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Recent investigations by others have indicated that the dynamic response of automotive brake rotors in the squeal frequency range involves the classic flexural modes as well as in-plane motion. While the latter set creates primarily in-plane displacements, there is coupling to transverse displacements that might produce vibrational instabilities. This question is investigated here by analyzing a modal model that includes two modes of the rotor and two modes of the pad and caliper assembly. Coupling between in-plane and transverse displacements is explicitly controlled. Results from this model indicate that the coupling does create vibrational instabilities. The instabilities, whose frequencies are in the squeal range, are characterized by power flow through the transverse motion of the rotor.

This paper first gives a brief review on brake squeal mechanisms and then studies in-plane modes/friction process and their contribution to disc brake squeal. Pulsed laser electronic speckle pattern interferometry was used to acquire the operational deflection shape (ODS) of a disc brake when it was squealing. Laser vibrometry was used to obtain mode shapes of brake discs/rotors including both the out-of-plane (transverse) modes and in-plane (radial or tangential) modes. The rubbing friction process with a non-rotation rotor under a free-free boundary condition was used to simulate friction-induced vibration. The coupling between in-plane modes and out-of-plane modes/vibration is believed to be the key to produce squeal. The in-plane modes tend to control the squeal frequency, and the out-of-plane modes/vibration are efficient to generate noise. Many case studies have shown that high frequency disc brake squeal occurs at one or some of its rotor in-plane resonant frequencies.