Research and/or Engineering Questions/Objective:
NVH associated with the friction brake system has big impacts on the assessment of a cars comfort and the perception of its brands quality. Therefore, related phenomena are unwelcome for drivers and OEMs. In this context, creep groan has become more relevant over the last decade in particular. Its presence is favoured by current trends in mobility such as shift to cars with automated and/or electrified drivetrains, increasing urban stop-and-go traffic or rigorous comfort requirements. In order to evaluate remedial technical measures against creep groan earlier in vehicle projects, its fundamentals have to be understood and appropriate simulation methods based on well-parameterised models are necessary.
Systematic experiments have been conducted for an automobile MacPherson corner setup with floating calliper brake at a drum driven suspension and brake test rig. An objective evaluation method is utilised to evince and analyse creep groan events regarding a test matrix of brake pressures and drum velocities. Moreover, interface forces between test rig and corner setup have been measured and evaluated together with characteristic static stiffness curves of the axle elastomer components. For the purpose of transient simulation with virtual evaluation of creep groan, a simplified FE-model of the tested system has been elaborated. The computations are performed using an explicit time-integration scheme. Two significantly different operating points have been simulated. In addition, a parameter study in terms of the non-linear static stiffness curve of the lower control arm hydro bushing is involved.
Parts of the test matrix experiments reveal operational parameter combinations with a clear tendency for creep groan phenomena. If such occur, signatures hinge strongly, but not exclusively, on the brake pressure. The dependence is explainable by different interface force levels which emphasise other operating ranges in the hydro bushing static stiffness curve. This variability results in specifically tuned and/or pronounced natural frequencies and related stick-slip cycles. Hence, this component parameter should be considered in a non-linear manner within transient simulations. Consequently, computations of dynamic system responses or stick-slip oscillations correlate better with comparable experiments.
Limitations of this study:
The introduced FE-model will need improvements for more compliance to the tested setup, e.g. concerning modelling and parameters of the suspension strut assembly or regarding non-linear damping of the elastomer components. Since tribological conditions are of main relevance, a carefully considered friction model should be implemented into profound studies.
What does the paper offer that is new in the field in comparison to other works of the author:
The utilised systematic experimental characterisation procedure for creep groan has already been published. Concerning simulations, a paper about low-frequency limits of modal methods has been disclosed. This work provides further experimental investigations of creep groan across a wide range of operational parameter combinations. Moreover, it introduces a simplified vehicle corner FE-model and applies promising explicit time-integration strategies.
This article deals with investigations of creep groan on vehicle corner level. Both the introduced model and the explicit schemes of time-integration in non-linear FEA are suitable to reflect a creep groan behaviour comparable to the test rig experiments. Thus, a helpful contribution is made towards accurate, efficient and robust procedures which can be applied for simulative evaluations of automobile brake creep groan.
Manuel Pürscher, Severin Huemer-Kals, Peter Fischer, Institute of Automotive Engineering, Graz University of Technology, Austria