Bump Steer

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‘Bump Steer’ is where the car steers without the drivers involvement due to hitting a bump in the road surface.

The front wheels toe-in or toe-out, as the suspension compresses to absorb the bump. Bump steer can affect handling much as toe-in.

Bump Steer is undesirable and it is important to address it during the suspension design stage. As the front wheels move up and down, the front wheels should maintain the desired direction. This is more important when negotiating a turn.

image: Bump Steer

The steering rack’s ‘Tie Rod’ should be of the correct length and be mounted at the correct height. The length must be equal to the distance formed by (1) and (2) below. This can get a little complicated with some set-ups as the ball joints may not form a single line and the chassis mounts form a plane because of the front and rear mounts (like with anti-dive geometry). In cases like these, the maths can get very complicated and really needs to be modelled in 3D.

The ratio of a/b should be equal to c/d.

In the above diagram the following imaginary lines are drawn:

  1. Upper Ball Joint pivot fulcrum to Lower Ball Joint pivot fulcrum.
  2. Upper A Arm pivot fulcrum to Lower A Arm pivot fulcrum
  3. Upper Ball Joint pivot fulcrum through Upper A Arm pivot fulcrum to intersect point (IRC – Instant Roll Centre/IC  – Instant Centre)
  4. Lower Ball Joint pivot fulcrum through Lower A Arm pivot fulcrum to intersect point (IRC – Instant Roll Centre/IC  – Instant Centre)

Shown above is the simplest bump steer configuration. However, with certain setups bumpsteer might not be so simple. Changes to anti−dive or moment center, may be change your bump steer characteristics. This is particularly true for set-ups with large amounts of travel.

Most cars never truely achieve absolute ‘Zero Bump Steer’. A good set-up will have 0.1mm of bump per 1cm of travel (less than 0.4mm of toe movement).

Condition 2

The tie rod length requirement does not mean the tie rod needs to be positioned laterally exactly between (1) and (2), it can be offset as long as it is still pointed at the instant center.

image: Near Zero Bump Steer

When the tie rod is not aligned with the IC and/or the length is wrong, Bump Steer will result. Vertical movement of the wheel will turn the steering wheel left or right.

Bump In

In a system where the tie-rod line passes below the IC, the wheel will bump-in as the wheel travels up (toward the centre-line of the car) then bump-out when the wheel travels down.

image: Bump in

Bump Out

If the tie-rod line passes over the level of the Instant Centre, the steering wheel will bump-out as the wheel travels up then bump-in when the wheel travels down.

image: Bump Out

Things that affect Bump Steer

Bump steer is not normally a problem in a production car, yet in a ‘kit car’ made from several manufacturers components, it can be a big issue. For instance, the track width of a donor car is often different to required and choice of steering racks widths is a limiting factor.

Anti-Dive / Pro-Dive and Bump Steer

With anti-dive, the upper ball joint moves toward the rear of the car. This rotates the ‘Steering Axis’ counterclockwise from a right-side view. This rotation moves the outer tie-rod end upwards, changing the angle of the tie rod. In this position it no longer points toward the IC. Adding Anti-Dive or Pro-Dive to a car not originally design for it will introduce bump steer. With Anti Dive or Pro-Dive bump steer needs to be modelled and analysed in 3D.

image: Anti Dive Geometry

Bump Steer Corrections

Symptom Cure
Toes out in compression and in on rebound all in one direction. Fewer spacers on outer tie rod or lower the inner tie rod.
Toes in on compression and out in rebound all in one direction. More spacers at outer tie rod or raise the inner tie rod.
Always toes in both compression and rebound. Lengthen the tie rod as it is too short.
Always toes out on compression and rebound. Shorten tie rod as it is too long.
Toes out on compression, then in on rebound and then starts back towards out with more rebound travel. Less shim at outer tie rod and shorten tie rod.
Toes in on compression, then moves out on rebound and then starts back towards in with more rebound travel. More shim at outer tie rod and lengthen tie rod.
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