prior to any Fishhook test -
one inner tube for each of the vehicle's tires. Inner tubes
should be appropriately sized for the test vehicle's tires.
Installation
of inner tubes is not required prior to Slowly Increasing
Steer tests, regardless of vehicle or load condition.
2.4
Data Collection
All data
is to be sampled at 200 Hz. NHTSA's signal conditioning consists
of amplification, anti-alias filtering, and digitizing. Amplifier
gains are selected to maximize the signal-to-noise ratio of
the digitized data. Filtering is performed with two-pole
low-pass Butterworth filters with nominal cutoff frequencies
selected to prevent aliasing. The nominal cutoff frequency
is 15 Hz (calculated breakpoint frequencies are 18 and 19
Hz for the first and second poles respectively).
Data collection
is initiated manually by the test driver immediately before
the start of the maneuver or automatically by "Handwheel Command
Flag" signal from the steering machine (refer to Section 3.2.4.2.2,
Handwheel Command Flag).
2.5
Instrumentation
Each test
vehicle is to be equipped with sensors, a data acquisition
system, and a programmable steering machine. Equipment location
and weight specifications are presented in Table I.1 and Figure
I.1.
2.5.1
Sensors and Sensor Locations
Table
I.2 lists the sensors required by NHTSA's dynamic rollover
propensity test procedure. A brief description of these sensors
is provided in this section.
Table
I.2. Recommended Sensor Specifications
| Type |
Output |
Range |
Resolution |
Accuracy |
| Multi-Axis
Inertial Sensing System |
Longitudinal,
Lateral, and Vertical Acceleration
Roll,
Yaw, and Pitch Rate |
Accelerometers:
±2 g
Angular
Rate Sensors: ±100 deg/s |
Accelerometers:
≤10 ug
Angular
Rate Sensors: ≤0.004 deg/s |
Accelerometers:
≤0.05% of full range
Angular
Rate Sensors: 0.05% of full range |
| Angle
Encoder |
Handwheel
Angle |
±800
deg |
0.25
deg |
±0.25
deg |
| Ultrasonic
Distance Measuring System |
Left
and Right Side Vehicle Height |
5-24
inches |
0.01
inches |
±0.25%
of maximum distance |
| Load
Cell |
Brake
Pedal Force |
0-300
lbf |
N/A |
N/A |
| Radar
Speed Sensor |
Vehicle
Speed |
0.1-125
mph |
0.009
mph |
±0.25%
of full scale |
| Infrared
Distance Measuring System |
Wheel
Lift |
13.75-33.5
inches |
0.01
in., short range
0.3
in., long range |
±1%
of full scale |
Data
Flag
(Handwheel Command Flag) |
Pauses
in commanded steering inputs |
0
- 10 V |
N/A |
Flag
should respond within 10 ms |
Data
Flag
(Roll Rate Flag) |
Indication
of ± 1.5 deg/s roll rate |
0
- 10 V |
N/A |
Flag
should respond within 10 ms |
2.5.1.1
Handwheel Angle
Handwheel
position is measured via an angle encoder integral with the
programmable steering machines.
2.5.1.2
Vehicle Speed
Vehicle
speed is measured with a non-contact speed sensor placed at
the center rear of each vehicle. NHTSA has had good experiences
with the use of Doppler radar based sensors. Sensor outputs
are to be transmitted not only to the data acquisition system,
but also to a dashboard display unit. This allows the driver
to accurately monitor vehicle speed.
2.5.1.3
Chassis Dynamics
A multi-axis
inertial sensing system is used to measure linear accelerations
and roll, pitch, and yaw angular rates. The position of the
multi-axis inertial sensing system must be accurately measured
relative to the C.G. of the vehicle in the Nominal Load and
Multi-Passenger Configurations. These data are required to
translate the motion of the vehicle at the measured location
to that which occurred at the actual C.G to remove roll, pitch,
and yaw effects. NHTSA uses an independent laboratory to
measure the C.G. of it's test vehicles.
The following
equations are used to correct the accelerometer data in post-processing.
They were derived from equations of general relative acceleration
for a translating reference frame and use the SAE Convention
for Vehicle Dynamics Coordinate Systems. The coordinate transformations
are:
x″corrected
= x″accel - (Θ′ 2 +
Ψ′ 2)xdisp + (Θ′Φ′
- Ψ″)ydisp + (Ψ′Φ′
+ Θ″)zdisp
y″corrected
= y″accel + (Θ′Φ′ +
Ψ″)xdisp - (Φ′ 2
+ Ψ′ 2)ydisp + (Ψ′Θ′
- Φ″)zdisp
z″corrected
= z″accel + (Ψ′Φ′ -
Θ″)xdisp + (Ψ′Θ′
+ Φ″)ydisp - (Φ′ 2
+ Θ′ 2)zdisp
where
x″corrected,
y″corrected, and z″corrected
= longitudinal, lateral, and vertical accelerations, respectively,
at the vehicle's center of gravity
x″accel,
y″accel, and z″accel = longitudinal,
lateral, and vertical accelerations, respectively, at the
accelerometer location
xdisp,
ydisp, and zdisp = longitudinal, lateral,
and vertical displacements, respectively, of the center of
gravity with respect to the accelerometer location
Φ′
and Φ″ = roll rate and roll acceleration, respectively
Θ′
and Θ″ = pitch rate and pitch acceleration, respectively
Ψ′
and Ψ″ = yaw rate and yaw acceleration, respectively
NHTSA
does not use inertially stabilized accelerometers for this
test procedure. Therefore, lateral acceleration must be corrected
for vehicle roll angle during data post processing. This
is discussed in Section 4.12.
2.5.1.4
Roll Angle
An ultrasonic
distance measurement system is used to collect left and right
side vertical displacements for the purpose of calculating
vehicle roll angle. One ultrasonic ranging module is mounted
on each side of a vehicle, and is positioned at the longitudinal
center of gravity. With these data, roll angle is calculated
during post-processing using trigonometry.
2.5.1.5
Wheel Lift
Wheel
lift is measured individually with two height sensors attached
to spindles installed at the wheel. Using trigonometry,
the output of the two sensors can be used to resolve the camber
angle of the wheel, and remove its influence from the uncorrected
height sensor output. Information on NHTSA's wheel lift measurement
system is documented in [4].
2.5.1.6
Brake Application
Brake
pedal force is measured with a load cell transducer attached
to the face of the brake pedal. While brake pedal force is
not explicitly required by this test procedure, it is important
to monitor the driver's braking activity during testing.
No test included in this procedure requires brake application.
If the driver applies force to the brake pedal before completion
of a test, that test is not valid, and should not be considered
in further analyses.
