There are many fixes to eliminate brake noise and many models that explain the system dynamics when the brake becomes unstable. What appears to be missing from most proposals is why brakes become dynamically unstable in the first instance. The principle reason is that the process from a quiet brake to full noise may take only 250ms. Even if researchers are able to measure that build-up event it does not guarantee that the initiation did not take place well before instability could be detected. In general NVH investigations may only measure frequencies within the audible range a maximum of 20kHz. It is suggested that brake instabilities may always exist within a brake system but initially exist at very high frequencies, possibly up to 100kHz, and even higher. These frequencies may emanate from the microscopic particle interactions providing a white noise to excite the total system. These high frequency instabilities only emanate as a noise frequency because of some trigger mechanism within the brake. If the engineer can understand that important initiation or trigger process then the noise problems may be resolved from the outset and within the design process. The purpose of this paper is to present a philosophical proposal that explains why instability can arise when light braking takes place (often a necessity for noise) and why noise disappears under heavier braking. The proposal considers disc in-plane vibration, abutment finger deflection, pad shear and the influence of the Mu/velocity curve of the friction material. As such it requires research engineers to consider a different approach to understanding instability dynamics and seek to measure relevant brake characteristics during a braking event. The paper goes further to explain why these in-plane deflections lead to disc out-ofplane movements. The proposal may be aligned to provide an explanation of why some design changes resolve noise problems typically added mass to calliper fingers and the application of grease to the pad back-plate.

Fieldhouse John. Advisor to Industry, UK.