When you and your cycling pals are planning a road trip to hit some gnarly trails but you are just one tray short for the extra bike, easily attach the MHS Duo 1-Bike Add-On tray to your MHS 11, 21, or 31 base. The dual-arm holders are compatible with wheel sizes from 20-29in and wheelbase up to 53in, securely holding your bike in place without even touching the bike's frame. The MHS Duo 1-Bike Add-On comes fully assembled, and features a tool-free adjustable wheel block.