The derailleur hanger is a small aluminium bracket between frame and derailleur. It is designed to bend — it is the sacrificial part that fails instead of the frame when the bike falls on its drive side or the derailleur catches something.
It does its job by deforming, and it deforms far more easily than most people expect. A bike laid on its side hard enough is sufficient.
The symptoms
Indexing that will not hold across the cassette. Set the barrel adjuster so the middle gears are perfect and the extremes are wrong. Fix the extremes and the middle goes.
This is the signature. A correctly aligned drivetrain indexes across the whole cassette from one adjustment; a bent hanger makes the derailleur's position error vary with sprocket, so no single adjustment satisfies all of them.
Noise in one or two specific gears with the rest quiet.
Hesitant shifting in one direction only — usually reluctant to move to larger sprockets.
Visibly non-parallel. Sight from behind: the derailleur cage should sit parallel to the cassette sprockets. A hanger bent inward tips the cage toward the wheel.
Check before adjusting anything
This is the practical rule that saves evenings. Before touching limit screws or the barrel adjuster on a bike that was shifting fine yesterday, check the hanger — especially after any crash, fall, or transport in a car.
Shifting problems that appear suddenly on a previously well-adjusted bike are far more often mechanical than adjustment drift. Cables stretch gradually; hangers bend instantly.
Alignment
The proper tool is a derailleur alignment gauge — it threads into the hanger and uses the rim as a reference, measuring at several points around the wheel. That is the only way to know how far out it is and in which plane.
Bending it back by hand is possible and imprecise. It works often enough to be worth trying on a ride, and it is not a substitute for measuring afterwards.
Aluminium work-hardens. A hanger straightened several times becomes brittle and eventually snaps — usually into the spokes, which is a considerably worse outcome. A hanger that has been straightened more than once or twice should be replaced rather than straightened again.
Carry a spare
Hangers are model-specific. There are hundreds of shapes, and the one for your frame is often not stocked by a shop in another town.
A spare costs little, weighs nothing, and is the difference between a delayed ride and an ended tour. Buy it when you buy the bike, while the model is still current.
UDH
SRAM's Universal Derailleur Hanger is a single standard shape adopted across many manufacturers, so one spare fits many frames. It also underpins direct-mount derailleurs that bolt through the frame rather than to a hanger.
If your frame takes UDH, spares are available anywhere. If it does not, you are back to the model-specific problem.
FAQ
How do I know if my derailleur hanger is bent?
Indexing works in some gears and not others regardless of barrel adjustment, and the derailleur cage does not sit parallel to the cassette when viewed from behind.
Can I bend a derailleur hanger back by hand?
You can, imprecisely, and it is worth doing on a ride. Check it with an alignment gauge afterwards.
How many times can a hanger be straightened?
Once or twice. Aluminium work-hardens and eventually snaps, often into the spokes.
Why does my bike shift badly after a car journey?
Bikes on racks and in car boots catch their derailleurs. Check hanger alignment before adjusting anything.
What is a UDH hanger?
A universal shape adopted by many frame makers, so one spare fits many bikes and direct-mount derailleurs become possible.