UNIOR: Features: Hanger Genie is foldable and precise tool, used for re-aligning rear derailleur hangers and maintaining precise gear shifting. It´s compatible with all wheel sizes from 20? to 29?+ and screws directly into rear derailleur hanger threads. Precise measuring gauge can be locked in place, so you don´t lose you measurement when you push it against the wheel in different positions. Thanks to rotating bottom part of the tool, you can easily clear the frame when you work your way around the wheel.