Bump Steer Explained

What is Bump Steer?

Bump steer is the tendency of wheels to steer themselves without driver input. This undesirable effect is caused by bumps in the road or track surface, as well as suspension travel due to braking and cornering, and their interaction with improper angles in the steering/suspension geometry.

From the factory, cars are designed so that bump steer effects are largely negated. However, when cars are lowered this geometry is compromised and must be corrected for. On a completely stock vehicle, bump steer compensation is not adjustable since proper geometry is engineered into the vehicle’s suspension.

For zero bump to be achieved, the tie rod’s motion must follow the arc of the suspension travel.

Simply put, bump steer is the amount of toe change throughout suspension travel.

Mustang Bump Steer Explained

Prep for Initial Bump Steer Measurement

Since the front suspension acts together as a system, you should have every parameter set before bump steer is adjusted.

  • Set Ride Height
  • Proper size wheel & tire
  • Camber is set
  • Caster is set
  • Toe is set
  • Tie rod length is set.
  • Steering is centered (wheels pointing forward)
  • Steering locked static
  • Ideally, sway bar & springs are disconnected. This is not necessary although it makes adjustment easier.

Jack the suspension on one side through ~3” of travel up and down, and record the changes in toe.

Make Bump Steer Corrections

Shimless Bumpsteer Kit
A shimless bump steer kit offers easier and more precise adjustment

Once you know your baseline level of bump steer, you are now ready to make changes to the level of bump steer compensation by changing the length of the outer tie rod.

On a classic “shim-style” bump steer kit, this adjustment is made by removing the rod end and changing the amount of shims between the rod end and the spindle.

On an FTR Shimless Bump Steer kit, this adjustment is made by simply loosening the jam nut and rotating the vertical adjuster to the desired location.

Symptom Correction (Shim-Style) Correction (Shimless)
Toe out on compression, in on rebound, in the same direction Remove shims between outer tie rod and spindle Turn adjuster clockwise
Toe in on compression, out on rebound, in the same direction Add shims between outer tie rod and spindle Turn adjuster counter-clockwise

How Much Bump Steer Do I Want?

In an ideal world, you want as little bump steer as is physically possible given your suspension geometry. This will keep your steering predictable over uneven surfaces commonly experienced in autocross events, and even some road courses.

A bit of bump out (toe out in compression) will make the car more stable upon corner entry, but bump in is almost always not wanted. Small amounts of bump steer will create an Ackerman type effect during a corner causing the inside tire to turn a bit further, increasing stability.

As a general rule of thumb, run a small amount of bump out (toe out on compression), and do not allow any bump in.

 

Further Reading:
Bumpsteer Explained – Mustang 360
The Ultimate Guide to Suspension and Handling – Bump Steer/Toe Steer – MotoIQ


Leave a Reply

Your email address will not be published. Required fields are marked *