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In order to determine the mechanism of brake squeal generation, six natural squeal odes were identified and the vibrations of the rotor, backplate and lining were measured for each mode by means of accelerometers. The results show that an important factor is the phase difference between the change in friction surface displacement and the change in lining thickness. In the two high frequency modes, it was confirmed that the exciting energy is generated by fluctuations in friction force due to fluctuations in surface pressure. And it was concluded that squeal generation is dependent on the ratio of change in frictional force rather than in the μ-v characteristics.

If a vehicle is left in a humid environment, the coefficient of friction between the brake pads and discs increases, generating a discomforting noise during braking called brake squeal. It is assumed that this increase in the coefficient of friction in a humid environment is the effect of moisture penetrating between the brake friction surfaces. Therefore, this paper analyzes the factors causing coefficient of friction variation with moisture between the friction surfaces by dynamic observation of these surfaces. The observation was achieved by changing the disc materials from cast iron to borosilicate glass. One side of the glass brake disc was pushed onto the brake pad and the sliding surface was observed from the opposite side by a charge coupled device (CCD) camera. First, a preliminary test was carried out in a dry state using two pad materials with different wear properties to select the appropriate pad for observing the friction surfaces.