A power transmission system used in connection with a vehicle power wheel of a bicycle or other vehicle which has a fixed axle and which utilizes a rotating chain sprocket engaging an endless chain for receiving an input torque and transmitting an output torque. The system includes a transmission ratio varying apparatus that is operably connected to the wheel by the endless chain. The transition ratio apparatus incorporates a drive collar fixedly secured to a sprocket and having one or more input and output drive rings. A drive body is displaceable with respect to the input and output drive rings and includes input and output pawls pivotally connected to the drive body and engageable with the input and output drive rings. A shifter housing is moveable with respect to the fixed axle cam to rotatably shift the drive body with respect to the drive ring and change the relationship between the input and output pawls and thereby vary the torque transmission ratio. An operator lever and a cable connecting the operator lever with the shifter collar functions to move the shifter housing with respect to the fixed axle cam.