2.5.2
Additional Mnemonics
2.5.2.1
Handwheel Command Flag
Refer
to Section 3.2.4.2.2, Handwheel Command Flag.
2.5.2.2
Roll Rate Flag
Refer
to Section 3.2.4.2.3, Roll Rate Flag.
2.6
Steering Machine
A programmable
steering machine is used to generate handwheel steering inputs
for all test maneuvers. The machine must provide at least
35 lbf-ft of torque at a handwheel rate of 720
deg/sec, be able to move each vehicle's steering system through
its full range, and accept angular rate sensor feedback input
for roll rate-induced steering reversals (refer to Section
3.2.4). It is recommended that the steering machine be capable
of initiating steering programs at a preset road speed, and
have the convenience of changing the steering program during
test sessions.
3.0
TEST MANEUVERS
3.1
Slowly Increasing Steer
The Slowly
Increasing Steer maneuver is used to characterize the lateral
dynamics of each vehicle, and is based on the "Constant Speed,
Variable Steer" test defined in SAE J266 [5]. The maneuver
is used to determine the steering that produces a lateral
acceleration of 0.3 g. This handwheel angle is used to define
the magnitude of steering to be used for the NHTSA Fishhook
maneuver.
3.1.1
Maneuver Description (Option #1)
To begin
this maneuver, the vehicle is driven in a straight line at
50 mph. The driver must attempt to maintain this speed during
and briefly after the steering is input using smooth throttle
modulation. At time zero, handwheel position is linearly
increased from zero to 270 degrees at a rate of 13.5 degrees
per second. Handwheel position is held constant at 270 degrees
for two seconds, after which the maneuver is concluded. The
handwheel is then returned to zero as a convenience to the
driver. The maneuver is performed three times to the left
and three times to the right for each load configuration.
Figure I.6 presents a description of the handwheel angles
to be used during Slowly Increasing Steer, Option #1 tests.
3.1.2.
Maneuver Description (Option #2, Preferred)
Historically,
NHTSA has used Slowly Increasing Steer tests to measure linear
range and maximum quasi steady state lateral acceleration.
While maximum lateral acceleration data is interesting, it
is not a required metric when determining a vehicles NCAP
rollover resistance rating. For this reason, NHTSA recommends
use of an "abbreviated" Slowly Increasing Steer maneuver.
The handwheel angles used in this abbreviated procedure only
steer the vehicle enough to assess its linear range lateral
acceleration performance.
To determine
the most appropriate Slowly Increasing Steer handwheel angle
for a given vehicle, a preliminary left steer test is performed.
The test speed during this test was held constant at 50 mph
via throttle modulation, and the steering input ranged from
0 to 30 degrees, applied at 13.5 degrees per second. The
magnitude of this input was selected because it was believed
to be capable of producing a steady state lateral acceleration
within the linear range for any light vehicle. Using the
ratio of steady state handwheel position and lateral acceleration
established by this test, the maximum steering input for the
abbreviated Slowly Increasing Steer test was derived using
the below equation:
Equation
3.1
where,
ay,30
degrees was the raw lateral acceleration
produced with a constant handwheel angle of 30 degrees during
a test performed at 50 mph
dSIS
was the steering input that, if the relationship of handwheel
angle and lateral acceleration was linear, would produce a
lateral acceleration of 0.55 g during a test performed at
50 mph
Note:
ay,30 degrees is "raw" data, not corrected for
the effects of roll, pitch, and yaw. NHTSA acknowledges the
relationship of handwheel angle and corrected lateral
acceleration data is often not linear at 0.55 g. However,
previously collected data indicates the magnitude of raw 0.55
g acceleration data is typically reduced by approximately
9.6 percent to 0.497 g, when corrected for roll, pitch, and
yaw, just outside of the linear range for most vehicles.
Removing the effect of accelerometer offset (error due to
the accelerometer not being positioned at the vehicle's actual
center of gravity) typically reduces the magnitude of these
data by an additional 0.07 percent. The importance of Equation
3.1 is that it simply provides experimenters with a direct,
"in-the-field" way of determining an appropriate steering
input for which to proceed with further tests for a given
vehicle.
Figure
I.7 presents a description of the handwheel angles to be used
during the abbreviated Slowly Increasing Steer, Option #2
tests.
3.1.3
Measured Parameters
Analyses
of Slowly Increasing Steer tests output overall average handwheel
position at a specified lateral acceleration
When lateral
acceleration data collected during Slowly Increasing Steer
tests is plotted with respect to time, a first order polynomial
best-fit line accurately describes the data from 0.1 to 0.375 g.
NHTSA defines this as the linear range of the lateral acceleration
response. A simple linear regression is used to determine
the best-fit line, as shown in Figures I.8 and 1.9.
Using
the slope of the best-fit line, the average of handwheel position
at 0.3 g is calculated using data from each of the six Slowly
Increasing Steer tests performed for each vehicle. This
average handwheel position is used to calculate NHTSA Fishhook
maneuver steering inputs, as described in Section 3.2.
3.2
NHTSA Fishhook Maneuver
3.2.1
Maneuver Overview
To begin
the maneuver, the vehicle is driven in a straight line at
a speed slightly greater than the desired entrance speed.
The driver releases the throttle, and when at the target speed,
initiates the handwheel commands described in Figure I.10
using a programmable steering machine. Following completion
of the countersteer, handwheel position is maintained for
three seconds. As a convenience to the test driver, the handwheel
is then returned to zero.
Each Fishhook
maneuver test series contains two sequences (with exceptions
noted in the following sections): tests performed with left-right
steering (first sequence), and tests performed with right-left
steering (second sequence). The sequence of left-right tests
always precedes those performed with right-left steering.
3.2.2
Default Procedure
Fishhook
maneuver handwheel angles are calculated with lateral acceleration
and handwheel angle data (d) collected during a series of
six Slowly Increasing Steer tests (a total of three left-steer
and three-right steer tests are performed). For each Slowly
Increasing Steer test, a linear regression line is fitted
to the lateral acceleration data from 0.1 to 0.375 g. Using
the slopes of these regression lines, the handwheel angles
at 0.3 g are determined for each individual test (d0.3
g). The six handwheel angles are then averaged to produce
an overall value (d0.3 g, overall).
d0.3
g, overall = (│d0.3 g, left (1)│
+│d0.3 g, left (2)│ +
│d0.3 g, left (3)│+ d0.3 g, right
(1) + d0.3 g, right (2) + d0.3 g, right
(3)) / 6
The Fishhook
maneuver steering angles are calculated by multiplying d0.3
g. overall by a steering scalar (SS). The default steering
scalar is 6.5.
dFishhook
(Default) = 6.5 x d0.3 g, overall
3.2.2.1
Maneuver Entrance Speed
For the
sake of driver safety, and as a final step in the tire scrub-in
procedure, each Default Procedure sequence begins with a Maneuver
Entrance Speed (MES) equal to 35 mph. The MES is measured
at the initiation of the first steering ramp, and is increased
until a termination condition is satisfied. The order of
MES for a sequence is, in mph: 35, 40, 45, 47.5, 50. For
each test run, the actual MES must be within 1 mph of the
target MES.
