Originally Posted by
berninicaco3
Finally, what's the role of the hanger --except maybe as a spacer, cause it seems some derailleurs don't need hangers
Well, some derailleurs bolt onto a hanger that's part of the frame, and some of them have built-in hangers which are held in place by the right-hand axle nut and a bolt and specially-shaped nut that goes through the dropout. There are also hangers which attach in the same way, so you can fit a bolt-on derailleur to a frame that doesn't have a hanger built in.