opensus
Bump steer: what causes it and how to measure it
Bump steer is the change in toe as the wheel moves up and down, with the steering wheel held still. It makes the car steer itself over bumps and as it rolls, so most designs aim for very little at the front, while a small, deliberate amount at the rear can be used to tune stability.
What causes it
The upright is located by the arms, and its steering angle by the tie rod. As the wheel travels, the tie rod's outer end must follow the path the upright gives that point. If the tie rod is the wrong length, at the wrong height, or at the wrong angle, its end swings on a different arc and pushes or pulls the steering arm: the wheel steers.
For zero bump steer on a double wishbone, the tie rod should behave like a third arm with the same instant center:
- In the front view, the tie rod line should pass through the front-view instant center.
- Its inner pivot should sit on the line through the upper and lower arms' inner pivot axes, so the tie rod has the right length.
Measured example
In the docs example double wishbone the tie rod runs parallel to the lower arm with its inner pivot on the lower arm's pivot axis, so both conditions hold: toe stays at 0.000° over ±25 mm of travel.
Moving only the tie rod's inner pivot shows how sensitive it is:
| Tie rod inner pivot | Toe at -10 mm | Toe at +10 mm | Bump steer |
|---|---|---|---|
| Correct height (Z = 150) | 0.000° | 0.000° | 0.000° / 10 mm |
| 5 mm high (Z = 155) | -0.108° | +0.106° | +0.107° / 10 mm (toe-in in bump) |
| 5 mm low (Z = 145) | +0.108° | -0.106° | -0.107° / 10 mm (toe-out in bump) |
Positive toe is toe-in. Here each millimetre of tie rod height error is worth about 0.021° per 10 mm, and the error is almost perfectly linear in both directions. Over 25 mm of bump a 5 mm error steers the wheel 0.26°: enough to feel through the wheel and to unsettle the car in a bumpy corner.
Reading it in opensus
- Kinematics → Heave → Toe (bump steer) plots toe against wheel travel; Bump steer rate plots its slope in degrees per 10 mm.
- The readout shows toe at the current position as you move the Heave slider under the 3D view.
- Kinematics → Roll shows roll steer: the same effect as the body rolls.
Related
- Reducing bump steer: a step-by-step fix
- Compliance: the toe change that comes from loads rather than travel