Everything you need to know about rear derailleur hangers

Small.

Easily overlooked.

Critical to gear function.

Yup, derailleur hangers fly under the radar when it comes to your drivetrain. A lot of bike riders don’t even know that they have a hanger.

But while you’re drooling over a beautiful cassette, fancy jockey wheels or a shiny derailleur, this little piece of metal is quietly making sure that these components actually work together properly.

So what exactly is a rear derailleur hanger, and what does it do?

Let's dive into how hangers work and why they’re so important, and what to do if yours is out of whack.

What is a derailleur hanger?

A replaceable rear derailleur hanger is a machined piece of metal which attaches the derailleur to the frame.

Not all derailleur hangers are replaceable, though.

Some frames have hangers which are integrated into the frame itself, like this one:

If a frame isn't designed to use a rear derailleur, such as track bikes or internally geared models, it won’t have a hanger at all, as you can see on this singlespeed frame:

These frames are well in the minority, however; most bikes will have a hanger, whether replaceable or integrated.

For the sake of this post, when we say ‘hanger’, we’re talking about replaceable ones that bolt on to the frame.

Right! Now that’s out of the way...

What does a rear derailleur hanger do?

A derailleur hanger has two important jobs:

1. To attach the rear derailleur to the frame.
2. To protect the rear derailleur and frame in the case of an impact.

If the derailleur suffers a hit, the hanger bends to take the force of the blow.

Think of it like a helmet for your gears.

(Did you know that bicycle helmets are actually designed to collapse or even break upon impact? If you hit your head in a crash and your helmet isn’t damaged, it’s not doing its job. But we digress.)

But why is a derailleur hanger even necessary? Why can’t the derailleur just take the hit?

The answer lies in its history.

Why do rear derailleur hangers exist?

In the early days of cycling, when bikes were made primarily of steel, the derailleur hanger was just part of the frame. If you hit your derailleur and bent the frame, it could usually be bent back into shape.

When carbon frames came on the market in the 1980s, however, things changed.

To fit a derailleur to a carbon frame required the use of an alloy dropout bonded into the carbon tubes, like this one:

The problem was that if you cracked or broke the hanger, the entire frame would need to be scrapped.

The solution? Replaceable hangers – you could either straighten the old hanger or simply whack on a new one instead of having to trash your lovely carbon frame.

A hanger is also a lot cheaper to replace than a rear derailleur.

What are derailleur hangers made from?

Replaceable rear derailleur hangers are usually made from an aircraft-grade aluminium of some sort, but the quality can vary wildly.

To the surprise of no-one, low-quality frames tend to have low-quality hangers.

This doesn't mean that high-quality frames have high-quality hangers, though. We've seen some very expensive bikes with hangers which have the consistency of a fish fillet.

How are rear derailleur hangers made?

Derailleur hangers are primarily made via two methods:

1. Cast – metal is melted, poured into a mould and cooled.
2. CNC-machined – the hanger is cut/machined out of a single piece of stock material.

As CNC-machined hangers tend to look better, people immediately assume that they’re better quality. Some manufacturers even run their cast hangers through a CNC machine so they look like—and are sold as—CNC-machined hangers instead. However, this aesthetics = quality theory isn’t strictly true.

CNC-machined hangers actually have a higher failure rate than cast hangers.

Let’s examine why.

Are CNC-machined hangers better than cast hangers?

In machining, the ‘grain’ refers to the material’s internal structure – the alignment of its crystals.

When working with aluminium, perfectly aligned grain is crucial for strength and performance. Aligned grains ensure that stress is distributed evenly throughout the material, which helps prevent failures or weaknesses. Material with aligned grain structure is also easier to machine; unaligned grain structure can cause irregular cutting forces, resulting in rough surfaces, tearing or excessive burrs.

The quality of a CNC-machined hanger relies on the quality of its original stock material, and it’s very difficult to tell whether stock grain is perfectly aligned or not. If flaws are missed (and they can be microscopic), the end result is a faulty hanger. And you don’t know you have a faulty hanger until it snaps.

The stock aluminium issue is a big one for CNC-machined component manufacturers; we know of several high-end brands who have gone through multiple aluminium foundries, trying to find one which can consistently provide high-quality stock material.

Cast hangers, on the other hand, don’t have this issue with grain alignment because they’re made with molten metal; as the metal cools, the grain naturally aligns.

(And, on that point, now you know why some prefer just to cast their hangers and then make them look CNC-machined.)

Does a derailleur hanger have to be super-strong?

The perfect hanger has a balance of stiffness and bendability, rather than being incredibly strong.

The hanger has to be useably soft, so it can be bent straight again if need be, but not so soft that it instantly breaks during a minor impact. It also has to be stiff enough to withstand the load of changing gears.

Some cast hangers can be super-rigid, so they’ll bend to protect the rear derailleur on impact, but then break during the straightening process. They effectively generate a memory for the bent position, then crack while being realigned. (Metal is weird.)

