Derailleur Hanger Repair

Impacts to a bike's rear derailleur can bend the derailleur hanger, which is the piece that attaches it to the frame. Most mountain bikers have probably seen this happen, as well as anyone who routinely drops their bike on the drivetrain side:

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Notice the angle on the derailleur's cage? It might be tempting to just pull it back into shape, but that will only make things worse (you might bend the derailleur instead of hanger). The real solution is to remove the derailleur:

You usually only need a 5 or 6mm allen key to do this, and you don't need to remove the chain or the cable. Just let it hang while you fuss with the hanger:

Park Tool makes a very expensive alignment tool, and a competent bike shop mechanic would be able to do a precise job realigning the hanger, but an adjustable wrench works pretty well too:

I just gently bend it out until the derailleur lines up straight again. It helps to have a good feel for how metal bends, but just be careful and patient and you can probably get it straight:

I wouldn't do this to a high end racing bike, but for any normal bike this should be just fine. I've repeatedly bent the hanger on my Trek mountain bike without breaking it, so no worries on a steel hanger (heat treated aluminum might be a bit iffy though). The last step is to check the gear indexing and adjust the set screws if necessary.