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On a Roll: Roll rates in a Vehicle
This could be treated as part 2 of the spring rates post.
When the vehicle goes through the corner, it experience the roll motion when the roll center of the vehicle doesn’t coincide with the vehicle center of mass(which usually is the case). Vehicle roll motion affects the wheel loading which in turn affects the available tire lateral/longitudinal forces and hence the performance of the vehicle.
This body roll during the corner is opposed by a roll stiffness. Roll stiffness can be defined as the roll resisting moment of the body (sprung mass) about the roll axis. It is usually expressed in (torque) N-m/degree of roll.
It can be provided by various components in the vehicle: springs, ARBs , tires and compliances with the primary stiffness coming from springs and Anti-Roll Bars. The vehicle level properties like location of center of mass, roll center location and axle track width also plays a big part. Roll stiffness of the complete vehicle is the sum of the separate roll stiffness rates of all vehicle elements.
Roll Gradient
The stiffness in roll is also conveniently expressed in normalized form for the entire vehicle as degrees of roll per unit lateral acceleration. (deg/g) This is called roll gradient. Some typical roll gradient values are :
Large Passenger Cars: 7.0 degrees/g
Small Passenger Cars: 5.0 Degrees/g
Sports cars: 4.0 Degrees/g
These are indicative values only. Usually if the springs alone are not able to provide the roll stiffness required, Anti roll bars or ARBs are deployed. You usually have some roll gain target in mind for the entire vehicle and try to match that number using different components. The spring rates/ride rates come from the ride frequencies/ wheel travel considerations and then the remaining roll stiffness (if applicable) from ARBs (as ARB and springs are the two main contributors to roll stiffness).
Anti Roll Bars
An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) wheels…
