Bump Steer

Bump steer is the front wheels steering themselves as the suspension moves through travel, with no input from the steering wheel. It comes from tie-rod geometry that does not match the control arms.

Also called:

bumpsteer, B/S

In practice

Longacre calls it “when your wheels steer themselves without input from the steering wheel,” caused by “improper length or angle of your suspension and steering linkages.” Each tie rod should point at that side’s instant center; a tie-rod line passing below the IC makes the wheel bump in as it travels up. Measure with a bump steer gauge and dial indicator, at ride height, reading each inch through two to three inches of travel. Targets differ even within one maker: Longacre’s back-to-basics article says “up to 0.030-inch of bump is acceptable” and that “most teams try to reduce bump to under 0.010-inch,” while their other guide says run .005 to .015 inch of bump out and never any bump in. Check your rulebook for what your class allows.

Why it matters

Bump steer is why a car darts over bumps and changes its mind mid-corner, and it will not show up on a car sitting still on the setup pad.

Go deeper

Read the full post:

How to Build a Race Car Setup Sheet That Actually Finds Speed

Sources

Bump Steer — Longacre Racing — gauge maker’s definition, the gauge-and-dial-indicator measuring procedure, and its bump-out target range.
Bumpsteer Tech — Back to Basics — Longacre Racing — same maker on the instant-center cause, decimal inches of bump per inch of travel, and the 0.030-inch and 0.010-inch figures.
Front Suspension Geometry Explained — Speedway Motors — parts retailer calling bump steer “a dangerous condition caused by the front suspension steering itself as it travels up and down.”