Bump steer: what it is and how to fix it
Bump steer is the wheel steering itself as the suspension moves up and down. It happens when the tie rod swings through a different arc from the wishbones, so the steering arm is pulled or pushed as the wheel travels.
What it feels like
- The car darts or tramlines over bumps and ruts.
- It changes direction under braking or acceleration as the nose dips and lifts.
- One-wheel bumps mid-corner make it twitch or push wide.
Where it comes from
For the wheel not to steer, the tie rod must move in step with the wishbones. In the front view, that means two things:
- The tie rod points at the instant centre. A line through the tie rod's inner and outer joints should pass through the same instant centre as the upper and lower wishbones.
- The tie rod has the right length. Its inner pivot should sit where the wishbone geometry says it should, so its arc matches the arc of the steering arm.
Get the height wrong and the toe changes steadily in one direction through travel. Get the length wrong and the toe curves, going one way in bump and the other in droop.
How to measure it
Remove the spring, set the car at ride height and lock the steering straight ahead. Fix a flat plate to the hub and measure its distance from a fixed reference at the front and rear edge with dial gauges. Move the wheel up and down in steps of about 10 to 25 mm and record the toe change at each step. Plot toe against wheel travel: a flat line is the goal.
The usual fixes
| Symptom | Likely cause | Common fix |
|---|---|---|
| Toe changes steadily one way through travel | Tie rod at the wrong height | Move the outer joint up or down with spacers or a different steering arm, or raise or lower the rack |
| Toe goes one way in bump and the other in droop | Tie rod the wrong length | Change the rack width or the inner tie-rod pickup |
| Both of the above | Height and length both off | Fix the height first, then the length |
A small, controlled amount of bump steer is sometimes designed in on purpose. Decide it deliberately; don't inherit it from where the rack happened to fit.
See your bump steer before you build
Enter your wishbone and tie-rod pickup points in the free FrameScore suspension calculator and see the toe change through travel, with the roll centre and camber gain alongside.
Open the suspension calculatorRelated: Roll centre explained