Bent Derailleur Dropout (Derailleur Hanger)

The rear derailleur is attached to the bike frame at the derailleur hanger, aka derailleur dropout. On some frames, the dropout is replaceable. A replaceable dropout is usually made from relatively weak aluminum, and it is designed to bend or break under stress before anything else is damaged.

The derailleur hanger must be precisely aligned with the centerline of the bike. The gears of the cogset must also be aligned with the bike's centerline. On a well-functioning bike, the rear derailleur jockey wheels will be immediately under the cogset gears and will perfectly align with them on the same plane. If the dropout is bent, proper shifting is impossible.

Stand behind your bike and study the way the derailleur hangs relative to the gears. The jockey wheels should be directly under a gear. Does derailleur point off to the side? Does it look a little twisted? If yes, your derailleur dropout is probably bent. It's possible that the derailleur itself is bent, and if so you will have to replace it.

If you can adjust your rear derailleur so it works great shifting in one direction, but no matter what you do, it shifts poorly in the other direction, suspect a bent derailleur hanger.

If you have a replaceable derailleur dropout, buy a new one and replace it. It's smart to always have a replacement on hand, especially if traveling. Replaceable derailleur dropouts are not standardized. The specific dropout you need may be hard to find, especially when your bike gets old. So buy a spare.

A bent derailleur hanger can be straightened by a professional mechanic at a bike shop. An expensive tool is necessary, and it takes some experience. In an emergency you can try to straighten a derailleur dropout by removing the derailleur and bending the dropout with an adjustable wrench, but you'll probably just screw it up worse.

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