Bump steer causes a vehicle to turn itself when one wheel is raised or lowered by a bump or hole in the road. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. For example, if a vehicle were not to have optimised bump steer and if the front left wheel were to roll over a bump it would compress the suspension on that corner and automatically rotate to the left (toe out), causing the car to turn itself left momentarily without any input from the driver.

It is typically measured in degrees of steer per metre of upwards motion or degrees per foot.

Zero Bump Steer

In order to accomplish near zero bump the steering arm must intersect the line of the kingpin (an imaginary line that runs from the upper ball joint through the lower ball joint), the line of chassis (an imaginary line that runs through the upper control arm pivot and the lower control arm pivot) and the center line of the steering arm must intersect with the instant center.

The racing aspirations calculators suggest the best line for the steering arms based on the above equation.

Calculating Bump Steer

The racing aspirations calculators give you a bump steer reading that equals the amount of lateral movement measured in mm, cm or inches that would be applied to the upright at the point the steering arm attaches to the upright. There will invariably be an offset between this point and the line of kingpin.

The racing aspirations bump steer reading (s) divided by the offset (r) will give you the amount of steer angle applied to the wheel.

Did you know

You can download the bump steer readings for further analysis in a spreadsheet.

1. Open the Macpherson Geometry Calculator or the Suspension Geometry Calculator
2. Click  so it becomes  if it isn’t already.
3. Click  to open settings.
4. Scroll down and select Bump Steer Left and/or Bump Steer Right
5. Click apply.
6. Click  to download a csv which can be opened with Excel or another spreadsheet program.

For more info see report data download

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