Research Questions / Objective: With diminishing background noise and shifting boundary conditions in the wake of electrification, autonomous driving and the introduction of brake emission regulation, the requirements on a brake’s NVH behavior are more demanding than ever. Problems with low-frequency creep groan, which occurs at very low speeds near standstill, will therefore likely persist or even gain more relevance. However, integrated development processes are so far not established for creep groan. This paper presents approaches and methods to test, evaluate, and rate creep groan in every stage of vehicle development. Methodology: Extensive tests on test bench and full vehicle level build the base for all evaluations. Operational deflection shapes during different creep groan vibrations are compared to check the validity and underlying mechanisms. A measurement setup including accelerometers and microphones is used to generate vehicle test data. Data handling and storage topology is discussed. Practical issues of creep groan detection, also in comparison to other low-frequency phenomena such as squeak noise, are discussed. Objective rating methods based on machine learning methods are applied on different test system levels. Results: Operating-Deflection-Shape results show the interaction between torsional and longitudinal vibration patterns during creep groan. Reduced setups therefore loose fidelity here, while they are often still able to depict a disadvantageous behavior of the friction components itself. The developed measurement equipment is able to detect and analyze creep groan events at each of the four brakes in real time. Therefore, an objective assessment of the subjective impressions is available at the same time as the noise event. Objective rating scales delivered comparable values on different system levels and can be even further refined with increasing test data. Limitations of this study: The presented methodology was developed based on one single car model. Future tests will show, how well objective results for other car models will fit to the subjective annoyance and whether small adaptions will be necessary. What is new in this paper? The vibration signature of test bench creep groan and vehicle creep groan was directly compared. A novel measurement setup was developed, which allows the detection and rating of creep groan in addition to already existing squeal measurements. The developed rating methods based on accelerometer data allow the objective assessment of creep groan annoyance on any test bench level. When testing on the full vehicle level, this annoyance can additionally be evaluated for the outside world and passengers via wheel house microphones and interior microphones. Conclusions: Creep groan will persist to be a significant noise and vibration problem, especially with changing boundary conditions within the automotive industry. Due to the increasing complexity of requirements on brake components, tackling the problem as soon as possible can be a crucial advantage in order to find effective and economic solutions. Therefore, the presented measurement equipment and accompanying methods represent an essential toolkit to test, evaluate, and rate creep groan. Ultimately, noise comfort and vehicle quality can be increased.
Dr.-Ing. Karl Häsler, Development Engineer Brakes, Control, Pedals, Mercedes-Benz AG; Dr.-Ing. Andreas Bender, Managing Director, STAC Elektronische Systeme GmbH; Dipl.-Ing. Severin Huemer-Kals, University Assistant, Institute of Automotive Engineering, Graz University of Technology; Dipl.-Ing. Martin Zacharczuk, Brake Systems Engineer, Mercedes-Benz AG; Mr. Jonas Weber, Software Development, STAC Elektronische Systeme GmbH; Prof. Dr. Peter Fischer, Head of Institute, Institute of Automotive Engineering, Graz University of Technology