These replacement arms are designed to be used with the CURT hitch-mounted bike rack #18021, allowing it to be transformed from a 2-bike model to a 4-bike model as needed. The arms easily and securely bolt into place and feature rubber cradles that hold the bike frame firmly in place during travel, without scratching the finish. To protect the arms against the elements and the trail ahead, they come with a durable black powder coat finish.