Lee, S., Jeon, J., Jeong, J., Park, B. et al., "Disc Brake Squeal vs. Disc Pad Compressibility-Caliper Stiffness Interactions: Low-Frequency Squeal and High-Frequency Squeal vs. Differential Pad Wear," SAE Technical Paper 2017-01-2528, 2017.
It is widely believed or speculated that higher pad compressibility leads to reduced brake squeal and that caliper design can affect brake squeal. After encountering anecdotal contradictory cases, this investigation was undertaken to systematically generate basic data and clarify the beliefs or speculations. In order to adjust pad compressibility, it is common to modify pad molding temperatures, pressures and times, which in addition to changing the compressibility, changes friction coefficient and physical properties of the pad at the same time. In order to separate these two effects, NAO disc pads were prepared under the same molding conditions while using different thicknesses of the underlayer to achieve different compressibilities, thus changing the compressibility only without changing the friction coefficient and physical properties of the pad. Test results show brake squeal occurrences increasing with increasing compressibility and increasing damping, contrary to the common belief. A combination of higher compressibility pads and higher caliper stiffness leads to increased brake squeal occurrences. The inner pad and outer pad wear rate difference affects disc squeal or caliper-knuckle assembly squeal generation. As the differential becomes smaller and smaller, more disc squeals occur and as the differential pad wear becomes larger, more caliper-knuckle squeals occur. There are indications that when the inner pad high pressure point is well matched against the outer pad high pressure point, in-plane disc squeals occur and when not matched, out-of-plane disc squeals occur.