A sprocket assembly mounted to a hub has at least eleven sprockets. The hub is able to rotate about a rotation axis and has a plurality of spoke attachment seats and a fastening portion to a frame of the bicycle. On the hub a predetermined axial distance is defined between a plane, substantially perpendicular to the rotation axis, comprising the points of the spoke attachment seats closest to the freewheel body and a plane, substantially perpendicular to the rotation axis and defined in the fastening portion to the frame, comprising the attachment points to the frame closest to the freewheel body. All of the sprockets of the sprocket assembly, when associated with the freewheel body, have an overall axial width that is lower than or equal to the predetermined axial distance. The chain has a maximum thickness between 4.6 mm and 5.8 mm.