On stock bikes , a rear hanger ( sometimes called a mech hanger ) is a metal tab that attaches a derailleur or chain tensioner to a frame's dropout .

They are available in differing sizes and types ( classified by an alphabetical system , for example " K - Type " ) depending upon the manufacturer of the frame . Some are interchangeable so as to fit several frames . The hanger itself is mounted using a shortened chainring bolt and is made of soft metal so as to bend in an impact so thus avoiding damage to the frame itself .