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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.

Disc brake squeal is a widespread cause for discomfort. The purpose of the work is to gain insight into the disc brake squeal problem and to investigate the mechanisms that generate instabilities that leads to audible noise. This paper presents a mathematical model for high frequency disc brake noise. Information of the flexible properties of the components is used to generate a small yet appropriate model. By using the technique of “Assumed Modes Method”, a model incorporating the flexible properties of a continuous disc can be accomplished resulting in only a limited number of degrees of freedom. Knowledge of the modal properties of disc and pads are used to select assumed modes for the model components. The basic concept of mode coupling is the starting point: Two orthogonal modes are present in a disc at the same frequency owing to it's rotational symmetry.

Finite element simulations are widely used for simulating disc brake squeal and the aim of this paper is to further increase the understanding of the effect of insulators. An earlier paper has presented an experimental technique for measuring the properties of the viscoelastic materials [1] and it has been shown how these data can be used in simulating brake response [2]. This paper deals with the sensitivity of a FE brake model to frequency dependent shim material properties and it is documented that with the current options for modeling shims in complex eigenvalue analysis it is only possible to accurately simulate response in a narrow frequency range. A procedure to find optimized parameters for a current damping model is discussed. The best α and β values for a Rayleigh damping model is found by obtaining a least square best fit in a frequency range of interest.

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.

One of the most important means used for suppressing squeal noise in disc brakes is the application of shims on the pad backplates. In many cases this proves a very efficient tool depending on the type of shim applied in the specific cases. Building up knowledge on the effects of shims have been ongoing for several years, and measuring the important parameters characterizing the shims is crucial for understanding how to develop and implement the shims in an optimal way. Several methods are described in literature for measuring the constrained layer damping effect and one method is described for direct measurement of the shear stiffness and shear damping properties. However, up to now no method has been available that can measure and characterize the normal stiffness and damping properties of shims. This is one of the most important properties of shims as it controls the de-coupling effect in the direction of the normal forces.

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.

A mathematical model for representing the vibration behavior of a disc brake is presented. Special emphasis has been put on the need to simulate the high frequency response by the use of continuous flexible elements, to represent the most important components of the brake. The simple mathematical modeling approach allows for conceptual understanding of the squeal generation mechanisms. In this study, the effect of the variation of system parameters over the length of the components, is investigated.

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.