Abstract: An adjustable sprocket wheel and transmission mechanism, the sprocket wheel having two toothed wheel segments each having a sector angle of substantially 180.degree., the wheel sections being interconnected by a lever system arranged to incrementally increase the diameter of the sprocket wheel defined by the wheel segments, one of the pedal cranks being connected to each of the wheel segments by crank pivots mounted on projections lying adjacent the periphery of the wheel segments, the wheel segments each having a recess positioned to receive the projection of the other wheel segment when the segments are in a retracted position to define a circular sprocket wheel, the lever system including levers pivotally connected to each of the two wheel segments, the levers being selectively engagable in recesses in the other wheel section, the transmission mechanism including actuating cams arranged to incrementally move the levers to selectively change the diameter of the sprocket wheel.