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Motorcycle engines have a higher speed than ordinary automotive engines. High engine speed often causes breakage or other mechanical troubles in reciprocating parts. It is important to analyze piston, connecting rod and other parts under actual operation to enable design of high speed motorcycle engines with higher reliability, better performance, and lower fuel consumption. This paper describes measurement techniques for reciprocating parts and measurement stress in the connecting rod. A linkage device which can pick up signals from the reciprocating parts of motorcycle engines has been developed. Measurement of the stress applied to the connecting rod was carried out with this device. With these measurements, we have been able to clarify how the connecting rod moves under high-speed operation. By measuring connecting rod stress in the high-speed range, it has become possible to clarify stress conditions which have been uncertain until now.

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.