All vehicles have inherent characteristics that decrease the performance of the vehicle and can create a dangerous scenario for the principal. One of those characteristics is the vehicle’s static stability factor (SSF).
One of the concerns is the vehicle’s susceptibility to a rollover. The SSF is how the National Highway Traffic Safety Administration determines a vehicle’s rollover probability. It is the vehicle’s at-rest calculation of its rollover resistance. The number is based on the vehicle’s 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 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.
A good SSF would be 1.5, not good would be 1.1. Not good is defined as once the vehicle starts to slide, there is a 28 to 30 percent chance of a rollover.
SLIDE VERSUS ROLL
When the limit is reached in a passenger vehicle, they tend to slide, and when they slide, they will under or oversteer. In an SUV, when the limit is reached, they also will slide and go into an under/oversteer mode. However, along with sliding, they tend to lift. The reason for the lifting is a simple matter of the laws of physics – the higher the center of gravity of the vehicle and the smaller the track (the distance from the centerline of the left front tire to the centerline of the right front tire) the more likely the SUV will lift.
Using SSF to Determine The Vehicle’s Center of Gravity
A critical characteristic of all vehicles is its center of gravity. The vehicles SSF supplies security drivers with another useful tool, determining the vehicle’s Center of Gravity. That can be accomplished by reworking the equation and solving for CG. When you do that you get this equation
CG = Track/(2 x SSF)
This chart is an example of the SSF and resultant CG height of various vehicles.
|Vehicle||Track||SSF||CG Inches||CG Feet|
When loading an SUV, for example, the Suburban, any additional weight above the 2.51 Foot center of gravity point will increase the weight above the Suburban’s CG and raise the possibility of a problem. Combine this added weight above the vehicle CG with low tire pressures, and the difficulties increase significantly. As we have mentioned many times, check the tire pressures, and do not rely on the vehicle Tire Pressure Monitoring System (TPMS).
The scores. NHTSA combines the SSF and dynamic test to assign a rollover-resistance score of one to five stars. Five stars represent rollover likelihood in a single-vehicle crash at 10 percent or less; one star predicts a rollover likelihood of 40 percent or more. The SSFs underlying the star ratings vary from about 1.0 to 1.5. (The higher that number, the better.) SUVs usually measure out at 1.0 to 1.3, and cars frequently fall in the range of 1.3 to 1.5.
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