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Brake squeal is an NVH issue experienced by brake systems and vehicle manufacturers for decades. This leads to customer dissatisfaction and the questioning of the quality of the brake system. Advanced testing tools, design modification, dynamometer testing, vehicle validation etc., are performed to study, analyze and eliminate this problem. But still it continues to exist nowadays. One of the most important reasons is the complexity of the brake pad having non-linear material properties. Therefore, it is imperative to understand the behavior of the brake pad in terms of its dynamic properties (eigenfrequencies, damping and mode shapes) under varying boundary conditions. Experimental Modal Analysis (EMA) is used to study the dynamic properties of any structure and is generally performed under free-free boundary conditions. An approach to study brake pads under pressure condition is a step towards reality, as brake pads squeal only during braking events.

Predominant brake noise evaluation and rating was developed many years ago and no longer fulfills the need of modern development work. An extended description of a noisy brake event (European expert group guideline EKB 3006) and a standardized test data exchange format, allowing the comparison of different source test results (EKB 3008) are presented. Today's noise rating systems are described and compared by selected examples. The paper proposes an open 4 level noise rating system (EKB 3007). It starts with simple occurrence statistics, noise rating based on sound levels, situational noise rating including duration and finally based on the human perception, described by psychoacoustics.

The development of a new method to evaluate the NVH quality of diesel combustion noise bases upon following questions by regarding typical driving modes: Driving behavior with diesel vehicles Which driving situation causes an annoying diesel combustion noise Judgment of diesel combustion noise as good or bad A suitable test course was determined to regard typical driving situations as well as the European driving behavior. Vehicles of different segments were tested on that course. The recorded driving style and the simultaneously given comments on the diesel combustion noise results to a typical driving mode linked to acoustics sensation of diesel combustion noise. The next step was to simulate this driving mode on the chassis dynamometer for acoustical measurements. The recordings of several vehicles were evaluated in listening test to identify a metric. The base of metric was objective analyses evaluating diesel combustion noise in relevant driving situations.

Since the brake colloquium in 2004 the role of climatic conditions and their relations to noise occurrence, sound pressure level and friction coefficient level is widely discussed in the US and European working groups on brake noise. A systematic study has been started to investigate the influence of relative humidity, absolute humidity and temperature on brake noise and the corresponding friction coefficient level. In this study an enormous effort was taken to keep the influences of the brake parameters, e.g. lining material, Eigenfrequencies and dimensions of the different components as small as possible to investigate the climatic influence only. Strategic humidity and temperature levels were tested according to the Mollier-Entropy-Enthalpy-Diagram which are corresponding to the seasons in the various international regions. A regression analysis evaluates the correlation and the influence of each parameter to noise and friction coefficient level.

Today several international brake acceptance tests exist, like the Los Angeles City Traffic test (LACT) or the Mojacar Noise Route in Spain. During these tests noise evaluation is done subjectively by test drivers, which can cause discrepancies. Sometimes noise data are recorded and evaluated by different, mostly company-specific methods but a procedure that considers the human perception of brake squeal is missing. To fill this gap, the procedure BONI (Brake Objective Noise Index) detailed in this paper is developed based on subjective ratings acquired in hearing tests. It provides reliable prediction of squeal annoyance with high correlation to human perception.

Brake squeal is a self-excited vibration caused by the dynamic instability of the brake system. Extensive research effort has been undertaken on understanding and elimination of brake squeal. However, due to the complexity of the brake system and the fugitive nature of the phenomenon, there is no systematic prevention method. Brake squeal continues to be a major warranty cost to automotive OEM and suppliers. Complex eigenvalue analysis is a widely used analytical tool to tackle brake squeal. In this paper, based on this method, a new approach combining component contribution and eigenvalue sensitivity analysis is proposed for brake squeal. The modal contribution factor is used to quantify the influence of different real system modes on the unstable mode. The real system modes are projected in component mode sub-vectors to identify the most contributing components.

As an effort to solve high frequency brake squeals, increasing attentions have been placed on the relationship of the in-plane (IP) and the out-of-plane (OP) modes of a disc brake. However, there exists wide confusions between the frequency lineup and the mode coupling that largely hinders the efforts on correct solutions for brake squeals. This paper investigates the relationship between disc brake IP-OP modes interaction and brake squeal by both analytical and experimental approaches. It starts with a systematic discussion and categorization of the modal patterns of a typical disc brake rotor. Then, the relationship between IP-OP mode and high frequency squeal is evaluated by means of disc mode indicating function (DMIF) testing and dynamometer testing for three rotor designs. At last, comprehensive correlation study is performed for 19 different front and rear brake systems from the results of 60 dynamometer noise tests.

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.

In the present work, the finite element method is used to analyze the friction-induced vibration of brake systems, which may lead to squeal. A new approach is proposed to model the friction interaction at the rotor/pad interface, leading to the dynamic equations which well represent the dynamic characteristics of brake systems. The complex eigenvalue analysis method is then employed to detect the unstable modes of the system. The analytical method presented can be applied to both disk brake and drum brake systems. Some application results are presented and discussed.

A new NVH matrix for brake noise test rigs is presented which combines the specifications of the European drag mode test developed by the AK Noise (‘Arbeitskreis Geräusch’) with US-American in-stop tests. Additionally parts, of the AK Master lining selection dynamometer program are included in the proposed procedure to correlate friction and noise. The NVH matrix has been assigned a SAE concept number J 2521. To aid data evaluation and correlation of dynamometer and vehicle results, a post-processing system has been developed offering high flexibility and fast data visualization.