Mobility measurement in six DOFs applied to the hub of a car

• The mechanical mobility was measured in 6-DOF for a complex mechanical structure.
• The hub of a car was chosen as measurement object.
• The quality was evaluated using reciprocities, coherence functions and random error.
• Reciprocities and multiple coherences indicate a good quality between 25 and 500 Hz.

Complex models are usually needed to predict functional performance of mechanical devices such as noise and vibration levels. A typical model is divided into substructures where each substructure is described either as a simulation or as an experimental model. When coupling substructures, information is needed that describes the boundary conditions at the attachment points of the various substructures. This can be hard to achieve, especially when full structural behavior including rotational degrees of freedom (DOFs) are needed. The objective of this study was to obtain the mechanical mobility matrix in six DOFs for a wheel hub of car, as an example of a complex mechanical structure, through an experimental approach based on the multiple-input multiple-output technique. Further, evaluation of the measurement quality was conducted without requirements for numerical simulation comparisons. A specially designed brake disc was fabricated for direct attachment of shakers and transducers. The quality of the 6-DOF mechanical mobility matrix was evaluated using reciprocities, coherence functions, and random error in the gain-factor estimates. The results showed good quality in the reciprocities, and the multiple coherences in each degree of freedom were close to one in the frequency range of 25–500 Hz. The random error can be reduced by increasing the number of averages.