A Study on Trigger of Small Friction Noise in Disc Brake Squeal

Disc brake squeal is caused by flutter of dynamic unstable systems under small disturbance. Therefore the research of small disturbance is very important technical issues for disc brake squeal reduction. The experimental set-up for basic research was developed based on the actual disc brake squeal vibration mode in experiment and its theoretical analysis, which replaces actual caliper and pads to cantilever type pad-caliper. Disc brake squeal depends on the natural frequency of cantilever type pad-caliper, which occurred at 2.7 kHz, 2.0 kHz and 5.3 kHz for each cantilever length L=40 mm, 50 mm and 80 mm in each friction test. In friction tests L=40 mm and 50 mm, cantilever type pad-caliper vibrates with the 1^st order bending vibration mode and disc vibrates with the 1^st or 2^nd order bending vibration mode. In friction test L=80 mm, cantilever type pad-caliper vibrates with the 2^nd order bending vibration mode and disc vibrates with the 4^th order bending vibration mode. Theoretical analysis is shown here for considering disc brake squeal and its trigger. The vibration modes in each disc brake squeal are not same, but each friction point behaviour per 1 cycle during brake squeal generation is the same oval shape. These simulation results mean the same mechanisms of dynamic energy increase. On the other hand, small friction noise occurred around 1 kHz in this friction test of cantilever length L=80 mm. Then its cantilever type pad-caliper vibrates with the 1^st order bending vibration mode but disc vibration amplitude is so small. This small friction noise is considered to be self-excited vibration in friction coefficient dμ/dv<0, which depends on not only friction characteristics but also the natural frequencies of cantilever type pad-caliper and disc. As disc brake squeal is considered to be triggered by small friction noise, disc brake squeal can be reduced by reduction of small friction noise.