The derailleur hanger is the replaceable part of the frame that the rear derailleur attaches to. It is designed to break before the frame is ruined if the derailleur receives a smack. There are many different hangers each unique to the bike brand and model. iCycle keeps many of the most popular hangers in stock and can quickly order hangers we don't have. There is hundreds of different hangers out there for the various types of bikes.