Note:
NHTSA's experience with the Fishhook maneuver indicates that
an incremental increase in MES of 5 mph, up to 45 mph, minimizes
tire wear without compromising test driver safety. However,
when a MES greater than 45 mph is used, the severity of the
responses produced with some vehicles can increase substantially
from that observed at lesser entrance speeds. This is especially
true if a vehicle has a propensity to oscillate in roll, and/or
is able to produce two-wheel lift slightly less than NHTSA's
threshold criterion of two inches. In some of these cases,
the driver and/or experimenter may not be comfortable with
a final 5 mph upwards increment in MES, and might, for the
sake of driver safety, deviate from a test procedure that
requires it. Generally speaking, such a deviation typically
involves the experimenter's use of a more gradual 2.5 mph
increase in MES.
To promote
driver safety while also eliminating inconsistencies in the
way NHTSA's Fishhook maneuvers are performed, the test procedure
requires a MES increment equal to 2.5 mph be used above
45 mph if a test performed at 45 mph does not produce two-wheel
lift, regardless of the vehicle being evaluated.
3.2.2.2
Outrigger Contact
If either
safety outrigger contacts the pavement without two-wheel lift
during a Fishhook maneuver test run, the affected outrigger
is raised 0.75 inches and the test is repeated at the same
MES. If both safety outriggers contact the pavement without
two-wheel lift, both outriggers are raised 0.75 inches and
the test is repeated at the same MES.
3.2.2.3
Termination and Conclusion Conditions
A test
sequence is terminated if the MES capable of producing
two-wheel lift is observed and the MES is 45 mph or lower.
If two-wheel lift is observed during a left-right sequence
at 45 mph or lower, the [entire] series is terminated.
If no two-wheel lift is observed during a left-right sequence,
right-left tests are performed. If two-wheel lift is observed
during a right-left sequence performed with a MES of 45 mph
or lower, the test series is terminated.
If the
MES capable of producing two-wheel lift during a left-right
or right-left sequence is 47.5 mph or higher, a new set of
tires is installed on the vehicle and the procedure described
in Section 3.2.3.1 is implemented.
A test
series is terminated if rim-to-pavement contact or
tire debeading is observed during any test performed with
either test sequence.
A test
series is deemed complete if both test sequences within a
given series have been performed at the maximum maneuver entrance
speed without two-wheel lift, rim-to-pavement contact, tire
debeading, or outrigger-to-pavement contact. If the Default
Procedure is completed without encountering a termination
condition, Supplemental Procedure Part 2, described in Section
3.2.3.2, is implemented.
The flowchart
presented in Figure I.11 describes the sequence of events
for the Default Test Series.
3.2.3
Supplemental Procedures
Note:
If the results of the Default Test Series require the implementation
of the Supplemental Procedure Part 1, neither Supplemental
Procedure Part 2 nor Part 3 is used.
Note:
Depending of the response of test vehicles to elements of
the Fishhook maneuver protocol, Supplemental Procedure, Parts
1, 2, and 3 may require a change in the steering scalar.
The steering machine used by NHTSA has the capability for
making such changes in vehicles during test sessions via selection
of a pre-programmed steering schedule and the adjustment of
overall steering angles.
3.2.3.1
Supplemental Procedure Part 1
Following
the tire scrub-in procedure outlined in Section 4.6, tests
are performed with handwheel angles equal to dFishhook
(Default), as explained in Section 3.2.2. The steering
combination (i.e., either left-right or right-left) that produced
two-wheel lift in the Default Test Series is used. The first
test is to be performed at a MES of 35 mph. This test is
performed to ensure any mold sheen remaining from the tire
break-in procedure has been removed from the tires. The second
test is to be performed at the MES at which two-wheel lift
had been previously observed (i.e., with the previous tire
set). If two-wheel lift is produced during the test performed
with handwheel angles equal to dFishhook (Default),
the tip-up will be reported in the vehicle's NCAP Rollover
Resistance Rating and the test series is deemed complete.
If two-wheel lift is not produced and the MES is 47.5 mph,
the MES is increased to 50 mph. If two-wheel lift is produced
during the test performed with MES equal to 50 mph, the tip-up
will be reported in the vehicle's NCAP Rollover Resistance
Rating and the test series is deemed complete.
If two-wheel
lift is not produced at 50 mph with handwheel angles
equal to dFishhook (Default), tests are performed
with steering angles calculated by multiplying d0.3 g.
overall by a steering scalar of 5.5.
dFishhook
(Supplemental) = 5.5 x d0.3 g, overall
After
the application of the reduced scalar, a test is to be performed,
using the same steering combination (i.e., either left-right
or right-left), at the MES at which two-wheel lift had been
observed in the Default Test Series. If two-wheel lift is
produced during the test performed with handwheel angles equal
to dFishhook (Supplemental), the tip-up will be
reported in the vehicle's NCAP Rollover Resistance Rating
and the test series is deemed complete. If two-wheel lift
is not produced and the MES is 47.5 mph, the MES is increased
to 50 mph. If two-wheel lift is produced during the test
performed with MES equal to 50 mph, the tip-up will be reported
in the vehicle's NCAP Rollover Resistance Rating and the test
series is deemed complete. If two-wheel lift is not
produced at 50 mph, the test series is deemed complete and
no tip-up will be reported in the vehicle's NCAP Rollover
Resistance Rating.
A test
series is terminated if rim-to-pavement contact or tire debeading
is observed during any Supplemental Procedure Part 1 test.
The flowchart presented in Figure I.12 describes the sequence
of events for the Supplemental Procedure Part 1.
3.2.3.2
Supplemental Procedure Part 2
If two-wheel
lift is not produced during tests performed with the Default
Procedure, the steering scalar is reduced from 6.5 to 5.5.
