Detection of critical mode-shapes in flexible multibody system dynamics: The case study of a racing motorcycle

In multibody system models, body flexibility is introduced when the structure compliance is not negligible for an accurate description of the system behaviour. Flexible multibody systems provide a more detailed description of system kinematics and dynamics, introducing the effects of the body dynamics deformations.The influence of component higher frequency harmonics is often undesired during system working conditions. Therefore, in the design of a flexible multibody system the components and assembly modal properties are optimised to avoid resonances in the operating condition frequency ranges. However, in the system optimisation phase or when the system is upgraded to a new release, the least modifications would be ideally performed to optimise the system for its working conditions. In this context, there are no straightforward methods available to recognise which component mode-shapes require structural optimisation to avoid undesired dynamics behaviours.Based on an assembly-to-components modal approach, this paper proposes a method to detect the component mode-shapes responsible for undesired dynamics in the assembled multibody systems; the method is suitable for both numerical model or experimental applications. The inputs to the detection method are the components, sub-structures and assembly mode-shapes, together with the main frequencies exciting the system in operating conditions, to which operative deformations are associated. The procedure reveals the critical components and their mode-shapes requiring modifications.The detection method is conceived especially for systems requiring fast and continuous optimisation, e.g. high-performance racing vehicles, which are at least annually upgraded, or new improved releases of industrial complex machines. Firstly, the proposed method is numerically applied to the flexible multibody model of a slider-crank mechanism, detecting high-frequency vibrations in the piston stroke. Then, it is applied to a race motorcycle chassis frame to detect the critical component mode-shapes in realistic working conditions. Independent application of the procedure to numerical and experimental data of the motorcycle chassis pinpoints the same component mode-shapes as critical, on which optimisation efforts would be required