What is a derailleur hanger? | Everything you need to know 

Derailleur hangers, SRAM UDH and Shimano direct mount explained

By Oscar Huckle

Published: Friday, 19 May 2023 at 12:00 am

The derailleur hanger may not be the most exciting component, but it performs a crucial function in keeping your gears shifting optimally and is designed to bend or break in an impact.

In this guide, we’ll take you through what a derailleur hanger is, its function, the different types available and how to align or replace a bent one.

We’ll also explain what SRAM’s Universal Derailleur Hanger is, as well as Shimano’s direct-mount standard.

What is a derailleur hanger?

"Jack
This little part plays a crucial role in the quality of your gear shifting.
Jack Luke / Our Media

The derailleur hanger (also commonly referred to as a mech hanger) attaches to the dropout, acting as the connection between the rear derailleur and the frame.

Before the derailleur hanger’s widespread use, the rear derailleur would mount on the frame dropout. Although that was generally fine when frames were made of steel, the move to aluminium or carbon fibre means they are more likely to snap or bend.

It’s still a wise decision to have a separate hanger on a steel frame, though, for easier repairs.

"You'll
This steel Surly Disc Trucker sees the rear derailleur bolted straight onto the dropout.
Dave Caudery / Our Media

Having to write off a frame is wasteful if the rear derailleur breaks off and thus enters the derailleur hanger.

What does a derailleur hanger do?

"Broken
Although a grisly sight, the derailleur hanger has performed its function.
Josh Evans / Immediate Media

A derailleur hanger is a sacrificial part designed to bend or break in an impact, protecting the rear derailleur or frame.

Accidents happen and it’s a lot cheaper to replace a derailleur hanger than a frame or rear derailleur.

It’s crucial the derailleur hanger is aligned perfectly to enable optimal gear shifting.

If the derailleur hanger is misaligned, the gears will shift poorly because the derailleur pulley is misaligned with the cassette cog. If it’s bent sufficiently far inwards, the derailleur could even shift the chain into the spokes, potentially damaging both the derailleur and wheel.

Although a strong impact on the derailleur can cause the hanger to bend, it can also happen under more innocuous circumstances.

"Tubeless
Don’t do this!
Jack Luke / Immediate Media

Avoid leaning your bike on the driveside and position the rear derailleur away from any points of impact if you’re loading the bike into a car or resting your bike alongside others at a coffee stop.

Damage to the driveside of the bike can be a clue the hanger has had an impact; it’s worth checking the derailleur hanger alignment before you adjust and index your gears.

It’s also a good idea to check the alignment on a new bike, because you’d be surprised how often they can be bent straight out of the box. A good bike shop will incorporate this into their pre-delivery inspection (PDI).

Why are there so many derailleur hangers?

"A
Derailleur hangers come in a range of shapes and sizes.
Jack Luke / Our Media

For many years, derailleur hangers were frame-specific. With the exception of some performance mountain bikes and a handful of gravel bikes that use SRAM UDH (more on this later), this is still largely the case.

Derailleur hangers come in all shapes and sizes, and you must use the correct hanger for your frame.

Differences can include whether they mount to the inside or outside of the dropout, and the number and type of fasteners the hanger uses. They can often have other unique features too.

"Jack
Hangers are frame-specific and there isn’t a substitute.
Jack Luke / Our Media

Derailleur hangers are almost always model-specific and it’s not uncommon for manufacturers to change the design of the hanger for a particular model mid-product cycle.

After-market brands such as Wheels Manufacturing, Pilo or RearMechHanger.com specialise in making hangers for most frames and offer guides on their sites.

It’s worth having the old hanger to spare so you can check carefully if the design of the replacement matches your old one.

Because you’re unlikely to find the correct derailleur hanger at a shop if you come a cropper, it’s a good idea to carry a spare with you when you’re out riding or on a bikepacking trip.

"Converting
A faux-singlespeed could be an option to get you home if the hanger breaks on a ride.
Haynes

If the derailleur hanger breaks on a ride and you don’t have a spare, it’s almost certainly a ride-ender. If the chain isn’t twisted, you could remove it from the rear derailleur and shorten it, to ride the bike home as a singlespeed.

Some brands, such as Wheels Manufacturing, make an emergency hanger, which you can install to get you home.

SRAM UDH explained

"SRAM
SRAM’s UDH is now commonplace on many performance mountain bikes.
Gary Walker / Our Media

Launched in 2019, SRAM’s Universal Derailleur Hanger (UDH) was designed to unify hanger standards.

Rather than bend or break, SRAM designed the hanger to pivot backwards in the event of the chain jamming. If the hanger pivots backwards, you can push it forward, often without needing to loosen the thru-axle.

It also sports a hooked lip to prevent the chain from jamming between the dropout and cassette.

SRAM made UDH an open standard and many brands have adopted it on their mountain bikes. It’s also beginning to find traction on some gravel bikes.

Given its popularity, this also makes it much easier to source a replacement.