Some cast hangers are badly made, and the resulting faults mean that they, too, will snap during straightening.

Some hangers—cast or CNC-machined—are so badly designed that they’re inherently soft and will bend in a stiff breeze. However, this is not the fault of the factory making the hanger, rather the bike’s design team. Why does a hanger need to be 1-2mm thick through a critical load point? It doesn’t.

Overall, the most critical aspect of a hanger isn’t massive strength: it’s design and straightness.

Why does a hanger need to be straight?

In order to change gears properly, the derailleur, jockey wheels and chain have to align perfectly with the cassette (rear cogs).

The rear derailleur hanger is the physical link between the rear derailleur and your bike. If it’s bent, the derailleur (and, as a result, the jockey wheels and chain) won’t line up correctly with your cassette.

This leads to everything from gears slipping under load to chains skipping across cogs or chattering between them.

So you want that sucker to be straight.

How does my derailleur hanger get bent?

Any time a bike takes a strike on the derailleur side, there’s a pretty good chance the hanger will bend.

Crashed your bike?

You’ve probably got a bent derailleur hanger.

Rock strike on your gears?

You’ve probably got a bent derailleur hanger.

Leant your bike against a wall, wandered off and then heard a crash?

You’ve probably got a bent derailleur hanger.

Put your bike gear-side down into your car? Stacked your bike with a bunch of others outside a café? Used a bike locker? Parked your bike in public? Left it unattended anywhere where other people can accidentally knock it?

You guessed it; you’ve probably got a bent derailleur hanger.

There is one other, less common way to get a bent hanger: poor shifting technique.

You probably already know that you shouldn’t change gears with a lot of load on the pedals. To add to this, some hangers are particularly soft, and the force of shifting under load can be enough to send them out of alignment.

Always take some load off when you’re changing gears until the gear engages. It’s good practice, even if your hanger isn’t soft as butter.

Are there any other bent derailleur hanger symptoms?

There's one other way to tell that your hanger is bent: if you're cycling and hear a ting ting ting noise.

If this happens, STOP RIDING.

That 'ting' sound is your rear derailleur brushing against your spokes. You are one pedal revolution away from sending your derailleur straight through your back wheel.

At best, you’ll rip your derailleur off.

At worst, you’ll rip your derailleur off, explode your rear wheel, and end up in hospital.

So consider ting! ting! ting! your bike’s alarm system, and pedal no further until you can get your hanger sorted out.

Which derailleur hanger do I need?

So your hanger is cactus and you need a new one. Just grab a replacement and you're ready to go, right?

Unfortunately it's not that simple.

A critical – and frustrating – thing about derailleur hangers is how many different ones there are.

We’re not talking about a handful of versions, either; we’re talking literally hundreds, possibly even thousands – the catalogues are like phone books (for those of you who remember what a phone book is).

Think we're kidding? Here's a small sample

This problem is due to the fact that each derailleur hanger is specifically designed to fit a particular bike frame model.

There’s no single Trek hanger or Specialized hanger, no ‘standard’ road bike hanger, no ‘regular’ mountain bike hanger, and the hanger that fits your bike won’t fit your friend’s unless they have exactly the same make, model and year of bike that you do.

The one exception to this is SRAM’s Universal Derailleur Hanger (UDH), SRAM’s attempt to make a single hanger that can work on multiple frame designs. Launched in 2019, the UDH can be used on a lot of different bikes, but is not truly universal – your frame still needs to be compatible.

Can I straighten a bent hanger myself?

If you’ve crashed your bike while out on a ride and you need to get home, you may need to try an emergency straightening.

Look at your hanger and derailleur from the rear. Do they look like this?

If so, carefully ease the hanger straight as best you can. Don't go crazy; you don't want to break it off and make the problem worse. You just need to make sure that the derailleur isn’t going to go into the spokes on your way back to your house/car/pick-up point.

This situation is why carrying a spare derailleur hanger with you is a great idea. It takes up practically no room in a saddle bag, and can be the difference between ending your ride at the finish line or waiting for a lift home.

Outside an emergency situation like this, we don’t recommend trying to DIY a bent derailleur hanger.

Hangers need to be perfectly straight in more than one plane, and if you don’t have the experience and technique to align them properly, you can end up creating more issues than you solve.

Hanger alignment tools are also notoriously unreliable, because of poor manufacturing techniques/tolerances.

Your best bet is to bring it in and make sure it’s done properly.

The derailleur hanger - demystified

Small, unobtrusive and inexpensive, most people never give a derailleur hanger a second thought. But it’s always there, working away to make sure that your gears change when they’re supposed to (and don’t change when they’re not meant to).

Look after it and it’ll look after you, because a perfectly aligned hanger is the only thing standing between crisp gear shifting and accidentally ripping your rear wheel apart.

And besides, what other part of your bike sacrifices itself for the greater good?

Still having gear problems? Hearing the dreaded ting ting ting? We'll get it sorted for you.