Aligning a Bent Derailleur Hanger

 In the past, I would carefully bend rear derailleur hangers back into shape with an adjustable wrench. There is a more precise method of re-aligning them, however. The threads inside a derailleur hanger are M10x1.0. A rear wheel with a quick-release axle also has M10x1.0 threads. You can use another rear wheel to precisely align a bent derailleur hanger.

 First unbolt the rear derailleur:

 

 

Next, screw the axle from your spare wheel into the hanger, and tighten it in place with a wrench:

 

 

Now you can measure the distance between the two wheels in all directions (top, bottom, left, and right). 

 Compare these measurements, and bend the hanger back into shape using the wheel as leverage. When the measurements are the same all around (or at least very close), then you can reinstall the rear derailleur. I would also check the limit screw adjustments to make sure it doesn't over-shift, since the derailleur will be in a new position.