Friction behaviour and squeal generation of disc brakes at low speeds

Brake squeal is a subtle phenomenon, apparently occurring according to some random behaviour. Two stops during nominally equal conditions may act completely differently concerning brake squeal. Further, without changing the nominal conditions, a brake may shift from silent to squealing. This confused behaviour, in the otherwise stable and well-controlled brake system, can only be explained by the characteristics of the friction between the pad and the disc. A test series was tailored in order to increase the understanding of the friction characteristics with respect to two suggested squeal mechanisms. It included braking with continuously increasing and decreasing brake pressure at constant speed and vice versa. This concept minimizes the influence of temperature and run-in on the coefficient of friction. It was found that no squeals were generated below a coefficient of friction of 0.4, that braking conditions with a high coefficient of friction were related to more frequent squeal generation and that pads with a negative friction-velocity behaviour did not invariably show a higher squeal propensity. Furthermore, it is concluded that squeal-related friction characteristics can only be measured in a timescale similar to that of a squeal, i.e. on the 0.1 ms scale, owing to the different speeds of processes influencing the friction force. A higher lateral resolution over the pad of the friction measurement than currently used in brake testing is also required.