The center of roll is a line around which a vehicle rotates, and its position changes depending on the vehicle’s suspension and center of mass. Engineers use this information to predict vehicle performance and make safety improvements, such as changing suspension systems or lowering the center of mass.
A center of roll is a line around which a vehicle rotates. For a car or truck, when sitting still, the center of roll would be a line, front to back, toward the center of the vehicle. If the vehicle is moving, the center of roll may change position. How much it changes depends on the vehicle’s suspension and its center of mass. Engineers consider a vehicle’s center of roll, along with the center of mass, to predict vehicle performance under certain road and driving conditions.
Roll hubs say little about the cars handling. To use roll centers to make performance predictions, they must be considered in conjunction with the vehicles center of mass. The center of mass of a stationary vehicle is usually the same as the center of roll from left to right, but add another line from top to bottom. It is the movement of these two lines, when compared to each other, that determines the rolling performance of a vehicle.
When a vehicle is in a maneuver that applies a rolling force, such as that created by centrifugal force when turning a corner, the vehicle may lower on the outboard lateral suspension. This changes the force exerted on the wheels of the vehicle. While the vehicle’s center of mass remains constant, the force increases on the outside wheels and decreases on the inside wheels. Unless another force, such as a suspension system, offsets this difference, the vehicle’s center of roll migrates to the outside of the turn.
The new location of the roll center will cause the center of mass to react to it in a way called the drive arm. In this situation, the centrifugal force of the spin uses the difference between the center of mass and the center of the roller to create a fulcrum. This fulcrum places a rotational force, or torque, on the outer wheels of the car perpendicular to the direction of travel. If the centrifugal force is strong enough, it will transfer through the drive arm, using the outer tires as a fulcrum, then lift and rotate the center of mass up and over the outer tires, or roll the vehicle.
By looking at these various forces and their interaction with the rolling centers of vehicles, engineers can make safety improvements to cars. One of the ways they can do this is by changing the suspension systems. They can stiffen the suspension so vehicles don’t squat to one side and shift their roll centers.
However, only so much can be done with suspension systems. Often, as is the case with vehicles like 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.
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