Bump Steer - Part 1 - The Theory

Right Get a cup of tea first.... This might take some time!

Bump steer is a phenomenon that occurs when the inner track rod end ball joint does not lie on the plane formed by the upper and lower wishbone inner fulcrums. This means that as the suspension moves the track rod will effectively lengthen/shorten relative to the wishbone pivots thus causing the associated wheel to toe in/out. In practice this means that driving on bumpy roads can cause an undesired steering effect and will make the car feel "twitchy".

I got my replacement track rod ends and set up the front as I had done before - using the method of visual alignment of two straight edges to check the bump steer. This has been the only part of suspension geometry that I've not been able to physically measure.

Now I'm an engineer - I like to measure things - it's in my nature!

So how do you measure bump steer without buying an extremely expensive bump steer gauge?

The answer to this question is not as simple as it would first seem. In order to measure something you need a datum to measure the thing relative to. So that's easy then, just find a datum. Hmmmm.... easier said than done!

There are a number of problems with this.

1. As the suspension moves from full droop to full bump the ends of the wishbones (and therefore the hub) move in a vertical arc. This means that the hub will move initially away from the chassis and then (after the wishbones have passed the horizontal) towards the chassis. You cannot therefore use a point on the chassis as a reference as the datum would not be constant (remember we are trying to measure small changes).


2. Due to the unequal length of the wishbones the camber measurement will change as the two wishbones describe different vertical arcs. If you used a point on the wishbone as a datum, any measurement to the hub would be affected by this.


3. The measurements need to be taken with the suspension at different heights - therefore the system of measurement needs to be independent of change in suspension height.

Now this one has had me puzzled for a few days..... but maybe there is an answer.

We need a datum independent of the suspension, but also one that is not affected by the position of the suspension.

So then, let us assume we fix a mirror on the brake disc and shine a laser at it. This allows us to use the reflection of the beam to double the angle measured and therefore increase the accuracy of the measurement - it would look something like this viewed in plan:

The above diagram is exaggerated - the angle of the laser needs to be as square to the mirror as possible (just enough to let the reflected beam back past it) otherwise as the hub moves in/out relative to the laser the beam will be deflected and give a false reading. In elevation the only key thing would be that the laser needs to be horizontal:

If there was no camber change and zero bump steer then the angle of the mirror would not change (although its vertical position would as the height of the suspension changes) and the laser dot on the graph paper would therefore not move.

If the camber changes then the laser dot would be deflected upwards or downwards on the graph paper.

If bump steer is present then the toe in/out angle would change and the laser dot would be deflected left or right on the paper.

Taking measurements at different suspension heights would give a number of points on the graph paper which can be joined to form a curve. Any horizontal deflection of this curve would represent a measure of bump steer and any vertical deflection would represent a measure of camber change.

If the distance of the graph paper from the mirror is known, then the measurements on the graph paper can be converted to actual angular measurements.

The further the graph paper is from the mirror then the greater the deflection of the laser beam and therefore the greater the accuracy.

Thinking about it there are two other criteria: the graph paper must be vertical and must be aligned perpendicular to the reflected beam (in order to be able to calculate the angular deviation).

Seems like a cunning plan - so back up on the axle stands - off with the wheels and remove the springs.

Watch out for Bump Steer - Part 2 - The practice!