Dynamic condensation techniques, usually applied to simplify the dynamic models of mechanical systems, are based on the choice of master degrees of freedom and the selection criterion entails important characteristics on the equivalent reduced models. Starting from geometrical data of analytical models or experimental setup and corresponding modal data, the present paper presents a modal-geometrical selection criterion of master degrees of freedom that can minimize the ill-conditioning of mass and stiffness matrices. Some analytical models are tested, by computing with the System Equivalent Reduction Expansion Process (Serep) methodology the equivalent dynamic reduced systems. Both the modal properties and the ill-conditioning of mass and stiffness matrices of reduced models are compared. In particular, by increasing the order of the reduced model, the ratios between the maximum and minimum eigenvalue of the matrices models computed through the modal-geometrical selection criterion are matched up with the best and worst choice of master nodes between all possible solutions. Therefore, performance and restrictions of the proposed criterion is discussed for increasing weight of models and modal properties until to consider complex finite element (FE) models of automotive subsystems, such as tire, wheel clamp and exhaust pipeline.

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