A roll center is a line around which a vehicle rotates. For a car or truck, when sitting still, the roll center would be a line, from front to rear, down the center of the vehicle. If the vehicle is moving, the roll center can change position. How much it changes depends on the vehicle’s suspension and its center of mass. Engineers consider a vehicle’s roll center, along with the center of mass, to predict the vehicle’s performance under certain road and driving conditions.
Roll centers say little about cars' handling. In order to use the roll centers to make performance predictions, they must be considered along with the vehicles’ center of mass. A static vehicle’s center of mass is typically the same as the roll center from left to right, but adds another line from top to bottom. It is the movement of these two lines, when compared to one another, that determines the roll performance of a vehicle.
When a vehicle is in a maneuver that applies a rolling force, such as created by centrifugal force when turning a corner, the vehicle may dip down on the outer side suspension. This shifts the force placed on the vehicle’s wheels. While the vehicle’s center of mass remains constant, force increases on the outside wheels, and decreases on the inside wheels. Unless another force, such as a suspension system, counters this difference, the vehicle’s roll center migrates toward the outside of the turn.
The new placement of the roll center will cause the center of mass to react to it in a manner called a momentum arm. In this situation, the centrifugal force of the turn uses the difference between the center of mass and the roll center to create a fulcrum. This fulcrum places a rotating force, or torque, on the outside wheels of the car perpendicular to the direction of travel. If the centrifugal force is strong enough, it will transfer through the momentum arm, using the outside tires as a fulcrum, then lift and rotate the center of mass up and over the outside tires, or roll the vehicle over.
By looking at these various forces and their interaction with vehicles’ roll centers, engineers can make safety improvements to cars. One of the ways they can do this is by changing suspension systems. They can stiffen the suspension so that vehicles do not dip on one side and shift their roll centers.
There is, however, only so much that can be done with suspension systems. Often, as is the case with vehicles such as SUVs that have unusually unstable roll centers, the only course of action is to lower the vehicles' centers of mass. In this way, engineers can minimize the forces applied in unwanted interactions with roll centers.