US Patent 7469611 Vehicular power unit with improved transmission structure

A transmission structure of a power unit for a vehicle includes a clutch mechanism that is provided between a crankshaft and a main shaft, and is installed at the end of the main shaft outside a crankcase. The transmission includes a shift drum for driving shift forks for selectively establishing any of plural speeds. Both ends in an axial direction of the shift drum are rotatably supported by the crankcase. In the transmission structure, a sealed structure dedicated to the shift drum is not required, the number of parts is reduced and the assembly time is reduced. The shift drum, arranged in the maximum outside diameter of the clutch mechanism when viewed in a plane perpendicular to the axis of the main shaft, is assembled in the crankcase from the side of the clutch mechanism.