Squeal Noise in Disc Brakes and the Influence of Damping on the System Dynamic Stability

Abstract

Squeal noise in vehicle disc brakes is perceived by consumers as both annoying and as an indication of a problem with the braking system, dramatically affecting quality, satisfaction ratings and warranty costs. Among the methods that have been used to control squeal noise, increasing the system damping has been shown to be very effective. The most commonly used method to increase system damping consists of attaching multi-layer laminates on the back of the brake pads. Such laminates, also known as brake shims, are composed of different layers of metal and viscoelastic materials. In this paper, the dynamics of a disc brake system are discussed. At first, the squeal noise mechanism is broken down and a simple analytical model is used to exemplify the modal coupling phenomenon and how damping affects it. A technique to measure brake system damping as a function of brake lining pressure and temperature is explained and the results with and without brake shims are discussed. One of the conditions believed necessary for the occurrence of squeal noise in some disc brake systems is related to the relative position of the rotor and pad critical mode shapes. This condition, addressed in some previous papers [1, 2], is also studied here by using the Sound Intensity technique. Measurement of the properties of viscoelastic materials as a function of temperature and frequency is discussed. Results are shown for different materials (rubber, acrylic and silicone). Modal damping is also measured for different brake shim configurations as a function of braking temperature and lining pressure using the above mentioned technique. It was found that the modal coupling plays an important role in noise generation for certain brake system designs. For the brake system analyzed, the anti-nodes of the rotor mode related to the squeal frequency are fixed in space, i.e., they no longer rotate, remaining in the same position along the rotor diameter. Also, measuring system damping can be effectively used in the design and selection of brake shim configurations. However, in practice, the technique presented in this paper showed the best results for specific squeal noise problems where rotor and pad bending modes are the main root cause of the noise.