Glossary
What is Bump Steer? Causes, How to Measure It and How to Fix It
Bump steer is the change in toe angle that happens as a wheel moves up and down through its suspension travel, without any driver steering input. It occurs when the tie rod's arc does not match the arc of the suspension arms. The car then steers itself over bumps, under braking dive and in roll, which feels like instability or inconsistent turn-in. The target on a race car is usually under 1 mm of toe change per 25 mm of wheel travel, and often close to zero.
| Behaviour | Front axle effect | Rear axle effect |
|---|---|---|
| Toe-out in bump | Outside wheel toes out in roll: faster turn-in, nervous under braking | Oversteer in roll |
| Toe-in in bump | Outside wheel toes in: stable but lazy turn-in | Stabilising, common design target |
| Zero | Predictable; steering only from the driver | Predictable |
Measuring bump steer
Remove the spring, set the car at ride height on a bump steer gauge (a plate against the wheel with two dial indicators), lock the steering straight ahead and move the hub through its travel in 10 or 25 mm steps. Record the toe change at each step and plot it. A straight, nearly flat line is the goal; a curve means the tie rod length is wrong, a slope means the tie rod height is wrong.
Correcting it
If the toe changes linearly with travel, raise or lower the outer tie rod end with shims until the slope is flat. If the curve bends, the tie rod length differs from the ideal and the inner pivot must move. Re-check after any change to ride height, camber or rack position because all of them move the geometry.
Frequently asked questions
Can lowering a car cause bump steer?
Yes. Lowering changes the control arm angles but not the tie rod angle, so a car that was fine at stock height can develop significant bump steer.
Is a little bump steer ever useful?
Some engineers run slight toe-out in bump at the front to sharpen turn-in as the outside wheel compresses, and slight toe-in in bump at the rear for stability. It must be small and deliberate.
Does bump steer affect the rear?
On independent rear suspension with toe links, yes, and it is often more important there because rear toe changes directly affect stability.
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