Research and/or Engineering Questions/Objective: The NVH performance of passenger car friction brake systems is an important product quality criterion. Brake creep groan, which is favoured by technological trends such as electrified powertrains and/or automated driving functionalities, has become a frequent inconvenience in that field. In order to develop and evaluate corrective measures against creep groan phenomena by simulative approaches, extensive and well-parametrised models are essential. Axle bushings, which are an important part of this challenge, are treated within this work. Methodology: On one hand, creep groan of a MacPherson corner setup with floating calliper brake system has been investigated at a drum driven suspension and brake test rig. Experiments were conducted for different combinations of brake pressure and drum velocity. In order to estimate the in-situ characteristics of built-in axle elastomer components, interface forces between test rig and corner setup as well as related accelerations were measured. On the other hand, transient simulations with a non-linear corner FE-model have been carried out. Two typical manifestations of creep groan at different operating points were computed. These manifestations’ sensitivity towards viscous damping of the lower control arm hydro bushing has been analysed by a simple simulative parameter study. Results: Most of the test matrix experiments led to creep groan phenomena. The measured signals’ characteristics show a strong relation to the applied brake pressure. The interface forces indicate this via specific dynamic amplitudes at variable static levels. Thus, all involved axle bushings operate in different performance ranges. The radial direction of the lower control arm hydro bushing is affected in particular. High relevance of its non-linear static elasticity concerning creep groan vibrations is already known from a former study. The current work also shows strong influences of the non-linear dynamic stiffness and viscous damping respectively. Limitations of this study: The simulative side offers potentials for a better physical conformance such as modelling and parameterisation of the suspension strut, determination and consideration of non-linear dynamic bushing parameters or implementation of a well-parametrised friction model. What does the paper offer that is new in the field in comparison to other works of the author: Several works regarding creep groan test rig experiments have been published. In terms of its transient simulation, one paper has dealt with the basic FE-model as well as the relevance of non-linear static stiffness curves. Based on that, the present study provides additional experimental/simulative results and treats the influence of dynamic axle bushing properties. Conclusion: In order to establish accurate virtual creep groan evaluation methods, holistic understanding of all relevant system influences is crucial. This study shows that axle bushings are of high relevance. Correlating results of the systematic test rig experiments and the transient non-linear FEA indicate strong effects of the non-linear behaviour of hydro bushings.
Severin Huemer-Kals Institute of Automotive Engineering, Graz University of Technology, Austria Peter Fischer, Manuel Pürscher