A drive member selection mechanism comprising: at least one drive member and at least two selector members (3a, 3b) arranged about a common axis in such a manner that the position of each respective said selector member along the common axis may be changed; wherein: the or each drive member comprises a first and second face, and further comprises, on each face, at least one projection (7c, 7d), and the selector members comprise, on at least one face thereof, at least one complementary projection (7a, 7b) arranged to be selectively engaged with a said projection of a drive member; and the arrangement is such that for the or each respective drive member at least one said projection on the first face thereof may be drivingly engaged with at least one said projection of a selector member in a first torque connection after moving that selector member along the common axis into engagement with that drive member in use and at least one said projection on the second face thereof may be drivingly engaged with at least one said projection of another selector member in a second torque connection opposed to the first torque connection after moving that selector member along the common axis into engagement with that drive member in use; the drive member selection mechanism further comprising a shift mechanism arranged to control the position of the selector members along the common axis, the shift mechanism having at least two rotatable drums each provided with a circumferential track, where each said rotatable drum is substantially identical and mounted with a rotational displacement relative to the or each other rotatable drum, and where each respective said selector member is mounted in association with a separate said track such that rotation of the corresponding rotatable drum determines the position of the associated selector member along the common axis. at least one drive member and at least two selector members (3a, 3b) arranged about a common axis in such a manner that the position of each respective said selector member along the common axis may be changed; the or each drive member comprises a first and second face, and further comprises, on each face, at least one projection (7c, 7d), and the selector members comprise, on at least one face thereof, at least one complementary projection (7a, 7b) arranged to be selectively engaged with a said projection of a drive member; and the arrangement is such that for the or each respective drive member at least one said projection on the first face thereof may be drivingly engaged with at least one said projection of a selector member in a first torque connection after moving that selector member along the common axis into engagement with that drive member in use and at least one said projection on the second face thereof may be drivingly engaged with at least one said projection of another selector member in a second torque connection opposed to the first torque connection after moving that selector member along the common axis into engagement with that drive member in use; the drive member selection mechanism further comprising a shift mechanism arranged to control the position of the selector members along the common axis, the shift mechanism having at least two rotatable drums each provided with a circumferential track, where each said rotatable drum is substantially identical and mounted with a rotational displacement relative to the or each other rotatable drum, and where each respective said selector member is mounted in association with a separate said track such that rotation of the corresponding rotatable drum determines the position of the associated selector member along the common axis.