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.

On modern cars the front suspension and steering links are designed so that during a turning maneuver, 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 tires 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 that 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.

Bump steer causes a vehicle to turn itself when one wheel hits a bump or falls down into a hole or rut. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. For example, if the front left wheel rolls over a bump it will compress the suspension on that corner and automatically rotate to the left (toe out), causing the car to turn itself left momentarily without any input from the steering wheel. Another example, is that when most vehicles become airborne their front wheels will noticeably toe in.

Typical values are from two to ten degrees per metre, for the front wheels.

The linearity of the bump steer curve is important and relies on the relationship of the control arms and tie rod pickup points, and the length of each part. As the suspension goes through bump and droop, each part follows an arc resulting in a change of effective length. Whichever parts are longest tend to have less change in effective length because their arc radius is longer. This is the determining factor in designed bump steer. Another factor that affects bump steer is bushing compliance and deflection and arm bending. During a turn, if some or all of the bushings deflect then their pickup points have changed If any of the arms and tie rods bend then their effective length will change resulting in a change of toe.

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Wikipedia