Using the same tires used for tests performed with the Default
Test Series, tests are performed with steering angles calculated
by multiplying d0.3 g. overall by a steering scalar
of 5.5.
dFishhook
(Supplemental) = 5.5 x d0.3 g, overall
For the
sake of driver safety, the first test of the left-right sequence
with the reduced steering scalar applied is to be performed
at a MES of 45 mph. If this test does not produce two-wheel
lift, the MES is increased to 47.5 mph. If the test with
MES equal to 47.5 mph does not produce two-wheel lift, the
MES is increased to 50 mph (the maximum MES used for Fishhook
maneuver testing). If no two-wheel lift is observed during
the left-right sequence, the right-left test sequence is initiated
using the same process as the left-right sequence. If any
test in the Supplemental Procedure Part 2 test series produces
two-wheel lift, a new set of tires is installed on the vehicle,
and the procedure described Section 3.2.3.3 is implemented.
A test
series is terminated if rim-to-pavement contact or tire debeading
is observed during any test performed with either test sequence.
A test series is deemed complete if both test sequences within
the series have been performed at the maximum maneuver entrance
speed without two-wheel lift. The flowchart presented in
Figure I.13 describes the sequence of events for the Supplemental
Procedure Part 2.
3.2.3.3
Supplemental Procedure Part 3
Following
the tire scrub-in procedure outlined in Section 4.6, two tests
are performed with handwheel angles equal to dFishhook
(Supplemental). The steering combination that produced
two-wheel lift during Supplemental Procedure Part 2 testing
is used (i.e., either left-right or right-left). The first
test is to be performed at a MES of 35 mph. This test is
performed to ensure any mold sheen remaining from the tire
break-in procedure has been removed from the tires. The second
test is to be performed at the MES that had produced two-wheel
lift during Supplemental Procedure Part 2 testing (i.e., with
the previous tire set). If two-wheel lift is produced during
the test performed with handwheel angles equal to dFishhook
(Supplemental), the tip-up will be reported in the vehicle's
NCAP Rollover Resistance Rating and the test series is deemed
complete. If two-wheel lift is not produced and the MES is
45 mph, the MES is increased to 47.5 mph. If two-wheel lift
is not produced and the MES is 47.5 mph, the MES is increased
to 50 mph. If two-wheel lift is produced during any test
performed during Supplemental Procedure Part 3, the tip-up
will be reported in the vehicle's NCAP Rollover Resistance
Rating and the test series is deemed complete. If two-wheel
lift is not produced during Supplemental Procedure
Part 3, the test series is deemed complete and no tip-up will
be reported in the vehicle's NCAP Rollover Resistance Rating.
A test
series is terminated if rim-to-pavement contact or tire debeading
is observed during any Supplemental Procedure Part 3 test.
The flowchart presented in Figure I.14 describes the sequence
of events for the Supplemental Procedure Part 3.
3.2.4
Handwheel Inputs
3.2.4.1
Steering Rate
The handwheel
rates of the initial steer and countersteer steering ramps
are always to be performed with nominal steering rates of
720 degrees per second, regardless of what steering scalar
is used.
3.2.4.2
Dwell Time
The Fishhook
maneuver is designed to maximize the roll motion of the test
vehicle. When left-right steering is used, this is accomplished
by:
1. Steering
the vehicle with an input equal to dFishhook (Default)
or
dFishhook
(Supplemental)
2. Waiting
until the vehicle achieves maximum roll angle
3. Reversing
the direction of steer
4. Steering
the vehicle with an input equal to -dFishhook (Default)
or
-dFishhook
(Supplemental)
When right-left
steering is used, the sign conventions indicated in Steps
1 and 4 above are switched from positive to negative (i.e.,
for Step 1) or from negative to positive (i.e., for Step 4).
Dwell
time is defined as the time from the completion of the initial
steering ramp to the initiation of the steering reversal.
A roll rate "Window Comparator" is used to determine when
the vehicle has achieved maximum roll angle. Since the programmable
steering machine used by NHTSA has a mechanical overshoot
after completion of the initial steer, dwell time is not measured
directly with handwheel angle data. Rather, two signals output
from the steering machine are used: "Handwheel Start" and
"Roll Flag".
3.2.4.2.1
Steering Machine Window Comparator
As indicated
in Figure I.10, Fishhook maneuver steering reversals are commanded
after the completion of the initial steering ramp and when
the roll rate of the vehicle is very close to zero (because
it is the derivative of roll angle, when roll rate is equal
to zero at this point, roll angle is at its maximum). To
minimize the likelihood of erroneous reversals, the reversals
occur when the roll rate signal transmitted from a sensor
positioned near the test vehicle's center of gravity enters
the window comparator. The window comparator is defined as
±1.5 degrees per second, regardless of what steering scalar
was used.
Examples:
If an initial steer to the left is input, the reversal is
initiated when the roll velocity of the vehicle is equal to
1.5 degrees per second. If an initial steer to the right
is input, the reversal is initiated when the roll velocity
of the vehicle is equal to -1.5 degrees per second.
3.2.4.2.2
Handwheel Command Flag
The programmable
steering machine used by NHTSA outputs a "Handwheel Command
Flag" signal based on the machine's internal clock. The output
of the Handwheel Command Flag signal ranges from 0 to 10 volts,
and is binary. The signal is high (10 volts) when the steering
machine is in the process of executing a commanded input,
or low (0 volts) when the machine is not in use or a pause
is commanded during the execution of a commanded input, as
shown in Figure I.10. When the pause ends, and execution
of the commanded steering inputs are resumed, the Handwheel
Command Flag signal is once again set high. In a Fishhook
maneuver, the duration of the pause is the dwell time.
3.2.4.2.3
Roll Rate Flag
The "Roll
Rate Flag" signal output by the programmable steering machine
used by NHTSA is monitored. Like that of the Handwheel Command
Flag channel, the Roll Rate Flag output ranges from 0 to 10
volts, and is binary. The signal is high (10 volts) when
the roll rate of the test vehicle is within the window comparator,
or low (0 volts) when roll rate is outside the window comparator,
as shown in Figure I.10.
Fishhook
maneuver steering reversals are to be initiated by the steering
machine within 10 milliseconds of the roll rate entering the
window comparator. Initiation of the steering reversal is
defined as the instant the steering machine sets the Roll
Rate Flag signal high.
Note:
After completion of the initial steer, the instants that the
steering machine sets the Roll Rate Flag and Handwheel Command
Flag signals high should coincide.
3.2.4.3
Excessive Steering
In some
cases, the magnitude of dFishhook (Default) used
during the Default Procedure may be so great that
the vehicle reaches maximum roll angle before completion of
the initial steer. This is defined as excessive steering;
i.e., the vehicle cannot respond to the entire commanded steering
input.
Excessive
steering is also said to occur if the dwell time of a Fishhook
test performed with the Default Procedure results in a dwell
time less than 80 milliseconds. The mechanical overshoot
of the steering machine that occurs after completion of the
initial steer can prohibit the machine from accurately executing
dwell times less than approximately 80 milliseconds. In such
cases, the effect of the overshoot is that the actual dwell
time is equal to zero (an immediate steering reversal).
