After decades of investigating brake noise and despite the application of advanced tools and methods of investigation, brake squeal remains a general problem of the automotive industry. Finite Element Analysis (FEA) has been employed as the main tool in numerous studies recently, mainly using the Complex Eigenvalue Analysis (CEA) to predict the occurrence of brake instability and, hence, brake squeal. However it has been shown that not all instabilities predicted by CEA do occur in reality. The effective (negative) damping ratio (expressed in percentage terms) is used as a measure of the strength of instabilities. The level of effective damping ratio of the predicted instabilities below which the instability is not prone to cause noise is set at different levels by different researchers with 1% being an average value in the industry. However, in this study, the acceptable effective damping ratio percentage is set to be 0.5% and any instability predicted with an effective damping ratio of more than that is assumed to be prone to causing brake squeal when tested on the car. This study is based on the hypothesis that variation of frictional coefficient over the radius of the brake pad is effective in reducing the susceptibility of the brake instability to cause brake squeal. Various patterns of distribution of Coefficient Of Friction (COF) over the disc pad contact interface have been investigated, and results are illustrated in terms of four different scenarios. The successful scenario recommends that increasing the COF radially over the disc radius will lead to instabilities with an effective damping ratio in the acceptable range. Exaggerated variation in the COF in the disc-pad contact interface is set to cover the entire range of COF used in different analyses performed to examine noise level of a brake system, varying from μ=0.3 to μ=0.7. The proposed friction distribution pattern reduces the strength of all predicted instabilities over the frequency range to less than the target value of 0.5%. This suggests that instabilities predicted for the brake system using the proposed pad material design will not cause brake noise.
Esgandari, Mohammad; Olatunbosun, Oluremi; - University of Birmingham