To investigate the chassis strength of a motorcycle, the Finite Element Methed (FEM) is generally utilized. However, it is difficult to determine the load conditions for FEM analysis on rough terrain. A mathematical model and computer simulation system have been developed to predict the load conditions which is obtained by accelerations at several chassis locations. The dynamic behavior of a motorcycle is simulated on a bench test device composed of rotating drums fitted with protrusions.The model is constructed using flexible bodies (e.g., front fork and rear arm) in addition to rigid bodies. The flexible front fork model was made by combining beam theory with the substructural method. Also, the analysis includes a tire model which expresses the force characteristics when protrusion is enveloped.The calculated results of this model correspond with actual measurements very closely. The model is necessary and sufficient from the points of view of simplicity and accuracy.