There are number of possible causes for brake vibration or judder.

Incorrect brake bedding

As discussed in brake pad bedding, pads and rotors need to be ‘bedded’ in order to achieve optimal performance. If bedding is carried out incorrectly and the brake discs have uneven pad deposition around them, this can cause irregular friction levels or high-points on the discs which results in uneven friction levels as the disc rotates. The other cause of these high-spots can be pitstops during a race when the driver keeps brake pressure after the car is stopped with very hot brakes. The vibration caused will transfer through the brake pedal, but mainly via the steering wheel. If really bad this problem can feel very violent and is normally only fixed by cleaning the disc surface and re-bedding the brakes.

Pad ‘contamination’

If your discs have already been used on a different brake pad material and you put new pads over the top, you run the risk of the two materials being incompatible and causing a build up on the disc. This will have an identical effect as incorrect pad bedding. Always ‘clean’ the used disc with some emery paper to loosen the existing material on the dic as this will reduce the possibility of contamination and resultant vibration.

DTV (Disc Thickness Variation)

If the brake disc has been manufactured outside tolerances and is effectively thicker in some sections of the disc than others, this will also cause the vibration issue. This vibration is more commonly felt through the brake pedal, and partly via the steering wheel. This is only fixed by machining the disc flat or replacing it.