Bump Steer

Since we are talking about cornering and steering, let's discuss bump steer. Simply put, bump steer is the effect that immediate toe and camber changes have on wheel and steering angles. Generally speaking it happens, obviously enough, after hitting bump. In road cars the driver feels this as a wriggling of the steering wheel, but because they are not at the limits of adhesion, driving heavily damped suspensions, riding on massively compliant tire sidewall, little drama happens. In a race car the effect can be perilous.

Technically what happens is that the tire is connected to the chassis via suspension arms - so as the wheel travels vertically up/down it travels in a single arc that is the compromised sum of the arcs of the two suspension arms. With each degree of movement the effective length of the arms change. This reduction will pull the toe-in/toe-out measurements to a measurable degree. If the car is on the mechanical and aerodynamic limit of there will be no available driver inputs to correct the inevitable loss in traction and the driver must move into compensation mode - all with a corresponding loss in speed or track position.

Mechanically, since it can be measured, bump steer can be accounted for - and race car set up begins with this type of balance and trade off. Does the chassis engineer try to design/adjust-out the bump steer, do they make other adjustments to compensate for it, or do they try to use bump steer to their advantage? Bump steer is a complex function of the relative lengths of the A-arms to the steering rod, the angles they all go through when deflected, and the caster/camber angles the chassis designer built into the car. Bump steer cannot be totally eliminated, but by changing the angle of the track rod relative to the A-arms, the tendency to move the wheel assembly can be minimized.