In the planetary-gear train proposed, a sun gear (3) is indirectly mounted via a bushing (15) on a central shaft (1) with a centered flange (2). The sun gear meshes with several planet gears (4) which are secured by screws (12) to the planet-gear carrier (5) and to the flange (2), and hence to the shaft (1). The planet gears (4) mesh in turn with internal toothing (6') on a bell-shaped element (6) which is mounted on the shaft (1) and which represents the crown gear of the planetary-gear train. Mounted on the outside of the bell-shaped element are the various sprocket wheels of a chain-change sprocket assembly (9) via which muscle power can be added to that of the planetary-gear train crown gear. The drive is transmitted through the planet-gear carrier (5), which is designed as a mounting ring for a disc wheel (10). The gear train proposed requires little space, is simple in design and, at typical travel speeds, no energy losses due to rolling motion are incurred since under these conditions the sun gear and the crown-gear turn synchronously and the planet gears therefore stay still.