Frog Replaceable Derailleur Hanger A spare bolt-on rear gear hanger is invaluable if the original gets bent, say if the rear derailleur takes a whack. Worth carrying as a spare, especially on long trips and more epic rides. A Replaceable Derailleur Hanger is a sacrificial part. If this most vulnerable part of the frame takes a hit, the replaceable drop out bends or breaks. And, hopefully, not the derailleur. Or the frame.