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Straightening the Derailleur Hanger

It's pretty easy to bend the derailleur hanger when you fall over to the right side. The derailleur hanger is the thing that sticks down behind the right dropout (rear wheel axle slot in the frame) to attach the rear derailleur. On some bikes, it's part of the frame. On most upper-end bikes, it's a separate piece of metal, designed to be replaced if it's damaged. (Even if the hanger is a separate piece of metal, you can use this technique to correct a minor warp. But, my suggestion is that you always have a replacement hanger available.)

A bent derailleur hanger can make the chain jump between rear cogs, can create "chain suck" on the front chainrings, or will make the chain jump off the front chainrings when backpedaling.

Using a hex wrench, unscrew the derailleur mounting bolt from the derailleur hanger. Leave the chain in place on the derailleur, letting the derailleur fall away from the hanger.
The best tool for the job is a derailleur straightening wrench. But most of us don't have one in the toolbox. A 10 mm fine-pitch bolt can be used for emergency tweaks of the derailleur hanger. Costs about a buck at the hardware store.

Turn your 10 mm, FINE-PITCH thread bolt all the way into the derailleur hanger. (You can find a bolt like this at a hardware store. The longer the bolt is, the better it will work. Just make sure you get the fine-pitch thread, not standard thread.)

Grasp the bolt with your hand, or a wrench. Using the leverage at the end of the bolt, tweak the derailleur hanger until it appears straight.

Check the straightness of the hanger by confirming that it's 90 degrees from the main "plane" of the bike. Holding a carpenter's square against a 6-inch bolt, the square should parallel a string between the middle of the headset tube and the midpoint between the dropouts for the rear wheel.

Now replace the derailleur. Twist it up and back (clockwise) from its usual position, so the tensioning screw is clear of the hanger, then tighten the hex bolt. Check the derailleur for position. If it's not right, take the derailleur off and tweak the hanger a little more.

Check and adjust the shifting of the derailleur.


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