


E30 SemiTrailing Arm Geometry 

Page 3 
We have examined the camber and toe curves for the E30 semitrailing arm rear suspension through the entire
090 degree range of motion. Now let's zoom in on the actual range of motion that is of practical interest.
We convert the trailing arm rotation (in degrees) to actual movement of the rear wheel (in inches).
Note that his is only valid for a small range near the horizontal position of the trailing arm
(say 34 inches either up or down from the horizontal position).
As before we begin by looking at the camber curve: 
Semitrailing arm CAMBER curve vs. rear wheel movement in inches. 
After all the work required to generate this curve it turns out to be basically a straight line. In other words, in the range of 0"  3" of rear wheel travel the rear camber is linearly proportional to wheel position. A study of the curve reveals that near the horizontal position of the rear trailing arm you will gain about 0.9 degrees of negative camber for every inch of suspension compression. If a rear wheel moves up 2 inches then you will gain 1.8 degrees of negative camber. And so on... The same occurs as the rear trailing arm moves downwards from horizontal, except that you loose negative camber (i.e. gain positive camber) as the rear wheels droop. 
... close camber curve window 
Now let us take a look at the rear semitrailing arm toe curve: 
Semitrailing arm TOE curve vs. rear wheel movement in inches. 
The toe curve is a little more interesting as it is not simply a straight line in the range of small rear wheel travel. The vertical axis shows total rear wheel toe in inches. This is what would be of interest if one were lowering an E30 and wanted to know how the alignment had changed. I have used inches instead of degrees since this is what most people are familiar with. Note that when you talk about toe in inches then it matters where you measure it. In this case the measurement is referenced to the rim lip on a 17" wheel. 
What we learn from the curve is that if you lower your E30 chassis by about 3.0 inches, you will gain about 1/16" of total toein on the rear wheels. This is not a "huge" amount, especially considering that few of us will lower our cars that much. But it is nice to know what's happening back there. If you are considering the toe change on just a single rear wheel then divide the total toe numbers on the vertical axis in half. That's all there is to it! 
... close toe curve window 



