Abstract: A bicycle power train assembly includes a crank axle, a primary driving sprocket cluster mounted on one end of the crank axle, a rear wheel axle, a primary driven sprocket cluster mounted on one end of the rear wheel axle, a primary drive chain trained between the primary driving and driven sprocket clusters, a secondary driving sprocket mounted on the other end of the crank axle and rotating with the crank axle, a secondary driven sprocket mounted on the other end of the rear wheel axle, a secondary drive chain trained between the secondary driving and driven sprockets, an idle sprocket rotatably mounted on the rear wheel axle, and a ratchet clutch disposed between the rear wheel axle and the secondary driven sprocket to disengage the secondary driven sprocket from the rear wheel axle when the primary drive chain is trained between the primary driving and driven sprocket clusters and to engage the secondary driven sprocket and the rear wheel axle so as to drive rotatably the rear wheel axle when the primary