A study of brake noise and the influence of the centre of pressure at the disc/pad interface, the coefficient of friction and calliper mounting geometry

Abstract

It is generally accepted that the coefficient of friction between the friction pair, the pad and the rotor plays a significant role in the propensity of a brake to generate noise. Because of the ease with which changes may be made to the pad compound material it is this that is more often the subject of change when there is a problematic noisy brake with both the rotor and the calliper tending to remain ‘isolated’ from examination. This paper investigates the propensity of a brake to generate noise over a range of temperatures and pressures under conditions that allow a mechanically induced offset centre of pressure between the pad and rotor to be varied. It is shown that for a high-volume production car brake there is an increased tendency for it to generate noise when a very specific leading offset centre of pressure is engineered. With this condition it is shown that a situation is promoted whereby the brake becomes mechanically unstable, with system changes such as brake pressure and temperature variations having little influence on the brake to alter its tendency to generate noise. Furthermore, it is shown that the critical offset centre of pressure may be related to the coefficient of friction between the friction pair and the mounting geometry of the calliper. Confirmation of the findings are supported by a repeat investigation of the vehicle's alternative brake calliper. It is suggested that the source of a noisy brake may lie as much in basic mechanical design as inappropriate material choice. Further confirmation is demonstrated through the study of a commercial sliding fist-type brake, where the brake is significantly noisier when the centre of pressure and calliper mounting arrangement provide an appropriate spragging angle related to the friction coefficient.