Brake disc runout and thickness variation
Definition
Brake judder felt in the pedal and steering wheel is rarely a truly 'warped' disc; it is usually disc thickness variation (DTV), where the disc has worn unevenly so the pads grip harder on the thick spots. DTV develops from lateral runout - the disc wobbling as it turns because of a dirty or rusty hub face, uneven wheel bolt torque, a bent hub or a disc that was not seated flat - combined with pad contact at every rotation; it is also caused by pad material transferred unevenly to the disc after hard braking and holding the pedal while stationary. Measure lateral runout with a dial indicator on the installed disc (typically below 0.05-0.07 mm allowed) and thickness variation with a micrometer at eight points around the disc (usually under 0.01-0.015 mm); check the hub itself if runout is high. Fix by cleaning hub faces, torquing wheels in sequence, replacing or machining the disc and, if the hub is bent, replacing the hub or bearing. There are no fault codes for runout; related complaints are steering wheel shake at 80-120 km/h braking, pulsing pedal and uneven pad wear, which also point to a sticking caliper slider.