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When developing a motorcycle exhaust system, it is important to predict the fatigue durability of the exhaust system during the design stage. We have been predicting fatigue durability using our own methods [1]. In recent years, however, in order to meet stricter emission regulations, the installation position of a catalyzer has been changed and the temperature of the exhaust system has been increased. Accordingly, the required fatigue durability of mufflers is at higher temperatures than before. With such a change in situation, a prediction method with higher accuracy for fatigue durability that can handle a higher temperature range, was required. The exhaust system temperature distribution and the physical properties of the material change depending on the temperature. Therefore, in the simulation model developed this time, the temperature distribution of the exhaust system is calculated by a heat conduction analysis method applying FEM.

A method applicable in the design stage to predict fatigue strength of a motorcycle exhaust system was developed. In this prediction method, a vibrating stress, thermal stresses, stresses resulting from the assembling of the exhaust system components and a deterioration of fatigue strength of materials originated from high temperature were simultaneously taken into account. For the prediction of the vibrating stress, flexible multibody dynamics was applied to get modeling accuracy for vibration characteristics of the entire motorcycle and the exciting force delivered from engine vibrations. The thermal conduction analysis and the thermal deformation analysis based on finite element method (FEM) were applied for the prediction of thermal stresses in the exhaust system components. The temperature distribution on the surfaces of the exhaust system components is required for calculations of the thermal stresses.