Checking and Measuring Suspension Geometry

During setting up my various vehicles, it has often been desirable to check or adjust some of the axle alignments. Many owners just head for the local tyre shop to have tracking set, or get a fuller check of alignment. However I usually find I don't want to take the time out to stop work and go traipsing around looking for a tyre place where someone knows what they are talking about and they are prepared to work with a non-standard vehicle. Don't get me wrong, these places do exist, but my local qwik-fit isn't one of them!

I don't propose to define all the relevant terms, there are plenty of websites out there with all the info you need. Try Whiteline Automotive for a start. You could also read this post on which explains the terms quite well, but proves that mad scientist don't know much about cars in the real world ;-)

Tracking, or Toe, is the figure which most usually needs checking, and fortunately it's very simple to measure it on a Land Rover, at least to a reasonable degree of accuracy. It is simply a measure of how far the front wheels are away from 'straight ahead' when the vehicle is driving straight forward. For a coil sprung Land Rover the wheels are pointing outwards from the straight ahead position (toe out), I believe that leaf sprung Landies have toe-in.

To measure toe, I use a simple telescopic pole made from two rigid plastic tubes, one of which slides inside the other, secured with a jubilee clip. With the wheels in the straight ahead position, I support the rod on two axle stands (to keep it level) and set the rod to just fit between the flats on the inside of the wheel rim at the trailing edge, level with the wheel centre. With the rod moved to the front edge of the wheel, the gap between the rod and wheel is the toe out. (Remember that if you make any adjustment, only adjust out half the error - think about it!).

Since the measurement is small, it is advisable to rotate the wheels a half turn, and make both measurements at the same point on each rim, so that any run-out in the rim is not included in the measurement.

Camber is quoted as 0 degrees for a coil sprung Land Rover and is not adjustable. There is still good reason to measure it though, as an indication of more serious mis-alignment, and as part of Castor measurement.

To measure camber, I have a modifed spirit level with an attached inclinometer. The spirit level is modified by the attachment of a few bolts which fit against the wheel rim to allow consistent measurement. When the spirit level indicates vertical (in the plane of the wheel) the inclinometer indicates the wheel camber.

Castor is quoted as +3 degrees for a standard coilsprung Defender. While castor is generally not considered a very important alignment figure, it does have some significance for a non standard vehicle. Changes in suspension height and vehcile attitude will change the castor angle, easily reducing it to zero or worse. This will result in a less than pleasant driving experience!

A rough calculation shows that castor angle is reduced by around 1.5 degrees for every 1" increase in the front spring length, and reduced by 0.6 degrees for every 1" increase in rear spring height. Thus if no action is taken, even a simple 2" lift kit will reduce castor angle from 3deg to around 0.5 deg, which will give a noticeable reduction in high speed stability. These are approximate figures

Castor angle is not easily measured directly, but it can be deduced by some simple calculations. Since the castor angle results from the steering swivels being inclined from the vertical, the measured camber angle changes as the wheel is turned. Thus measuring the change of camber angle over a measured range of steering angles allows calculation of the camber angle.

There are a few websites which describe the procedure in detail, but basically for each front wheel I measure the camber angle at centre and two extremes of steering. I also mark a line on the workshop floor parallel with the wheel at each position. This allows an accurate measure of the steering angle.

The relevant formula is then :-

Caster = (180 / pi) * (camber1 - camber2) / (steer1 - steer2)

If you can steer 30 degrees either side of straight ahead, then the Castor angle is simply the difference between the camber angle at these points.

The change in camber angle should be measured independantly for each wheel, to expose misalignment.

I used this method of caster angle calculation when I installed a set of castor correction radius arms, to correct a low castor angle in my Ibex due to lifted suspension. Before fitting the new arms, I measured 1.0 / 1.0 caster (L/R). With the new arms fitted I measured 3.0 / 3.5 degrees. (My measurement system has a resolution of 0.5 degrees).

If you use these procedures to align your vehicle, remember this info is worth precisely what you paid for it - nothing! If you disagree with anything I've written, or can spot any glaring mistakes or omissions, please let me know.

(Article date approximate due to website updates)