- As a consumer, how should I use the star rating along with the percent chance of rollover and dynamic test results?
- What does the rollover "diamond and bar" graphic mean that I see on the Test Details page for each vehicle on the safercar.gov website?
- Are SUVs the only vehicles that roll over?
- How can I find out which vehicles have technologies like Electronic Stability Control (ESC)?
- How should I interpret NHTSA's new combined rollover resistance rating?
- How is a vehicle&rsquos star rating determined from the statistical model?
- What is a Static Stability Factor (SSF) and how is it computed?
- What is a good Static Stability Factor (SSF)?
- How is the dynamic maneuvering test conducted?
- How much effect does the dynamic test result have on a vehicle&rsquos star rating?
- If a tip up occurs during the dynamic test, why aren&rsquot entry speed of the maneuver and height of wheel lift reported?
- How are the rollover ratings on 2004 model year cars different from those for earlier models?
- What is the
risk of rollover with 15-passenger vans?
1. 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&rsquo 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.
2. What does the rollover diamond and bar graphic mean that I see on the Test Details page for each vehicle on the safercar.gov website?
This graphic is to be used as a supplemental piece of information to a vehicle&rsquos rollover star rating. The diamond represents the vehicle&rsquos 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).
3. Are SUVs the only vehicles that roll over?
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.
4. How can I find out which vehicles have technologies like Electronic Stability Control (ESC)?
You can select a vehicle on the 5-Star Crash Test and Rollover Ratings section of www.safercar.gov and view it's Safety Features chart, or you can order a Buying a Safer Car brochure by calling 888-327-4236.
5. How should I interpret NHTSA's combined rollover resistance rating for model years 2004 and later?
The rollover resistance rating system for model years 2004 and later predicts a vehicle's chance of rollover in single-vehicle crashes by combining: (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.
About 95% of rollovers are tripped - meaning the vehicle strikes something low, such as a curb or shallow ditch, causing it to tip over. The Static Stability Factor (SSF) is specifically designed to measure this more common type of rollover and thus plays a significantly larger role in a vehicle's star rating (for model years 2004 and later) than the results of the dynamic maneuvering test.
The SSF rating and the tip or no tip 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 in the 5-Star Crash Test and Rollover Ratings section of 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.
6. How is a vehicle&rsquos star rating determined from the statistical model?
The model uses two inputs &ndash static stability factor (SSF) and dynamic test results &ndash to determine a vehicle&rsquos chance of rollover and thus its star rating. A vehicle&rsquos 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.
- Enter the chart from the SSF=1.20 location on the x-axis.
- Go vertically to either of the two curves. Stop at the solid curve if the vehicle did not tip up during the maneuvering test. Stop at the dashed curve if the vehicle did tip up during the maneuvering test.
- Follow those points of intersection horizontally to the left to determine the percent chance of rollover if involved in a single vehicle crash and the vehicle&rsquos star rating.
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.
This graphic is to be used as a supplemental piece of information to a vehicle&rsquos rollover star rating. The diamond represents the vehicle&rsquos 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).
7. 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.
8. 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.
9. 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).
Fishhook Maneuver
10. How much effect does the dynamic test result have on a vehicle&rsquos star rating?
About 95% of rollovers are tripped - meaning the vehicle struck something low, such as a curb or shallow ditch, causing it to tip over. The Static Stability Factor (SSF) is specifically designed to measure this more common type of rollover and thus plays a significantly larger role in a vehicle's star rating (for model years 2004 and later) than the results of the dynamic maneuvering test.
11. If a tip up occurs during the dynamic test, why aren&rsquot entry speed of the maneuver and height of wheel lift reported?
Vehicle entry speed and height of wheel lift are interesting engineering variables that are measured and recorded for the purpose of controlling the test&rsquos repeatability and uniformity. However, they have no influence on the predicted rollover percentage and star rating and thus are not reported. The only factor from the dynamic test that affects the rollover percentage and star rating is whether or not the vehicle tips up.
12. How are the rollover ratings on 2004 model year cars different from those for earlier models?
Starting with 2004 model year vehicles, rollover ratings will 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 in the 5-Star Crash Test and Rollover Ratings section of www.safercar.gov.
For model years 2003 and earlier, rollover ratings will still have star ratings, but will be based on the SSF rating only. Consumers making cross-year comparisons of vehicles' rollover ratings need to be aware of this difference.
13. What is the risk of rollover associated with 15-passenger vans?
NHTSA research has found that the risk of a rollover crash increases dramatically as passenger load increases to full load 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/studies/15PassVans/
15PassCustomerAdvisory.htm.