NHTSA's
experience with the Fishhook maneuver has demonstrated the
effect of excessive steering on dynamic rollover resistance
is vehicle-dependent. While it may not allow the roll motion
of some test vehicles to be maximized, excessive steering
has been shown to contribute to an increased tip-up propensity
in others. For this reason, a test sequence for which excessive
steering is observed should not be terminated. Testing
should proceed as outlined in Section 3.2.2, Default Procedure.
If two-wheel lift is not observed during either Default Procedure
test sequence, the Supplemental Procedure beginning at Part
2, described in Section 3.2.3.2, is performed.
4.0
ITEMS PERTAINING TO TEST CONDUCT
4.1
Definition of Two-Wheel Lift
Two-wheel
lift is defined as the occurrence of at least two inches of
simultaneous lift of the inside wheels from the test surface.
NHTSA does not consider two-wheel lift less than two inches
when calculating a vehicle's NCAP rollover resistance rating.
Two-wheel lift great enough to require outriggers to suppress
further roll motion is to be reported simply as "two-wheel
lift" as long as at least two inches of simultaneous two-wheel
lift occurs before outrigger contact with the ground is made.
4.2
Vehicle Test Configurations
4.2.1
Load Configurations
All vehicles
are to be evaluated with one of the two load configurations
previously defined in Section 2.1.
4.2.2
Fuel Tank Loading
Prior
to beginning a Slowly Increasing Steer or Fishhook maneuver
test series, the fuel tank of the vehicle is to be completely
filled at the beginning of testing and may not be less than
75% of capacity during any part of the testing. This criterion
is in agreement with that defined in FMVSS 135.
4.2.3
Stability Control System
If equipped,
vehicles are tested with stability control systems active.
Stability control is not to be deactivated for any Slowly
Increasing Steer or Fishhook maneuver.
4.3
Road Test Surface
Tests
are conducted on a dry, uniform, solid-paved surface. Surfaces
with irregularities, such as dips and large cracks, are unsuitable,
as they may confound test results.
4.3.1
Pavement Friction
All maneuvers
are to be performed on a dry, high-mu road test surface.
Unless
otherwise specified, the road test surface produces a peak
friction coefficient (PFC) of approximately 0.9 when measured
using an American Society for Testing and Materials (ASTM)
E1136 standard reference test tire, in accordance with ASTM
Method E 1337-90, at a speed of 64.4 km/h (40 mph), without
water delivery. This criterion is in agreement with that
defined in FMVSS 135.
4.3.2
Slope
The test
surface has a consistent slope between level and 2%. All
tests are to be initiated in the direction of positive slope
(uphill).
4.4
Ambient Conditions
4.4.1
Ambient Temperature
The ambient
temperature shall be between 0° C (32° F) and 40° C (104°
F). This criterion is in agreement with that defined in FMVSS
135.
4.4.2
Wind Speed
The maximum
wind speed shall be no greater than 10 m/s (22 mph).
4.5
Calibration Data
It is
strongly recommended that calibration data be collected prior
to tests of each configuration to assist in resolving uncertain
test data. NHTSA typically records the following data at
the beginning of each test day for each test vehicle configuration.
- The distance
measured by the speed sensor along a straight line between the
end points of a surveyed linear roadway standard of 1000 feet
or more (observed and recorded manually from the speed sensor
display).
- Five to
fifteen seconds of data from all instrument channels as the
configured and prepared test vehicle is driven in a straight
line on a level, uniform, solid-paved road surface at 60 mph.
4.6
Tire Break-In Procedure
Prior
to each test series, the tires must be "scrubbed in" to wear
away mold sheen and be brought up to operating temperature.
Test vehicles are to be driven around a circle 100 feet in
diameter at a speed that produces a lateral acceleration of
approximately 0.5 to 0.6 g. Using this circle, three clockwise
laps are to be followed by three counterclockwise laps. Once
the six laps of the circle are complete, the driver is to
input, sinusoidal steering at a frequency of 1 Hz and a handwheel
amplitude (dss) corresponding to 0.5-0.6 g for
10 cycles while maintaining a vehicle speed of 35 mph. A
total of four passes using sinusoidal steering are to be used.
The handwheel magnitude of the final cycle of the final pass
is to be twice that of dss. These four sinusoid
passes typically require an area similar in size to that required
by the Fishhook maneuver. The steering machine should be
programmed to execute the sinusoids. There should be only
a minimal delay between the completion of the tire break-in
and the start of a test series to allow for the collection
of a static data file, steering machine and data acquisition
system adjustment, and final driver briefing.
4.7
Static Datums
At the
completion of the tire break-in procedure and before the start
of a test series, fifteen seconds of data are collected from
all instrument channels with the test vehicle at rest, the
engine running, the transmission in "Park" (automatic transmission)
or in neutral with the parking brake applied (manual transmission),
and the front of the test vehicle facing in the direction
of positive gradient (uphill) on the test surface. The static
data files are used in post processing establish datums for
each instrument channel.
4.8
Vehicle Gear Selection
All tests
are performed with automatic transmissions in "Drive" or with
manual transmissions in the highest gear capable of sustaining
the desired test speed (Slowly Increasing Steer) or Maneuver
Entrance Speed (Fishhook), with one exception:
Slowly
Increasing Steer tests may be performed with automatic transmissions
in lower gears if 50 mph cannot be maintained in "Drive" and
the gear selection does not result in engine overspeeding.
In some cases, 50 mph cannot be maintained through to the
end of the steering schedule regardless of the gear selection
due to low engine power or chassis responses that result in
the loss of traction or spin out. It has been NHTSA's experience,
however, that maximum lateral acceleration is generally achieved
well before the maneuver's maximum handwheel angle is attained.
Manual
transmission clutches are to remain engaged during all maneuvers.
4.9
Outrigger Adjustment
The initial
clearance between the road surface and the bottom of the NHTSA
outrigger skid pads is approximately 14 inches for the "standard"
outriggers and approximately 12 inches for the "short" outriggers
with the test vehicle at rest on a level surface. Note that
the Multi-Passenger Configuration may compress the suspension
more than the Nominal Load Configuration (reducing outrigger
clearance). As such, outrigger height adjustment may be required
when transitioning from one load configuration to the next.
Outrigger
height adjustment may be required during a test series. If
an outrigger skid pad contacts the road surface during a test
run wherein there is no two-wheel lift, the outrigger at the
effected end of the vehicle is raised 0.75 inches and the
test run is repeated at the same maneuver entrance speed.
