Abstract: An enclosed drive for a mountain bicycle features a chain case which replaces the conventional chain stay on the right side of the bicycle coupled with an improved twelve speed transmission using four countershafts, which is housed in a gear case inside the rear wheel hub. The gear case is bolted to a cylindrical extension of the chain case which mounts the rear wheel bearing on the right side. The chain case is press fit on a steel tube rotatably mounted in the bottom bracket, and the chain case cover is a close shear fit on the rear spindle which is clamped with over center quick disconnects in dropouts at the lower ends of conventional seat stays. To change the rear tire, the rear spindle is released and the chain case is rotated about the axis of the bottom bracket to swing the wheel clear of the frame. The wheel can then be separated from the bicycle without disturbing or exposing any drive train components by removing four wheel nuts and pulling the wheel off from the left side.