Bump/roll steer check on the rears for the first time, using a laser level on the brake disc and a mirror. I don't know why I haven't done this before. It turned out they (the rears) had quite a lot of toe in bump steer dialled in.
Apparently it is common to set up road cars for understeer and stability by inducing a bit of bump steer - toe in on the rears and toe out on the fronts. On race cars close to zero bump steer is preferred if possible, for maximum absolute grip and less scrub.
I reduced the bump steer by simply switching place of the bump steer spacers and it was significantly reduced. Not quite zero but I leave it there. Now the larger 9mm spacer is up and the smaller 5mm is down - opposite of the top image. (the top image is from the CSR Assembly Guide Supplement document)
I did the fronts for quite some time ago but without the mirror. I must say the mirror method is both simpler and more accurate. The basic idea is to put the laser level on the brake disc and point it forwards or rearwards to a mirror that reflect the beam back to the same point. If the point moves sideways relative to the origin when you raise the wheel - you don't have zero bump steer. Adjust the spacers and repeat.
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