Bump steer

Bump steer or roll steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke. It is typically measured in degrees of steer per metre of upwards motion or degrees per foot. Bump steer or roll steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke. It is typically measured in degrees of steer per metre of upwards motion or degrees per foot. On modern cars the front suspension and steering links are designed so that during a turning manoeuvre, body roll and suspension motions will not influence the car to turn more sharply than the driver anticipates. On modern cars, the leading edge of the front tyres move outwards as the suspension is compressed, and inwards as the suspension droops (extends). This is known as 'toe out' under bump and results in roll understeer. I.e. during a turn, body roll will cause the outside suspension (relative to the curve) to compress and the inside suspension to droop (extend). Rear suspension can be designed a number of ways. Many modern vehicles have rear suspension designs which are opposite of the front suspension: Toe in under bump, and out under droop. They can also be designed to have very little or no bump steer at all. Cars with rear live axles, also known as solid axles do not exhibit true bump steer, but can still cause some steering over one wheel bumps, see § difference between bump steer and roll steer. If both wheels on a live axle move upwards by the same amount, they tend not to steer.