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.

illustrative

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).

What is a Static Stability Factor (SSF) and how is it computed?

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.

What is a good Static Stability Factor (SSF)?

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.

How is the dynamic maneuvering test conducted?

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 maneuver—steering sharply in one direction, then sharply in the other direction—within 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).

What is the risk of rollover associated with 15-passenger vans?

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/.