What Is Bump Steer?
Bump steer is a change in the steering angle of the wheel when the wheel is in bump or droop without turning of the steering wheel or lateral movement in the steering rack. This is due to the arc patterns of the wishbones and the steering arm having different radii and start points. The bump steer angle is the difference in angle between that of the wishbone and that of the steering arm shown in the diagram below.
The Purpose Of Bump Steer
Bump steer is highly undesirable within racing especially on tracks or surfaces that contain bumps. If the suspension is constantly transitioning between bump and droop, then a car with bump steer will constantly be shifting the steering angle of the car and will make the front end very unstable at high speeds. The bump steer in the system will affect the precisely designed Ackermann steering geometry and will make the steering more unpredictable for the driver.
Bump steer can provide an advantage to the rear end of a car on a smooth circuit effectively providing “active toe” during cornering. This can be used on a front wheel drive car to produce toe out, when the suspension is in bump, at the rear of the car to encourage oversteer on turn in; removing understeer characteristics.