If both outriggers make contact with the test surface during
at test run wherein there is no two-wheel lift, both outriggers
are raised 0.75 inches and the test run is repeated at the
same maneuver entrance speed.
4.10
Videotape Documentation
It is
recommended that all test runs be documented on videotape.
NHTSA videotapes Slowly Increasing Steer tests from a viewpoint
several hundred feet outside the circular path of the test
vehicle. Fishhook maneuver tests are videotaped from a viewpoint
that facilitates observation of the inboard side of the vehicle
so as to best record instances of two-wheel lift. For both
maneuvers, it is recommended the zoom of the camera be adjusted
during each test such that the vehicle fills the view frame
to the greatest extent possible.
4.11
Summary Of Tests To Be Performed For Each Vehicle
For each
test vehicle, testing will be performed according to the following
plan:
1. Installation
of new tires
2. Tire
break-in
3. Slowly
Increasing Steer Maneuver test series in the Nominal Load
or Multi-Passenger Configuration
4. Tire
change
5. Tire
break-in
6. NHTSA
Fishhook maneuver test series in the Nominal Load or Multi-Passenger
Configuration with additional tire changes and break-ins as
indicated in the maneuver protocol
4.12
Summary of Metrics Measured For Each Vehicle
1. Overall
handwheel position at 0.3 g in the Nominal Load Configuration
2. Two-Wheel
Lift in NHTSA Fishhook maneuver in Nominal Load or Multi-Passenger
Configuration (Yes/No)
3. Rim-to-Pavement
Contact or Tire Debeading in Nominal Load Nominal Load or
Multi-Passenger Configuration (Yes/No)
4.13
Post Processing
Data are
filtered in post processing with a 6-Hz 12-pole, 2-pass, phaseless
digital Butterworth filter. All accelerations are corrected
for CG displacement (see Section 2.5.1.3). Laser height measurements
are filtered with a one-pass 200 ms running average technique.
Post processing
also includes roll effects correction for lateral acceleration
as follows.
ayc
= aymcosΦ - azmsinΦ
where,
ayc
is the corrected lateral acceleration (i.e., the vehicle's
lateral acceleration in a plane horizontal to the test surface)
aym
is the measured lateral acceleration in the vehicle reference
frame
azm
is the measured vertical acceleration in the vehicle reference
frame
Φ
is the vehicle's roll angle
Note:
The z-axis sign convention is positive in the downward direction
for both the vehicle and test surface reference frames.
5.0
REFERENCES
1. Forkenbrock,
G.J., Garrott, W.R, Heitz, Mark, O'Harra, Brian C., "A Comprehensive
Experimental Examination of Test Maneuvers That May Induce
On-Road, Untripped Light Vehicle Rollover - Phase IV of NHTSA's
Light Vehicle Rollover Research Program," NHTSA Technical
Report, DOT HS 809 513, October 2002.
2. Forkenbrock,
G.J., O'Harra, Brian C., Elsasser, Devin, "An Experimental
Examination of 26 Light Vehicles Using Test Maneuvers That
May Induce On-Road, Untripped Light Vehicle Rollover - Phase
VI of NHTSA's Light Vehicle Rollover Research Program," NHTSA
Technical Report, DOT HS 809 547, 2003.
3. NHTSA,
"NHTSA's Experience With Outriggers Used For Testing Light
Vehicle - A Brief Summary," Docket No. NHTSA-2001-9663, January
2003.
4. NHTSA,
"NHTSA's Set-Up Procedures for Wheel Lift Sensors - A Brief
Overview," Docket No NHTSA-2001-9663, April 2003.
5. SAE
J266, Surface Vehicle Recommended Practice, "Steady-State
Directional Control Test Procedures For Passenger Cars and
Light Trucks," 1996.
Figure I.3. Water dummy placement for vehicles with
two designated rear seating positions, excluding pick-up
trucks.
<
Appendix II. Development of a Rollover
Risk Model
In its study of our rating system for rollover
resistance (Transportation Research Board Special Report
265), the National Academy of Sciences (NAS) recommended that
we use logistic regression rather than linear regression for
analysis of the relationship between rollover risk and SSF.
We had considered a logistic regression model during the development
of the rollover resistance rating system used by NCAP for
2001 to 2003 vehicles, but we observed that it predicted rollover
rates that were systematically lower than actual rollover
rates for vehicles with low SSF. Our first step was to explore
the use of transformations of SSF to create a logistic regression
model that better matched actual rollover rates while following
the recommendation of the NAS.
A satisfactory logistic regression model using
SSF only was the starting point for developing a risk model
that used both a vehicle's SSF and its performance in dynamic
maneuver tests to predict its rollover rate. We used four
binary variables to describe whether or not the vehicle tipped
up in two dynamic maneuver tests each performed at two different
occupant load conditions. The final model required the results
of only the Fishhook maneuver test with the heavy five occupant
load and the SSF of a vehicle. The predicted rollover rate
determines the rollover resistance rating of the vehicle.
A. Improving the Fit of the Logistic
Regression Model with SSF Only
We had considered logistic regression during
the development of the SSF based rating system (66 FR 3393,
January 12, 2001), but found that it consistently under-predicted
the actual rollover rate at the low end of the SSF range where
the rollover rates are high. The NAS study acknowledged this
situation and gave the example of another analysis technique
(non-parametric) that made higher rollover rate predictions
at the low end of the SSF scale. In the NPRM, we discussed
our plan to first examine ways to improve the fit of the logistic
regression model to the actual rollover rates in the simpler
model with SSF as the only vehicle attribute before expanding
the logistic regression model to predict rollover rates using
maneuver test results and SSF as vehicle attributes. In this
way, the addition of maneuver test results is more likely
to have an effect that reflects the additional information
they represent on rollover causation.
A consultant to the Bureau of Transportation
Statistics who lectured on logistic regression suggested that
we use a transformation of SSF, like Log(SSF), rather than
SSF alone to change the shape of the trend line generated
by the logistic regression in our range of interest of SSF.
This technique is similar to what we used to improve the fit
of the linear regression model in the SSF rating system (Figure
II.1). Linear regression creates a "best fit" straight line
to predict the relationship between the independent variable,
SSF in this case, and the dependent variable, rollover rate
per single vehicle crash in this case. However, the observations
of rollover rate for groups of vehicles with a known SSF did
not appear to lie on a straight line. The relationship appeared
to be exponential with a reduction in rollover rate with increase
in SSF much greater at low SSFs than at high SSFs. We used
the transformation Log(SSF) to replace SSF alone in the linear
regression model so that it would compute a "best fit" exponential
curve instead of a best fit straight line in order better
fit the prediction line to the observations. We referred
to Figure II.1 in notices 65 FR 34998 and 66 FR 3388 as a
linear regression model because of the analysis technique,
but the NAS study refers to it as the exponential model because
of its curve shape.
