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On the majority of motorcycles, the body structure is a framed construction. In the process of structural design for the motorcycle frame, a numerical optimization assisted by computer is vital as effective measure to determine the best design specifications all the time. Conventionally, however, in nearly all of the examples in which sensitivity analysis and optimum design methods were applied to frame structure, the design variables were limited to the cross-sectional dimensions of the members. In the study presented here, a minimum mass design of the motorcycle frame was aimed in which the cross-sectional dimensions of members and even the positions of members could be employed as design variables.

A structural analysis for the connecting rod was performed in consideration of bearing crush, oil clearance and initial deformation of the big end bore caused by tightening force of the rod bolts. This analysis enabled to calculate not only the bearing pressure distribution but also the three-dimensional stress state with accuracy. A new fatigue criterion for multi-axial out-of-phase stress was proposed by using mean stress tensor and alternating stress tensor of complex number in this study. This criterion was developed to improve the classical multi-axial “yield criteria”. This criterion is simple, comprehensive and suitable for the evaluation of three-dimensional FEM results. The connecting rods exhibiting high reliability under severe conditions were investigated through the stress analysis and the new evaluation criterion.