Volume Six, Issue 8
Accident Reconstruction Newsletter
No. A rollover crash can happen in any type of vehicle. SUVs, like pickup trucks and minivans, typically ride higher off the ground than passenger cars and have higher centers of gravity, and thus are more susceptible to rollover if involved in a single-vehicle crash. See the vehicle class comparison chart. But while vehicle type does play a significant role, other factors such as driver behavior and road and environmental conditions also help determine whether or not a vehicle rolls over.
Even a five-star vehicle has up to a 10% risk of rolling over in a single-vehicle crash. In fact, certain five-star vehicles, such as sports cars, may have a higher number of rollovers per 100 registered vehicles than certain three-star vehicles, such as minivans, due to the aggressive way in which the vehicle is driven and/or the age and skill of the driver.Back to top
You can select a vehicle on the New Car Assessment Program (NCAP) web page at www.safercar.gov and view the Safety Features chart, or you can order a Buying a Safer Car brochure by calling 888-327-4236 (888-DASH2DOT).
The new combined rollover resistance rating system predicts a vehicle’s chance of rollover in single-vehicle crashes by: (1) an at-rest laboratory measurement known as the Static Stability Factor (SSF) which determines how top-heavy a vehicle is, and (2) the results of a dynamic maneuvering test that determines how susceptible the vehicle is to an on-road un-tripped rollover.
The SSF rating and the tip-up or no tip-up results of a vehicle’s dynamic maneuvering test are combined into one overall rollover star rating. A separate star rating for the individual tests is not given. However, the results for individual tests, where available, can be viewed on the test details web page for each vehicle at www.safercar.gov.
Vehicles with a No tip* result were not actually subjected to the dynamic test. Results from these vehicles are imputed (assigned) based on the testing of passenger cars with lower Static Stability Factors (SSFs) that did not tip up during the dynamic test. NHTSA will periodically test passenger cars to validate imputed results.
The model uses two inputs -- static stability factor (SSF) and dynamic test results -- to determine a vehicle's chance of rollover and thus its star rating. A vehicle's SSF is used to determine its location along the x-axis.
In the illustrative example below, an SSF value of 1.20 is used. This SSF value is typical of a more stable SUV or pickup truck. Remember: the higher the SSF value, the more stable the vehicle is and the less likely it is to rollover.
In this example, a vehicle with an SSF of 1.20 that does not tip up would have a 19% chance of rollover and would receive 4 stars. A vehicle with an SSF of 1.20 that does tip up would have a chance of rollover of 22% and would receive 3 stars.
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As a consumer, how should I use the star rating along with the percent chance of rollover and dynamic test results?
As a consumer, you should first look at the Rollover Star Rating when comparing vehicles' chances of rollover. Remember, 5-stars is the highest rating and represents those vehicles least likely to rollover. Rollover star ratings can be compared across vehicle classes and weights.
When two vehicles have the same star rating, consumers should then compare the Chance of Rollover (percentage) between vehicles. The lower the percentage the less likely a vehicle is to rollover. Remember, this percentage depends both on the static stability factor (SSF) and whether or not the vehicle tips up during the dynamic test.
This graphic is to be used as a supplemental piece of information to a vehicle's rollover star rating. The diamond represents the vehicle's percent chance of rollover if involved in a single vehicle crash. The bar represents the range of percentages for all vehicles tested in a given vehicle class (passenger cars, vans, pickup trucks, or SUVs) for the last three model years (the current model year plus the two earlier model years).Back to top
The Static Stability Factor (SSF) of a vehicle is an at-rest calculation of its rollover resistance based on its most important geometric properties. SSF is a measure of how top-heavy a vehicle is.
A vehicle’s SSF is calculated using the formula SSF=T/2H, where T is the "track width" of the vehicle and H is the "height of the center of gravity" of the vehicle. The track width is the distance between the centers of the right and left tires along the axle. The location of the center of gravity is measured in a laboratory to determine the height above the ground of the vehicle’s mass. The lower the SSF number, the more likely the vehicle is to roll over in a single-vehicle crash.
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A higher SSF value equates to a more stable, less top-heavy vehicle. SSF values across all vehicle types can range from around 1.00 to 1.50. Most passenger cars have values in the 1.30 1.50 range. Higher-riding SUVs, pick-up trucks, and vans usually have values in the 1.00 1.30 range.
Many of the higher-riding vehicles of previous model years are being redesigned to ride lower and with a wider track width, thus improving their rollover resistance and yielding a higher SSF rating.Back to top
The dynamic maneuvering test uses a heavily loaded vehicle, to represent a five-occupant load, and a full tank of gas. Using a fishhook pattern, the vehicle simulates a high-speed collision avoidance maneuversteering sharply in one direction, then sharply in the other directionwithin about one second. Test instruments on the vehicle measure if the vehicle’s inside tires lift off the pavement during the maneuver ("inside" meaning the left wheels if turning left, and the right wheels if turning right). The vehicle is considered to have tipped up in the maneuver if both inside tires lift at least two inches off the pavement simultaneously.
The tip-up/no tip-up results are then used with the SSF measurement as inputs in a statistical model that estimates the vehicle’s overall risk of rollover in a single-vehicle crash. The overall risk of rollover for the particular vehicle will fall into one of five ranges of rollover risk and thus determine its star rating (1 through 5 stars).
Starting with 2004 model year vehicles, rollover ratings combine both the stationary (at-rest) measurement known as the Static Stability Factor (SSF) and the tip or no-tip results of the dynamic maneuvering rollover test. These results are then combined for one overall star rating. However, individual test results, where available, can be viewed on the test details web page for each vehicle at www.safercar.gov.
For model years 2003 and earlier, rollover ratings will still have star ratings, but are based on the SSF rating only. Consumers making cross-year comparisons of vehicles’ rollover ratings need to be aware of this difference.Back to top
NHTSA research has found that the risk of a rollover crash is greatly increased when 10 or more people ride in a 15-passenger van. This increased risk occurs because the passenger weight raises the vehicle’s center of gravity and causes the center to shift rearward. As a result, the van has less resistance to rollover and handles differently from other commonly driven passenger vehicles, making it more difficult to control in an emergency situation. For more information on reducing the risk of rollover crashes in 15-passenger vans, visit http://www.nhtsa.dot.gov/cars/problems/equipment/15PassVans/.Back to top
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