Figure II.2 plots the actual rollover rates
as a function of SSF observed for 293,000 single vehicle crashes
involving 100 vehicle groups in six states from 1994 to 2001
(not all state's data available in every year). The point
designated "actual rate" at each value of SSF gives the proportion
of single vehicle crashes for vehicles of that SSF that resulted
in rollover. For example, the leftmost point shows that for
all single vehicle crashes observed for vehicles with an SSF
of 1.00, slightly less than 50% resulted in rollover. There
are fewer than 100 data points because the data at each SSF
often include the crashes of several vehicles with the same
SSF.
Figure II.2 also plots the rollover rates
predicted for the same 293,000 crashes by a logistic regression
model operating on SSF without transformation as the only
vehicle variable. The model was developed from a database
that contained the driver characteristic and road condition
variables in the state crash reports of 293,000 crashes in
six states. Data from Maryland, Florida, North Carolina,
Missouri, Utah and Pennsylvania were used because these were
the only states with electronic records available to NHTSA
in which we could identify the make/model of the vehicle and
could be sure whether or not a rollover occurred. The driver
variables were gender, age [young (less than 25), old (70
or older), neither], and evidence of alcohol or drug use.
The road condition variables were weather, speed limit, curve,
hill, darkness, wet or icy surface, and potholes or other
bad surface conditions. The SAS logistic regression program
used these driver and road variables, the vehicle SSF, the
State and the outcome (rollover or not) for each of 293,000
single vehicle crashes to compute the risk model. Figure
II.2 shows the exercise of inputting the driver, road, state
and vehicle SSF circumstances for each individual crash of
the 293,000 back into the risk model to test how well the
model can predict the actual rollover outcomes.
In similar fashion as the "actual rate" points
on Figure II.2, the "predicted rate" points at each value
of SSF give the proportion of single vehicle crashes for vehicles
of that SSF that resulted in rollover. The number and circumstances
(as well as can be described from state crash report variables)
of crashes represented by the actual and predicted rate points
are identical. However, in one case the rollover outcomes
are the actual outcomes reported in the state data. But in
the other case, the rollover outcomes are the predictions
of the risk model given the driver and road variables and
vehicle SSF for each actual the crash. The predicted rate
points do not lie on a continuous curve when plotted against
SSF because the distribution of driver and road variables
are different for the single vehicle crashes experienced by
each group of vehicles represented by its SSF value.
Figure II.2 shows that the risk model obtained
using the untransformed SSF computes predictions that match
the actual rollover rates well at SSFs higher than 1.3, but
its predictions are consistently low at the low end of the
SSF range. The predictions also tend to be too high in the
1.15 to 1.25 SSF range. For this reason we described the
form of the curve inherent to the logistic regression computation
as being too flat or lacking sufficient curvature to represent
rollover risk in our past notices.
Figure II.2 also lists an objective measure
of the goodness of fit of the predictions to aid in the comparisons
of models with and without using transformations of SSF.
It is the R2 value for linear regression between
the predicted and actual rollover rates. Figure II.3 is a
plot of predicted versus actual rollover rates taken from
Figure II.2. It shows how the R2 value was obtained.
A linear regression of the form "y = mx" computes the best
fit line that passes through the origin. The R2
value that describes the goodness of fit of the points to
the line "y = 0.9673x" is 0.752. A perfect set of predictions
would cause an R2 value of 1.0 on the line "y =
1.0x".
Figures II.4, II.5, and II.6 show the predictions
of a series of risk models obtained in the same way as that
shown in Figure II.2 except that transformations of SSF were
used as the vehicle variable instead of just SSF. The first
transformation, shown in Figure II.4, was Log(SSF). This
is the transformation currently used in the linear regression
rollover risk model. It makes a very small improvement both
to the under-predictions at the low end of the SSF range and
the over-predictions in the 1.15 to 1.25 SSF range. The R2
goodness of fit indicator increased to 0.7975.
Next we tried the transformation Log(SSF-
margin). Figure II.5 shows the predictions of a logistic
regression model with a margin of 0.85. The subtraction of
a margin from SSF makes a large improvement in the fit of
the predicted rollover rates to the actual rollover rates
in the SSF range of 1.0 to 1.25. The R2 goodness
of fit indicator increased to 0.8811 about the line "y =
1.0011 x" for the whole SSF range of data base (1.0 to 1.53).
This transformation caused a small sacrifice in the fit of
the model at the high end of the SSF range. However, a good
fit in the 1.0 to 1.25 SSF range is more important to a rating
system because most of the consumer requests for rollover
information involve vehicles in this range.
Figure II.6 shows the fit of the model with
a margin of 0.9. The R2 goodness of fit indicator
increased slightly to 0.8948 about the line "y = 1.0091 x",
but the sacrifice of fit at the high SSF end also increased.
Figure II.7 is a plot of predicted versus actual rollover
rates taken from Figure II.6. The use of the transformation
Log(SSF-0.90) instead of SSF alone in the logistic regression
gave us a risk model with the benefits of logistic regression
recommended by the NAS and a goodness of fit with the actual
rollover rate data at least equivalent to that of the linear
regression model we have been using.
Figure II.8 shows the best logistic regression
model (margin = 0.90) and the linear regression model we have
been using. In this presentation, the driver and road variables
of the crashes for each SSF were the same so that the differences
in predicted rollover rates along each line were a purely
a function of SSF differences, and the risk curve is continuous.
The common scenario of driver and road variables represented
the average conditions for the entire 293,000 single vehicle
crashes (only 20% of which resulted in rollover). The linear
regression model represents the same scenario.
The line in Figure II.8 representing the linear
regression model is described by the equation:
The line in Figure II.8 representing the logistic
regression model is described by the following equation:
B. Adding Dynamic Maneuver Test Results
to the Logistic Regression Model
The dynamic maneuver test results (tip-up
or no tip-up in each maneuver/load combination in Table 1
of the main body of the notice) were used as four binary variables
in the logistic regression analysis. They were entered in
addition to SSF to describe the vehicle. The same driver
and road variables from state crash reports discussed above
were used. The state crash report data for twenty-four of
the vehicles used in the logistic regression analysis with
dynamic maneuver test variables was a subset of the database
of 293,000 single vehicle crashes described above. One extra
vehicle was added for the maneuver tests that was not among
the 100 vehicle groups we had studied previously, but state
crash report data from the same years and states was obtained
for it. However, the database with SSF and dynamic maneuver
tests was much smaller than the 293,000 sample size available
for the logistic regression model with SSF only. Its sample
size was 96,000 single vehicle crashes of 25 vehicles including
20,000 rollovers.
