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Understanding, reducing, eliminating and preventing brake squeal is a challenging task. It involves many design variables in a complex brake system and there are many harsh operational and environmental conditions under which squeal may occur. Much progress has been made on understanding brake squeal mechanisms and causes. Squeal simulation and analysis methods have been significantly advanced. The evaluation and testing technologies have been noticeably improved, and reduction remedies and prevention measures have become more mature. Brakes have become much quieter. However, the recurring occurrence of disc brake squeal indicates that there are still many challenges ahead and brake squeal is still an elusive quality issue. This overview provides the summary of some current developments and discusses remaining challenges along with future technologies. In addition it emphasizes the real world counter measures for squeal reduction, elimination and prevention.

The modal characteristics of a brake pad are important factors affecting brake squeal. The most frequently used counter-measures for eliminating or reducing squeal, especially at high frequency, are the modification of: the modal frequencies, damping, contact modal shapes or patterns of a pad by making a chamfer or slot, or selecting a different under-layer, lining material or insulator. This paper describes the development of the methods for the measurement of pad modal characteristics such as modal damping, frequency and contact mode shape. It provides comparison among three methods: accelerometer-hammer, laser-hammer, and laser/non-contact shaker with test data and CAE simulation. Subsequently, laser/non-contact shaker was used to evaluate the process capability of pad manufacturing in terms of modal damping and natural frequency. This method was also employed to investigate the effect of pad chamfer, under-layer and the insulator on pad modal characteristics.

This paper provides measurement and analysis on rotor in-plane mode induced squeal. Methodology is presented to simultaneously acquire both temporal and spatial squeal operational deflection shapes (ODS). Rotor accelerations both in the in-plane and out-of-plane directions were measured during squeal along with rotor's normal ODS using a laser vibrometer. Modal measurement and analysis of the rotor and pad in the in-plane and out-of-plane directions were conducted as installed in system condition. The test results indicating rotor modal coupling in the in-plane are provided, and out-of-plane directions, and conclusions on in-plane mode induced squeal are proposed. In addition, the countermeasure for squeal reduction is discussed.

Disc brake squeal is a manifestation of the friction-induced self-excited instability of the brake system. One of known techniques in suppressing dynamic instabilities in nonlinear systems is by applying dither. The focus of this paper is to examine, through numerical examples, the feasibility and effects of dither on nonlinear systems as a means of quenching large-amplitude limit cycles. In particular, various ways of introducing the dither, either via modifications of the system characteristics or as external excitation, are explored. The investigation is extended to a disc brake system using finite elements simulations. Numerical results show that large-amplitude vibrations can be suppressed by dither and careful tuning of the amplitude and frequency of the dither can result in an effective quenching. The potential application of this technique to disc brake squeal control is also discussed.

This paper as part IV of a series articles first very briefly reviews squeal generation process in terms of energy transfer. A squeal reduction and prevention cascade chart including various contribute elements is formed. Subsequently, variation ranges of some key parameters of brake components and system due to manufacturing processes and operational/usage condition changes are given. Design concept of a broad stable and less vibration brake system is proposed and addressed in light of these variations. Robust design criteria and strategies are discussed. Design tools and methods are summarized. At last, some application examples are provided.

This article, as part III of a series, briefly reviews some of the representative literature on brake squeal testing and evaluation. It discusses the potential influence of variation within brake components and operational conditions on brake squeal dynamometer tests and their correlation to vehicle road tests. Roles and challenges of component/system parameter measurements such as brake pad damping, disc rotor in-plane mode and friction induced vibration characteristics, friction coefficient, moisture absorption and elastic constants of lining material, and contact stiffness are addressed. An application example of a reliability method to assure dynamometer test results are statistically significant is presented. The advantages of using laser metrology are also briefly described, especially the measurement of 3D squeal operational deflection shape. Lastly, general future research directions are outlined.

The understanding, prediction and prevention of brake squeal is a difficult and challenging problem because of the large number of design variables involved in a complex brake system and many operational and environmental conditions under which squeal may occur. The design variables may have different optimal values and different contribution trends for different brake systems. Since the 1930's, much progress has been made in gaining physical insight into brake squeal mechanisms and causes, and brakes have become quieter. However, the recurring occurrence of disc brake squeal indicates that our understanding of the phenomenon is both insufficient and incomplete, and that brake squeal is still a quality issue in the automotive industry and its prevention is far from reality. Part I of this series of articles first reviews the various hypotheses put forth for brake squeal mechanisms and causes.

More evidence has been found that rotor in-plane mode(s) and out-of-plane mode(s) alignment and coupling are the primary and inherent root cause for a disc brake to generate squeal at high frequencies. Eight different vehicles with different rotors have been tested considering known squeals. It has been found that the squeal frequencies are at the rotor in-plane mode(s) and out-of-plane mode(s) alignment and coupling frequencies. Rotor modal test results, vehicle squeal frequencies, and CAE operational simulation are presented.

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.

This paper provides a systematic approach to identify the root cause of the squeal noise of a disc brake by using advanced experimental tools. Modal analysis was used to identify the modal participation factors when the brake was squealing according to the reproduced squeal phenomenon and acquired operational displacement shape (ODS) using pulsed electronic speckle pattern interferometry. Modal coupling between the disc and pad/caliper assembly is found to be the key to produce squeal. It has been demonstrated that using mass loading/damping can de-couple the modes between the disc and pad/caliper assembly and reduce the assembly vibration from which the squeal is eliminated.