Sprung Back : Ride Rates in a Vehicle

Ride rate refers to the rate of change of tire normal force with vertical body movement and roll rate refers to the rate of change of tire normal force with body roll about its roll axis. The ride and roll rates of the car affect the wheel loading wrt the body position change. And the wheel load or tire normal force determines the available tire lateral/longitudinal forces which directly impacts the vehicle performance.

Individual suspension components like springs and antiroll bar affect the vehicle ride and roll. The ride performance of the car is many a times in conflict with the roll performance and hence choosing the correct springs and anti roll bar is often a balancing act. The main purpose of the suspension is to keep the tires on the ground while also isolating vibrations. This can be achieved by soft suspension. Too stiff suspension can make the tire airborne in trough cases or unsettle it while going over bump. However, some stiffness is required to limit body roll and improved handling and also to protect vehicle with low ground clearance. Race cars also use it to maintain their ride height.

Hence based on the requirements, arriving at the correct spring and anti-roll bar stiffness is a difficult iterative task.

Definitions:

Every suspension use some sort of spring to get the vertical stiffness in order to keep the wheels on the ground. Broadly speaking a car can be viewed as a lumped vehicle body mass (sprung mass) being balanced on the two left right…