"Yeti
T-Type Eagle Transmission integrates the hanger into the rear derailleur.
Yeti

In March 2023, SRAM launched its T-Type Eagle Transmission mountain bike drivetrains. Eagle Transmission forgoes the derailleur hanger altogether, and the rear derailleur is mounted directly to the dropout via a proprietary bushing.

Shimano direct mount explained

"Shimano
The derailleur attaches directly to the (in this case, minimalist) hanger.
Russell Burton / Our Media

Shimano introduced its direct-mount standard in 2012 when it released its Shadow-style mountain bike rear derailleurs.

Rather than design a universal-fitting derailleur hanger, the direct-mount standard was designed to create a stiffer interface between the frame and rear derailleur to improve shifting quality.

The rear derailleur bolts straight onto a specifically designed hanger, rather than via a ‘B-link’. However, Shimano still includes a B-link if you need to connect a direct-mount rear derailleur onto a conventional hanger.

The design was ported over to road bikes with Dura-Ace R9100 and gravel with GRX. Interestingly, Shimano has dropped the standard on its latest 12-speed mountain bike groupsets.

How to straighten a bent derailleur hanger

"Oscar
There isn’t a substitute for a derailleur hanger tool.
Kaden Gardener / Our Media

If the gears aren’t shifting properly and you can’t get them to index, a bent derailleur hanger can often be the culprit.

You can take a visual of the hanger to see if it is noticeably bent. Provided it isn’t too far gone, the hanger can be bent back to tolerance.

Bear in mind you run the risk of the derailleur hanger snapping, because you’ll weaken the material by bending it back.

As well as the correct-sized hex or Torx key to remove and reinstall the rear derailleur, you will need a derailleur hanger alignment tool for the job.

These can be expensive, but there isn’t a substitute. If you own multiple bikes, it’s worth investing in, or perhaps sharing, a tool among your riding friends.

Some suggest using the jaws of an adjustable spanner to bend it back in place by eye, but we wouldn’t recommend this – it’s more likely to break given the increase in force and it won’t provide an accurate gauge of where it is misaligned.

Step 1

"Oscar
Remove the rear derailleur.
Kaden Gardener / Our Media

Make sure the rear wheel is installed correctly into the frame dropouts.

This is particularly important with quick-release axles. Installing the wheel while the bike stands on the floor is best because gravity helps guide the axle in place so the wheel sits squarely in the dropouts.

Shift the chain into the smallest cog and remove the rear derailleur. Hold the rear derailleur as you remove it to prevent it from springing back.

Step 2

"Oscar
Thread the alignment gauge into the hanger.
Kaden Gardener / Our Media

Lightly grease the threads of the derailleur hanger alignment tool and install it into the hanger threads. Be careful not to cross-thread it – the threads will need re-tapping if you do. Most derailleur hangers use an M10 x 1.0 thread.

Step 3

"Oscar
Position the gauge of the tool with the valve at the 12 o’clock position.
Kaden Gardener / Our Media

Position the valve of the rear wheel at 12 o’clock. It’s best to use the valve as your reference point, because this takes wheel trueness out of the equation.

Move the arm of the alignment tool so the end of it touches the rim. Some tools include O-rings, which act as a visual guide for the hanger’s alignment.

Next, move the valve to the 6 o’clock position and, without adjusting the arm of the tool, note where it sits.

"Oscar
This gap is larger than 3mm and requires adjustment.
Kaden Gardener / Our Media

There will be three possible scenarios:

  • The end of the arm touches the rim. This means the vertical alignment of the tool is correct and you can move on to the next step
  • There is a gap between the end of the arm and the rim. If the gap is more than 3mm, you’ll need to perform an adjustment
  • The end of the arm wants to go further inwards towards the rim. Reset the arm of the tool so it touches the rim and use the 12 o’clock position as your second reference point
"Oscar
This is the end result.
Kaden Gardener / Our Media

If the hanger requires an adjustment, gently pull or push the tool in the direction the hanger needs to be bent in. Once adjusted, check the vertical alignment of the hanger again using the previous steps until there is a gap of 3mm or less.

Step 4

"Oscar
You can now move to the 9 o’clock position.
Kaden Gardener / Our Media

We’ll now move on to the horizontal alignment.

Without adjusting the arm of the hanger, check the hanger is aligned both in the 3 o’clock and 9 o’clock positions. Bend the hanger as above if you need to make an adjustment.

Step 5

"Oscar
Take care if you need to make an adjustment.
Kaden Gardener / Our Media

Check the derailleur hanger tool is aligned perfectly at all four points.

Step 6

"Oscar
It’s always wise to put some anti-seize on the rear derailleur’s mounting bolt threads.
Kaden Gardener / Our Media

Reinstall the rear derailleur, being careful not to cross-thread, and tighten it to the manufacturer’s recommended torque.

Step 7

"Oscar
You’ll want to check the gear indexing after straightening a bent hanger.
Kaden Gardener / Our Media

Check the gears function correctly. You may need to adjust the limit screws and indexing.

How to replace a snapped derailleur hanger

If the derailleur hanger is snapped or broken, you’ll need to replace it.