Somehow I bent my rear derailleur hanger, how would one suggest bending it back into line? In my case the rear derailleur hanger forms part of the frame and therefore if I snap it when bending it back, I could write off the bike.
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1If all else fails, the thread of hole for the derailer bolt is the same as a standard hollow axle, in most cases, so you can get an old axle and several matching nuts, screw in the axle and thread the nuts tight against the hanger, and then use the leverage of the axle to bend the hanger. While there are special tools to get it exact, you can come pretty close by eye, if you take your time.– Daniel R HicksJun 13, 2016 at 20:20
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2The other approach is to take a standard "Crescent" style adjustable wrench, tighten it onto the hanger, and use the wrench to bend. (The wheel would probably need to be removed while doing this.) This approach lacks subtlety, however.– Daniel R HicksJun 13, 2016 at 20:25
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1Sure, you might break the hanger bending it back into position. But how much use is the bike if you can't fit a derailleur? Trying to get 8 seed derailleur working with a cheap bolt-on derailleur is doable, but it won't shift very well and you'll likely need to replace your QR axle with a solid, nutted one to make the derailleur work at all.– MóżJun 13, 2016 at 21:45
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Related - HOW did you bend it in the first place? What can you do to stop it bending again? As others have pointed out, repeated bending will break it.– Criggie ♦Jun 14, 2016 at 3:59
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2 Answers
As I said in comments, the real question here is: do it yourself or pay someone else. You can't really ride the bike as it is, so you don't have a lot to lose if it turns out to be unfixable.
A simpler approach if you don't want to buy the expensive tool just for this is to use something else to bend the hanger straight. I normally use a large adjustable wrench. Clamp it tightly to the hanger and twist cautiously. I'd do it in two or three movements, first the vertical alignment, then twisting it, then possibly re-doing the vertical. You will have to remove and re-fit the derailler a couple of times, and you will probably struggle to get it exactly right. But 8 speed isn't too sensitive (with 10 or 11 speed you're probably not going to get it accurate enough by eye to shift reliably).
An old m10x0.1 axle with nuts can also work if you have one. The important thing is that you have nuts or washers with a surface area as large as the flat on the hanger, because you really don't want to be twisting the hanger using the threads - that is likely to ovalise the hole and damage the threads. So you really need a cone and some washers, but ideally an m10x0.1 nut.
That bend isn't too bad IMO, it should be simple enough to get it working again. My main caution is that if you find yourself in your third round of bending it, give up and get someone with the right tool to do the job. Bending it back and forth is how you will break it off.
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A (rear) wheel with a non-quick-release axle is a good tool. It allows you to check that the two rims are parallel meaning that the hanger is correctly aligned.– CarelJun 14, 2016 at 7:43
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Only if the axle is an m10x0.1mm axle. Otherwise trying to do that will just damage the threads in the frame, if you can get it in there at all. And you can only check two points, which isn't enough– MóżJun 14, 2016 at 8:26