Abstract: The specification discloses a transmission for varying the mechanical advantage in the power train of a bicycle. The transmission includes a drive sprocket attached to one wheel of the bicycle for driving the wheel and a fixed pitch sheave mounted on the bicycle frame. An endless chain is entrained between the drive sprocket and fixed pitch sheave. A rotatable variable pitch sheave is mounted to move relative to the fixed pitch sheave and is operatively connected to the fixed pitch sheave by a V-belt passing therebetween.A pedal sprocket is rotatably mounted on the bicycle frame and connected by way of a chain to the variable pitch sheave for rotating the variable pitch sheave. The rotation of the variable pitch sheave by the pedal sprocket causes the movement of the variable pitch sheave relative to the fixed pitch sheave to vary the mechanical advantage therebetween.