Drive member selection

A drive member selection mechanism, comprising:

• at least one drive member and at least two selector members arranged about a common axis in such a manner that the position of each of the at least two selector members along the common axis may be changed;
• wherein:
• the at least one drive member includes a first and a second face, and further includes, on each face, at least one projection, and the at least two selector members include, on at least one face thereof, at least one complementary projection arranged to be selectively engaged with said at least one projection of the drive member;
• the arrangement is such that, for the at least one respective drive member, the at least one projection on the first face thereof may be drivingly engaged with the at least one projection of one of the at least two selector members in a first torque connection after moving the one selector member along the common axis into engagement with the at least one drive member in use and the at least one projection on the second face thereof may be drivingly engaged with the at least one projection of another of the at least two selector members in a second torque connection opposed to the first torque connection after moving the other selector member along the common axis into engagement with the drive member in use;
• a shift mechanism arranged to control the position of the at least two selector members along the common axis, the shift mechanism having at least two rotatable drums each provided with a circumferential track, where each of the at least two rotatable drums is substantially identical and mounted with a rotational displacement relative to another of the at least two rotatable drums, and where each of the at least two selector members 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;
• and
• each track is symmetrical around each rotatable drum, the pattern of each track being repeated on both sides of each rotatable drum, and each of the rotatable drums further includes at least two shift arms, each of the respective two shift arms arranged in association with one side of each rotatable drum to engage with the track formed thereon.