A bicycle transmission assembly that is mountable to a bottom bracket shell of a bicycle frame. The bicycle transmission assembly includes an input crank assembly, an output chainring, a planetary gear mechanism, a control system and a mounting system. The input crank assembly is mounted to a rotatable crank axle mounted to the bottom bracket shell. The planetary gear mechanism is mounted to the bottom bracket shell and provides a plurality of transmission paths. The planetary gear mechanism includes a sun gear, a ring gear and a locking element operatively connecting the sun gear and the ring gear. The control system is operatively connected to the planetary gear mechanism for selecting one of the transmission paths.