Someone contacted me recently with a bent derailleur hanger on a steel frame. It got bent when the chain jammed between the two front chainrings.
His local bike store was reluctant to attempt a straighten as there is a possibility it may break. I pointed out that the owner has nothing to lose if he attempted the job himself; if it breaks then it will have to go to a framebuilder to replace the complete dropout.
Here’s a simple homemade tool that will possibly straighten and at the same time correctly re-align the hanger. The thread on a gear hanger is 10mm x 1mm thread pitch. See if you can find a machine screw of this size that is 25mm in length, either hex head or Allen cap head will work.
If you can’t find such a screw, then use the actual derailleur hanger bolt; as long as it is steel and not aluminum.
From your local scrap steel yard, buy a piece of ¾ inch square steel bar. ¾ x 1 inch rectangle bar would also work. You could even use a piece of ¾ square steel tube as long as the wall thickness is at least 1/8 inch. Anything thinner would probably not be strong enough. This bar or tube needs to be 13 inches long.
If you managed to find a 10mm. x 1mm.p. x 25mm. screw, and have checked that it will fit in the hanger threads; Drill a 10mm. clearance hole (13/32 in.) 12 ½ inches from one end of the bar. You need to drill this in a drill press as it is important that the hole is square with the surface of the bar. A hand drill won’t cut it.
If you are going to use the derailleur hanger bolt, you will need to drill a 12mm. clearance hole. (31/64 in.) Also if this is the set up you have decided on, you will need two, thin, ½ inch flat steel washers to make up the thickness. In either case it is important that the bar bolts up tight to the face of the derailleur hanger.
Remove the derailleur from the hanger and disconnect the cable so you can move it completely out of the way. Remove the rear wheel, and either completely remove the chain, or at least let it hang out of the way, under the chainwheel.
Bolt the bar tightly to the derailleur hanger, and if it is badly bent, hold the dropout faces in a vise and use the bar to bend it straight, eyeballing it to an approximate degree of straightness.
Replace the rear wheel (Without the chain.) and tighten the quick release; make sure the wheel is central between the chainstays. Loosen the bar and swing it into one of the three positions shown in the top picture; tighten the bolt and take a measurement from the inside edge of the bar to the wheel rim.
Swing the bar to the next position and repeat. Make sure you tighten the hanger bolt each time and tweak the hanger using the bar as leverage. The hanger is in alignment when the bar to rim measurement is the same in all positions.
Ignore the fact that the top picture shows the derailleur and chain in place, I just used a stock photo to illustrate. If the difference between two opposite measurements is for example 1/4 inch, remember you will only need to tweak it 1/8 inch (Half the amount.) to correct it.
Inspect the hanger for cracks. If it does crack, the crack can be grooved out and tig welded; this would only cause a minimum damage to the paint. After welding, recheck the alignment as the heat may have moved it.
An improved version of this tool and one that a bike shop might like to have in their tool kit will need a machine shop’s assistance to produce.
A piece of ¾ diameter round steel bar approximately 12 inches long, is turned down and threaded 10 mm. x 1 mm. pitch at one end. This screws into the derailleur hanger and is actually used to bend it straight. A "T" handle at the opposite end to the thread to facilitate tightening by hand is a nice addition.
You still need a piece of flat bar with a 10 mm. hole in one end to act as a straight edge to measure to the rim, but this can be much lighter as it is not used for the actual bending; a piece of flat steel stock ¾ in. x 3/8 in. would work.