Variable Ride-Height 

Depending on conditions such as vehicle speed and terrain, Variable Ride-Height Suspension (VRHS) raises or lowers the ride height of the vehicle while it is in motion. Some VRHS systems operate automatically, while others require the driver to select the appropriate mode. VRHS systems can have a favorable effect on a vehicle's likelihood to roll over because they lower the height of the vehicle's center of gravity and improve its Static Stability Factor (SSF) rating for highway driving.

Image: Suspension raises/lowers based on speed terrain.

VRHS systems exist under many trade names, including Electronic Height Control (EHC), Active Height Control (AHC), and pneumatic suspension systems.


In general, during off-road, lower-speed conditions, the vehicle will ride higher to allow for increased ground clearance. During on-road, higher speed conditions, the vehicle will ride lower. The vehicle riding lower by several inches is less top-heavy, has a higher SSF, and is less likely to roll over in a crash.

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.

Image: Dimensions used to calculate SSF.