Component mode synthesis (CMS) is nowadays a classical solution for modelling the flexibility of a body involved in a computer-aided dynamic analysis, the elastic deflections being expressed by a weighted sum of deformation modes. One is however confronted with the selection of the component modes because, if the number of retained modes increases, the accuracy is ameliorated, but the computational task increases as well. In modal analysis of structures, the component mode synthesis is classically applied, by using either a Craig-Bampton (CB) or a MacNeal-Rubin (MNR.) reduction, both composed of static and vibration modes, the latter being selected according to a frequency ordering scheme. The purpose of this paper is to analyse the selection procedure in the context of the computer-aided dynamics of multibody systems, which present, several specificities like the importance of inertia forces, the large number of possible configurations or the variety of efforts that can be applied to a body. A quality index describing the performance of a given modal base has been defined in the time domain by comparing at every moment, on one hand, a reference deformation obtained by simulating the whole multibody system with a very complete mode set for each flexible body, and on the other hand, the combination of the chosen component modes that is the closest to this reference deformation. This quality index has been applied to describe the validity of a new selection procedure, according to a participation factor, calculated for a specified motion of the multibody system. The determination of this participation factor is based on the efforts acting on each body during the considered motion, estimated from the corresponding rigid-body analysis.

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