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Disc brake squeal can be a major problem during development of new brake systems. Squeal can be loud and persistent or fugitive but nevertheless annoying. Increasing the knowledge of the mechanisms generating squeal is one important contribution to the extensive research and development work being performed in order to solve the problems. The vibration motions of the brake components during squeal, especially the disc, have been studied intensively, and the existence of standing or traveling waves and the direction of such waves have been debated. Several measurement techniques have been employed in order to reveal the nature of the disc motion, including holography, scanning laser vibrometry and rowing accelerometers in the disc. Also acoustic holography has been employed previously - see for example [2] - but this paper documents the ability of acoustic holography to create new knowledge about disc brake squeal through measurement of the disc motion.

High frequency brake squeal is often suppressed by applying an insulator to the shoe plate of the pad. This may increase the damping and change the coupling conditions in a favorable way, but detailed knowledge about which of the several effects of insulators that are most important is not at hand. A joint effort is needed to increase the understanding of the effects of insulators. This paper describes a new way of measuring the shear stiffness and damping of insulators. The method can be used to measure either the individual layers in an insulator or the complete insulator that is build up of several layers. The method does not rely on the resonant behavior of a structure and it therefore allows for measurements of the parameters over a wide frequency range. The measurement setup can be placed in a temperature chamber and this allows the parameters to be measured over a wide temperature range.

Disc brake squeal is a very complicated phenomenon, and the influence of insulators in suppressing squeal is not fully understood. The aim of this paper is increase the understanding of the effect of insulators. A previous paper [1] presented an experimental technique for measuring the frequency- and temperature- dependent properties of viscoelastic materials currently used in insulators. The present work continues by considering the coupled vibrations of the brake pad and insulator. A comparison of natural frequencies found from experimental modal analysis and finite element modeling indicates agreement to with 5%. Experimentally determined modal loss factors of the brake pad vary dramatically with frequency, changing by a factor of 2 over the frequency range 2-11 kHz. A method for including this frequency dependence, as well as the frequency dependence of the insulator material, in state-of-the-art finite element software is proposed.

Introducing a rubbercoated steel plate - a shim - on the backing plate of a brake pad often solves squeal problems in disc brakes. This may be attributed to one or more of several effects exerted by the shim: Isolation or decoupling of brake components Alteration of the force configuration The addition of damping The effect of damping may be active in either or both of the following ways: In the direction of the normal force exerted by the piston As a constrained layer damping of the bending vibration of the pad This paper will deal with the constrained layer damping effect introduced by applying a viscoelastic material and an additional steel plate on top of the backing plate. This paper reviews some theories on constrained layer damping and extracts information on the effect of varying important parameters such as layer thickness, stiffness of the constrained material etc.