What’s a derailleur hanger?
A derailleur hanger is a small piece of metal alloy used as the attachment point between your bicycle’s frame and the rear derailleur. The derailleur hanger is a sacrificial part, this means that it is designed to break before the frame does.
Derailleur hangers are usually made from inexpensive cast or machined alloy, and is affixed to the frame using bolts and forms part of the frame dropouts (where the wheel axles slot).
Due to the primary function of the derailleur hanger being to save the frame in the event of crash or an obstacle hitting the derailleur, it must be made of a soft material so that it breaks quickly on impact, leaving the frame unscathed. This also means that the hanger is very susceptible to bending on smaller impacts.
A bent hanger is one of the most common reasons for lousy shifting on a bike. It doesn’t take much to bend the hanger to the point of negatively affecting the shifting quality, especially since the length of the derailleur cage results in enough lever effect – when subject to a hit – that easily bends the delicate alloy of most hangers.
What happens when a derailleur hanger is bent?
A perfectly straight derailleur hanger should always be at a right angle to the rear axle when observed from the rear of the bike. When a derailleur hanger is bent it is said to be “out of alignment”.
A misaligned hanger results in poor shifting. That is, it becomes difficult or impossible to have the index adjustment resulting in the rear derailleur shifting the chain smoothly throughout the gear range, even if all other components are in a perfect working condition.
Sometimes, the symptoms of a bent hanger are similar to other culprits of poor shifting, such as cable tension. Accordingly, it is important to always perform a hanger alignment check as part of the checklist of shifting issues diagnosis.
How to diagnose a bent derailleur hanger
You can look at the derailleur hanger from the rear of your bike and try to check for bending or misalignment. Keep in mind that visual inspection is only possible if there is a significant bend in the hanger.
Sometimes, a slightly bent hanger can look perfectly straight while on the bike even to the trained eyes of an experienced bike mechanic, yet even a fraction of a millimeter can result in poor shifting.
Remove it from the bike and check against a flat surface
if you unbolt the hanger from the bike, remove any hardware and lay it flat on a table or a workbench, you can check for bending or misalignment that would be exposed by the hanger rocking or wobbling if you give it a gentle vertical nudge on one end.
For this method to work, your surface will need to be perfectly flat. Any irregularities in your table or workbench may lead to false positives.
The process of elimination
If you’ve tried everything and your shifting is still off, it’s probably the hanger. You’ve replaced the cables and made sure they have no sharp bends around the frame, reindexed the drivetrain, made sure your chainline is good and your gears are still not working as they should, then it is very likely the hanger.
The proper method, use a hanger alignment tool
The most accurate way to check hanger alignment is to do what professional bike mechanics do: use a hanger alignment tool. This tool bolts to the hanger in place of the rear derailleur and allows alignment checks against the rim.
This tool also allows you to correct the alignment by rebending the hanger. That said, this tool is usually on the pricey side for something you’re not likely to use very often, and not all home mechanics will have one in their toolbox.
Can I ride with a bent derailleur hanger?
You can certainly ride your bike with a bent hanger, but shifting will be suboptimal and there is a risk of further damage to the bike. At the very least, you will have drivetrain noise in some gears, or your chain will skip in certain gears or shift more than one gear at once.
In situations where the bent hanger affects the gears closest to the spokes, you risk the derailleur cage hitting and possibly catching on the spokes.
Furthermore, your chain can become jammed or stuck between gears and when this happens under load, this can result in chain breakage (modern 12-speed drivetrains have thin chains that are particularly prone to breaking under load) With this in mind, it is best not to ride the bike until you perform the required repair.
How to repair a bent derailleur hanger
You have two options here: try bending the hanger back into alignment, or replace it with a brand new hanger.
If you have a hanger alignment tool (Like the excellent Park Tool DAG 2.2), and assuming you know how to use it, you can try applying force to the hanger to straighten it out. This method often works, but not always.
As mentioned, the soft alloy most hangers are made from is not meant to be very malleable, and subjecting it to bending forces in different directions will eventually break or severely weaken it. If the bend in the hanger is significant, it is better to replace it than try to re-align it.
If the hanger must be replaced, how do I find the correct hanger for my bike?
Your best source for finding the right hanger is your bike’s manufacturer! Failing that (if it is out of stock or your bike is an older model), Wheels Manufacturing has an excellent hanger fit finder tool that you will find here.
Some more tips on derailleur hangers:
Prevention is better than the cure
To reiterate, hangers are very easily bent. Avoid leaning your bike against objects that might hit the derailleur or laying it down on the driveside.
Have a spare hanger in your bike repair kit
Whenever I buy a new bike, one of the first things I do is buy a spare hanger for it, especially if it is a mountain bike which often carries more risk of hanger damage due to collision with trail objects and debris.
Protect the derailleur
Some companies make derailleur hanger protectors to fit their bikes, like the Syntace Rockgaurd. You can also buy basic universal derailleur protectors that fit a variety of bikes, which is especially useful if you have a commuter bike that you leave in public bike parking where the close proximity of bikes, especially those improperly secured, can often result in component damage.
Hani Morsi is a seasoned multi-discipline cyclist with a particular liking for mountain and gravel bikes. Hani is also a mountain bike coach, trail builder and experienced bike mechanic.