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An off-road two-wheeled vehicle runs at the dart road surface which tends to be slippery including the steep slope and unevenness mainly. An analysis method of the movement limit of the vehicle and its operation conditions suitable for a movement analysis under these conditions was considered. It was focused on movement of a longitudinal direction of a vehicle and formulated the movement limit which considered the friction coefficient and the road surface slope by basic physical law. Data measurement by an all-Japan top-ranking rider was performed. It could be confirmed that data is distributed in the reach of the assumed movement limit. There is also a possibility that the movement limit of the vehicle and the operation conditions can be grasped by measurement in an off-road two-wheeled vehicle.

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It is thought that the first step to understanding vehicle movement is to measure and analyze that movement in order to confirm that it falls within the range of the laws of physics. In the case of a four-wheeled vehicle, in general, the friction limit is defined within the circle of longitudinal acceleration and lateral acceleration through the use of a G-G diagram. Then measurements are taken to confirm that the driving state is within that range. Two-wheeled off-road vehicles are mainly ridden on slippery dirt roads that include steep slopes and rough, uneven surfaces. An analysis method for the driving state and the vehicle movement limits that would be suitable for analyzing the movement of such two-wheeled off-road vehicles under these conditions was examined. These movement limits were then formulated by taking into consideration the coefficient of friction and the road surface gradient in accordance with the basic laws of physics and also by focusing on the vehicle movement in the longitudinal direction. Measurements were also taken during actual off-road riding by top-class Japanese off-road motorcycle riders. It was confirmed that this measurement data was distributed within the range of the assumed vehicle movement limits. Consequently, it was confirmed that it is possible to use such measurements to accurately grasp the vehicle movement limits and the associated driving state for two-wheeled off-road vehicles.