Abstract: An automatic transmission for mounting within the rear fork of a bicycle includes a plurality of pairs of constant mesh gears and a plurality of discs. Each disc is concentric with one of the gears in each pair and includes a notch for partially enclosing a centrifugally actuated pawl. The notches in adjacent discs face in opposite directions and all the discs but one include a spring for opposing the centrifugal force on each pawl to keep each pawl in a disengaged position. The one disc includes a spring for holding the pawl in an engaged position. The pawls are shaped to permit the bicycle to coast or freewheel.