The risk models combining SSF and dynamic
maneuver test results ("dynamic results" for short) are computed
in the same way as the logistic regression curve in Figure
II.7. The logistic regression analysis of the database of
96,000 state reports of single vehicle crashes along with
the dynamic results and SSF of each crashed vehicle provides
a mathematical relationship between all of the vehicle, driver
and road variables and a prediction of whether rollover will
occur in a single vehicle crash described by any combination
of the variables. Next, for the number of sets of driver
and road variables that define the average crash scenario
of the 293,000 single vehicle crash database, predictions
of rollover or no rollover in the crash are made at each combination
of SSF and dynamic results. The proportion of crashes that
are predicted to result in rollover is plotted at each SSF
and dynamic result. Continuous curves predicting rollover
rate versus SSF for each combination of dynamic results is
the form of the model. Since all of the predictions were
made with the same driver and road scenario, the changes in
rollover rate along each SSF curve or between dynamic results
are functions of vehicle attributes.
Figure II.9 illustrates the form of the model
with dynamic results. It shows the predicted rollover rate
as a function of SSF and whether or not the vehicle tipped-up
in the Fishhook maneuver with 5 occupant loading (fishhook
heavy or FH). It predicts a rollover rate that is strongly
dependant on SSF but higher for vehicles that tip-up in this
severe maneuver than for vehicles that do not tip up in the
test.
The intent of using dynamic results from four
tests was to provide tests with a range of severity to best
discriminate between vehicles on the basis of dynamic performance.
The Fishhook heavy maneuver was the most severe, and the J-turn
light was the least severe. The expectation was that tip-up
in the least severe maneuver would predict a greater rollover
risk than tip-up in the most severe maneuver.
Figures II.10, II.11 and II.12 show logistic
regression models using each of the other maneuvers as a single
variable for dynamic results. In Figure II.10, vehicles that
tip-up in J-turn heavy are predicted to have a slightly greater
rollover risk than those that do not tip. However, in the
Fishhook light and J-turn light maneuvers, the logistic regression
models of Figures II.11 and II.12 predicted a greater rollover
risk for vehicles that did not tip-up.
We do not believe vehicles that tip up in
the least severe maneuvers are actually safer than those that
do not tip up. A more rational interpretation is that the
numbers of vehicle tipping up in these maneuvers were too
few to establish a definitive correlation. Only three vehicles
tipped up in the J-turn light maneuver, and six vehicles tipped
up in the Fishhook light maneuver. Only one more vehicle
tipped up in the J-turn heavy maneuver than in the Fishhook
light, and the prediction of the model with J-turn heavy was
consistent with expectations that tip-up in the test predicts
greater rollover risk. However, the extra vehicle in the
J-turn heavy tip-up group was the Ford Ranger 2 WD with a
very large sample size of over 8,000 single vehicle crashes
(nearly 10 percent of the entire data base).
Next we computed a logistic regression using
both dynamic results variables, Fishhook heavy and J-turn
heavy, that were observed to have a directionally correct
result when entered into the model individually. The result
was that the variable, J-turn heavy, was rejected by the logistic
regression program as not statistically significant in the
presence of the Fishhook heavy variable. In other words,
the predictions based on tip-up in the Fishhook heavy maneuver
do not change whether or not the vehicle also tips up in the
J-turn heavy maneuver.
Figure II.13 shows the final model that uses
only Fishhook heavy of the dynamic results variables. The
printout of the SAS logistic regression procedure that establishes
the coefficients of the model has been docketed separately.
This model has a risk prediction for vehicles that tip up
in the dynamic maneuver tests based on the greatest number
of vehicles possible in our 25 vehicle data base. All 11
vehicles that tipped up in any maneuver are represented on
the tip-up curve, and the 14 vehicles without tip-up are represented
on the other curve. The logistic regression model based on
SSF only for 100 vehicles is included for reference. It is
very similar to the risk model with dynamic result variables
for vehicles that tip up in the Fishhook heavy maneuver.
This result is not surprising because the SSF only model was
optimized for best fit in the 1.00 to 1.25 SSF range that
included all vehicles tipping up in dynamic maneuver tests.
The SSF only model was based on a vehicle sample that included
10 of the 11 vehicles that tipped up in the dynamic tests,
but the sample included 90 additional vehicles. The fact
that the prediction based on the SSF of 100 vehicles closely
matches the prediction based on 11 vehicles that tipped up
in the dynamic tests suggests that the small sample has produced
a robust prediction although the predictive power of tip-up
in the dynamic test may not be great.
In Figure II.13, the equation of the line
representing the SSF only model (from the 100 vehicle database)
is:
The equations for the final model representing
a combination of SSF with dynamic scores for each of the dynamic
results (tip-up and no tip-up) are:
Figure II.1: Linear regression
model for 2001-2003 SSF Rollover Resistance Rating (rollovers
per single-vehicle crash estimated from six states adjusted
to national average road use and for differences in state
reporting)
Figure II.2: Logistic
regression model operating on SSF (w/o transformation)
100 vehicle database - SSF only
Figure II.3: Actual rollover
rate vs. Predicted rollover rate
(using model of Fig. II.2) - 100 vehicle database - SSF only
Figure II.4: Logistic
regression model operating on the LOG(SSF)
100 vehicle database - SSF only
Figure II.5: Logistic
regression model operating on the LOG(SSF-0.85)
100 vehicle database - SSF only
Figure II.6: Logistic
regression model operating on the LOG(SSF-0.90)
100 vehicle database - SSF only
Figure II.7: Actual rollover
rate vs. Predicted rollover rate
(using model of Fig. II.6) - 100 vehicle database - SSF only
Figure II.8: Logistic
regression risk model using SSF only and
Linear regression risk model for 2001-2003 NCAP Rollover Resistance
Figure II.9: Model with
single dynamic variable - Fishhook, Heavy
Figure II.10: Model with
single dynamic variable - J-Turn, Heavy
Figure II.11: Model with
single dynamic variable - Fishhook, Light
Figure II.12: Model with
single dynamic variable - J-Turn, Light
Figure II.13: Comparison
of Combined Model with SSF only Model
[1]
For brevity, we use the term "light trucks" in this document
to refer to vans, minivans, sport utility vehicles (SUVs),
and pickup trucks under 4,536 kilograms (10,000 pounds) gross
vehicle weight rating. NHTSA has also used the term "LTVs"
to refer to the same vehicles.
[2]
A broken hip with splintering of the bone is an example of
an AIS 3 injury.
[3]
NHTSA Research Note, "Passenger Vehicles in Untripped Rollovers,"
September 1999
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