The Static Stability Factor (SSF) is calculated using a formula based on a motor vehicle's center of gravity and track width. It is a measure of how top heavy a vehicle is. It has been used by the National Highway Traffic Safety Administration (NHTSA) in its Rollover Resistance Rating System for cars and light trucks.
static stability factor (dimensionless ratio) =
= (vehicle's track width based on tire midpoints) / (2 * (height of the vehicle's center of gravity above the horizontal)) =
= (distance from the midline of the vehicle to the midpoint of a tire on one side) / (height of vehicle's center of gravity above the ground)
The vehicle's center of gravity is measured in a Vehicle Inertia Measurement Facility (VIMF).
The track width for a vehicle is based on the average of the following 8 measures:
|
Distance |
Front Axle |
Back Axle |
|
inside of left tire to outside of right tire |
at front surface of tire |
at front surface of tire |
|
inside of left tire to outside of right tire |
at rear surface of tire |
at rear surface of tire |
|
outside of left tire to inside of right tire |
at front surface of tire |
at front surface of tire |
|
outside of left tire to inside of right tire |
at rear surface of tire |
at rear surface of tire |
Rollover Rating Scale (RRS)
|
Estimated Risk of Rollover |
RRS Stars |
|
> 40% |
1 |
|
30.1 – 40% |
2 |
|
20.1 – 30% |
3 |
|
10 – 20% |
4 |
|
< 10% |
5 |
From the "Rated Vehicle Details", the following correlation is seen between the SSF and Rollover Resistance Rating Scale:
|
SSF |
RRS Stars |
|
<= 1.03 |
1 |
|
1.04 – 1.12 |
2 |
|
1.13 – 1.24 |
3 |
|
1.25 – 1.44 |
4 |
|
>= 1.45 |
5 |
where:
• This table is based on the data in the "Rated Vehicle Details" table. No vehicles had values of 1.03, 1.24, or 1.44; I assigned these somewhat arbitrarily to the Star scale. Thus, a 1.03 may indicate a Star 2 rating; 1.24 and Star 4 rating; and 1.44 a Start 5 rating.
Limitations:
• Some view the SSF as overly simplistic and does not reflect real world conditions.
• The NHTSA is planning to use dynamic testing during actual driving to derive future measures of rollover tendencies.
Specialty: Emergency Medicine, Critical Care
ICD-10: ,
