Moan is a low frequency noise occurring during brake applications mainly at very low speed. Due to the low frequency range the axle components have an even more significant influence on the noise than during squeal. Today problems with moan noise often occur at late stage of the project when axle parts and brake components are already fixed and extended vehicle tests are performed. At this time the components can only be changed with an enormous amount of money. Thus the big challenge is to find a procedure to evaluate moan criticality, ideally based on simulation earlier in the project, and to find tools to effectively work on the problem.
Before starting with any kind of simulation or design guidelines to avoid moan it is necessary to identify the phenomena, or in other words, the physics behind the problem. In this paper measurements with a real series vehicle are shown. Besides the brake conditions the deflections of the relevant brake and axle components are measured synchronously in time domain and later evaluated with regard to signal growth and frequency characteristics. In addition a classical experimental modal analysis (EMA), an operational modal analysis (OMA) and an operational deflection shape analysis (ODS) are performed. These results are used to align the FE model. An evaluation of the moan problem is finally done with help of a complex eigenvalue analysis based on the FE model.
The time domain measurements show that the amplitudes of the different components simultaneously start to grow exponentially until they reach their limit cycle. Frequency domain evaluations show a mainly monofrequent stable signal. Modal analyses give an insight into the existing modes around the moan frequency. The identified modes are dominated by the stiffness and mass of the axle parts. Although test conditions during modal analysis were very close to driving conditions, no mode at the moan frequency but two modes one below and one above the moan frequency could be identified. The operational modal analysis shows how modes change while changing brake and driving conditions. The operational deflection shape analysis finally shows the mode shape during the moan event. The moan problem could be detected with the finite element model and a countermeasure could be derived.
This deep going and extensive study is firstly limited to one exemplary vehicle and brake system. Nevertheless the authors assume that the physics behind the system will be the same for similar problems. Mode shape analyses and frequency behavior of different projects show many similarities.
The results show that moan noise has significant similarities with the well known brake squeal and thus can be classified as a self excited vibration due to the coupling of two system modes. Moan noise can thus clearly be separated from creep groan. The methods used to find countermeasures against a known problem should be the same as for squeal.
Jens Bauer, Continental, Division Chassis & Safety; Matthias Körner, Volkswagen AG; Alexander Pfaff, Frankfurt University of Applied Sciences.