Brake judder is not necessarily the most technically challenging issue, but it can be a source of headaches for technicians, especially since fitment issues are often at the root cause of it.
By following a few simple guidelines, brake judder can be virtually eliminated:
1 Before refitting a disc, first check the disc’s thickness. Using a brake micrometre measure the disc’s thickness, 25mm from the outer edge, at eight equidistant points around its perimeter – never base the measurements on any one single spot. Compare the results with the specification. If it’s outside the manufacturer’s tolerances then it cannot be serviced and must be replaced. In general, the maximum allowable thickness variation is around 0.013mm.
2 Ensure that both the hub and disc mounting faces are scrupulously clean and free from rust scale or any other form of dirt or grease. Use a cloth and appropriate solvent to clean the disc, and a soft wire brush or emery paper
for the hub.
3Once fitted, use a dial indicator to check the disc’s lateral runout. The dial indicator must be securely mounted to a stationary, yet adjustable fixture, for example, a control arm. Position the dial indicator’s plunge tip about 25mm inside the disc edge, set to zero and rotate manually through 360°, recording changes in runout. Whilst tolerances will vary from make to model, 0.05 to 0.10mm is a good target for maximum runout limit. If runout is out of tolerance recheck the disc fitment onto the hub. If this is correct remove the disc and carry out a run out check with the DTI on the wheel bearing/hub unit.
4 Ensure that the caliper carrier is clean and that the slider pins are well lubricated, free of rust and moving freely.
5Torque all wheel fasteners in the correct pattern and at the correct torque settings.
6Always bed in new brakes according to the manufacturer’s instructions
7Prevent overheating the discs by inappropriate use of the brakes. Avoid both continuous, consecutive heavy braking from high speed and holding the car back with the brakes on long steep descents – use a lower gear to make use of engine braking instead.