a first carrier and a second carrier; wherein the first carrier has a first receiving groove and the second carrier has a second receiving groove; wherein the first carrier has a first through hole defined in an inner top of the first receiving groove and extending upwardly from the first receiving groove, the second carrier has a second through hole defined in an inner bottom of the second receiving groove and extending downwardly from the second receiving groove;

a first shift block and a second shift block; wherein the first shift block has a first through path and the second shift block has a second through path; wherein the first shift block is positioned in the first receiving groove of the first carrier and configured to shift with respect to the first carrier, the second shift block is positioned in the second receiving groove of the second carrier and configured to shift with respect to the second carrier; and

a positioning member; wherein the positioning member has a first end and a second end located at the first through hole and the second through hole, respectively; wherein the positioning member extends sequentially through the second through hole, through the second through path, through the first through path, and into the first through hole; wherein the connection unit is configured to be fixed to the inner wall of the bicycle tube or disengaged from the inner wall of the bicycle tube by rotating the positioning member to displace the first shift block and the second shift block.