I've bent (again) my derailleur hanger. It's a replaceable hanger but for this summer I can't get an original one and I know that the non-original one I can get is made in a harder material and could possibly damage the frame.
So I'm left with the choice of straightening this light aluminium alloy thing. How do I do that without breaking/weakening it?
Remove the derailleur and use an adjustable wrench to bend it back into shape on the bike. That way you can see when it's straight more easily. Remember to check both vertical alignment and fore-aft alignment. With 5/6/7 speed doing this by eye will be fine. You might get good enough for 8 speed, but with 9 and 10 speed you'll have to be lucky to get reliable shifting using this method. Those setups are just too fussy.
Bike shops have a derailleur hanger alignment tool that does this much more accurately by screwing into the derailleur mounting hole and letting you gauge position against the wheel rim. I've found that with 9 speed often a couple of millimetres at the rim can be the difference between smooth shifting and constant "not quite in gear" clicking in one or two gears.
Edit: this is a classic "trivial with the right tools, impossible otherwise" job on 9 and 10 speed systems. Put the bike in a mechanics workstand, attach the alignment tool, bend the hanger, reattach the derailleur, check the gears and it's done in 5 minutes. The bike shop will probably want to put in a new gear cable and housing for you (because otherwise if the shifting is less than perfect you might blame them rather than the friction in your old gear cable).
The fun part, BTW, is finding a replacement hanger if the bike manufacturer no longer stocks them. There are "clone" manufacturers but the catalog of available hangers is huge (often one bike model has different hangers in different years, so unless you know exactly which frame you have the label on the frame might not help much).
Also check out http://derailleurhanger.com when you just can't get it straight. The fact that there's a web site out there that only sells derailleur hangers is what makes the interwebs great.
I've had luck by removing the hanger and then using a vise to clamp the whole hanger flat. Whatever you do, remember that you've got one, maybe two shots at it...too many bends and it's gone.
A caveat... Aluminum is very susceptible to "work hardening". Some alloys are softer than others, and you may get away with it OK. I have done it on one of the department bikes... So far so good. However, there's always the danger the thing will just let go.
Better to replace if possible.
Video here shows how to do that using appropriate tools and just a simple hex key. The video is in Spanish but images are pretty much descriptive. Note the video has 2 parts. Second one starts automatically after the first one.
