How to Fix a Bent Rear Derailleur

A bent derailleur hanger will negatively affect shifting

After an accident, a rear derailleur will sometimes "bend" toward the frame of the bike. Actually, it's not the derailleur that's bent: rather, it is the derailleur hanger. This is a small, sometimes removable piece of aluminum that the rear derailleur attaches to. If the derailleur is struck hard enough, such as during a fall, the force will cause the hanger to become bent, in turn sparing the more expensive and less easily repaired derailleur the same fate. Straightening this hanger is the key to repairing a bent rear derailleur.

Things You'll Need

• 5-mm Allen wrench
• Derailleur alignment tool

Instructions

1. Shift the gears on your bike, so that the chain is on the largest front chain ring and the smallest rear cog. If the chain will not shift to the smallest cog, don't worry: Shift the gear anyway.

2. Use a 5-mm Allen wrench to unscrew the derailleur from the derailleur hanger. The derailleur securing bolt will be found in the center of the derailleur. Loosen this bolt.

3. Thread the derailleur alignment tool into the same hole on the derailleur hanger.

4. Rotate the alignment tool around the circumference of the wheel, measuring the space between the alignment tool gauge and the side of the wheel. The distance between the two should be constant.

5. Straighten a bent derailleur hanger by carefully pulling on the arm of the alignment tool. After pulling, once again rotate the tool to check for evenness between the alignment tool and the wheel.

6. Unthread the alignment tool from the derailleur hanger.

7. Reinstall the rear derailleur, by threading its fixing bolt back onto the derailleur hanger and tightening the bolt with the 5-mm Allen wrench.

References

• ParkTool: Rear Derailleur Hanger Alignment