Comparison of disc and drum brake rotor mode movement

Disc brake noise is generally accompanied by a movement of the rotor mode of vibration. It is shown that the speed of mode movement is related to the frequency of noise generation divided by the rotor vibration mode order, the direction being the same as disc rotation. The same type of movement is also seen in a drum brake generating low frequency noise. The backplate is shown to have a moving mode with a speed that is twice the noise frequency divided by the mode order. A possible drum mode is indicated and it is suggsted that this rotates at the frequency of noise and in the direction of drum rotation. It is proposed that the mechanism involved with disc mode movement and drum/backplate mode movement may have a common explanation. It is explained that this mode movement is the result of two contra-rotating waveforms of differing amplitudes - this being an explanation for both brake types as a feature of brake noise.