A complete guide to rear derailleurs

Derailleur compatibility, tooth capacity and much more explained

Everything you ever wanted to know about rear derailleurs

Is it time to upgrade or replace your rear derailleur? Do you sit awake at night worrying about what ‘tooth capacity’ is? Or have you ever simply wanted to know absolutely everything there is to know about buying a rear derailleur or thought which rear derailleur do I need? If so, you’ve come to the right place.

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While we certainly don’t recommend you break out this hot derailleur chat at your next social appointment, this is undoubtedly useful information if you’re looking to buy or upgrade a rear derailleur.

We must stress that this article only covers rear derailleurs because including front derailleurs would make this guide far too unwieldy. Otherwise, here’s everything you need to know about the rear derailleur.

Which brand of derailleur should I buy?

Which rear derailleur do I need? Shimano, SRAM and Campagnolo are the three main players in the drivetrain market.

Microshift, Box, Rotor, FSA and others all also make groupset components, but it would be beyond the scope of this article to cover all of these. 

As a general rule of thumb, it’s best not to mix and match drivetrain components from different brands. While things such as cranks, chains and cassettes are mostly inter-compatible between brands, generally speaking, shifters and derailleurs aren’t.

In brief, Shimano, SRAM and Campagnolo all use different cable pull ratios (the amount that a derailleur moves for every millimetre of cable pulled through by the shifter), and mixing parts will result in very poor shifting. 

Collage of black and silver bicycle groupset parts on a white background
Generally speaking, road and mountain bike groupset components will not work with each other.
Various

However, it’s more complicated than that. Mountain bike groupsets and road groupsets, even when they’re from the same brand, typically use different cable pull ratios.

Ratios may also change as the number of gears changes, and there may even be pull ratio differences between different generations of the same gear model. 

There are, of course, exceptions, and there are lots of bodged, Sramshimpagnolo mashups that can be cajoled into working, but for the sake of simplicity, we suggest you stick to the same brand as your shifters when buying a rear derailleur.

How many gears does my bike have?

Shimano 105 R7000 groupset review, cassette
To work out how many ‘gears’ your bike has, count the number of sprockets on your cassette.
Jack Luke / Immediate Media

Once you’ve settled on the brand, you must now determine the number of gears that your groupset has.

If you are replacing an existing derailleur on a bike, simply count the number of cogs on your cassette and you’re good to go.

If your drivetrain’s speed is an unknown quantity, you can count the number of steps that your shifter runs through and add ‘one’ to determine the number of gears your drivetrain has.

Derailleur compatibility explained

A complete guide to rear derailleurs
Compatability across Shimano’s road and mountain bike groupsets is a little bit complicated.
Steve Behr / Immediate Media

As a general rule of thumb, you shouldn’t mix and match groupset parts of different generations or mix mountain bike and road components, but there are a few exceptions. Below, we have broken down compatibility by manufacturer.

For clarity, the following information is applicable to both derailleurs and shifters.

Shimano derailleur compatibility

Most 8- and 9-speed Shimano mountain bike and road kit is inter-compatible because they both use the same cable pull ratio. This means that you can use an 8 -or 9-speed mountain bike derailleur with road shifters or vice versa. 

The only exception is pre-1997 9-speed Dura-Ace gearing, which won’t play nicely with anything because it uses a totally different cable pull ratio. 

Although it initially doesn’t appear to be the case, the situation is a bit more clear with the newest generation of both Shimano road and mountain bike components. Bear with us…

Shimano road bike derailleur compatibility

  • All 11-speed Shimano road components are inter-compatible – you could for example use a Dura-Ace 9100 derailleur with a pair of 105 7000 shifters
  • Shimano’s 11-speed GRX gravel groupsets are also cross-compatible
  • All 10-speed road components (except Tiagra 4700 and 10-speed GRX gravel, see below) are inter-compatible – you could for example use an old Ultegra 6700 derailleur with old 105 5700 shifters
  • Current 9-speed road components are backwards compatible with older 9-speed road and mountain bike components, excluding the aforementioned exception

The only exceptions to the above rules are Tiagra 4700 (and the associated RS405 hydraulic shifters) and GRX400 gravel mechs, which use the same cable pull ratio as current generation 11-speed road kit. 

This means you could, for example, use an 11-speed derailleur with 10-speed Tiagra 4700 shifters.

Shimano mountain bike derailleur compatibility

  • All 12-speed Shimano mountain bike components are inter-compatible – you could for example use an SLX M7120 derailleur with an XTR M9100 trigger shifter
  • All 11-speed Shimano mountain bike components are inter-compatible – you could, for example, use a XTR M9000 derailleur with a pair of SLX M7000 shifters
  • All 10-speed Shimano mountain bike components are also inter-compatible – you could, for example, use an old 10-speed XTR M986 rear derailleur with new Deore M610 shifters
  • Current 9-speed Shimano mountain bike components are compatible with older 9-speed road and mountain bike components, excluding the aforementioned exception

Just for clarity, current generation (10-, 11- and 12-speed) Shimano road and mountain bike components are not inter-compatible with each other because they use different cable pull ratios. For example, you couldn’t use a set of road shifters with a mountain bike rear derailleur.

It bears mentioning that cable pull converters from brands such as JTek, and Lindarets/Wolf Tooth Components do exist and will allow you to mess about with your drivetrain configuration to your heart’s content.

SRAM derailleur compatibility

SRAM GX Eagle mountain bike drivetrain
SRAM’s latest 12-speed GX mountain bike rear derailleur.
Russell Burton / MBUK magazine

The situation is a little more clear with SRAM and we’ve summarised the main points below.

SRAM road and mountain bike rear derailleur  compatibility 

  • 7-, 8- and 9-speed SRAM components are all inter-compatible, regardless of whether they are road or mountain bike parts
  • 10-speed SRAM components are inter-compatible, regardless of whether they are road or mountain bike parts – for example you could run road shifters with a mountain bike rear derailleur
  • 10- and 11-speed SRAM mountain bike components are not inter-compatible
  • 10- and 11-speed SRAM road components are inter-compatible – meaning you could run a 10-speed SRAM Red rear derailleur with a pair of SRAM 22 shifters
  • 12-speed SRAM components are not backwards compatible with 11-speed. The exception is eTap batteries, which work with all 12-speed AXS and 11-speed components

SRAM has also been at the forefront of the push to ever-wider gear ranges and its newer rear derailleurs are designed to handle wider range cassettes. So it’s important to consider whether a replacement mech can handle all the cassette sprocket sizes that you might want to run.

For clarity, despite visual similarities, SRAM’s Force 1 11-speed groupsets are not compatible with 11-speed mountain bike components because they each use different cable pull ratios (though it will work with 10-speed road shifters).

Campagnolo derailleur compatibility

Campagnolo Centaur groupset
Campagnolo’s entry-level Centaur groupset.
Russell Burton / Immediate Media

In un-typically Campagnolo fashion, cross-compatibility between different generations of groupsets is fairly easy to understand because there are no mountain bike groupsets (yes, we know it made Euclid and a host of other off-road bits way back) to contend with. 

But as is typical of Campag, there’s the odd flourish of awkwardness.

Campagnolo rear derailleur compatibility 

  • All 8- and 9-speed Campagnolo groupsets before mid-2001 used the same pull ratio and are compatible with each other. This generation of parts is often referred to as ‘Campy old’
  • After mid-2001, Campagnolo started using a revised pull ratio for its newer 9-speed kit, and these and all 10-speed (and 11-speed) groupsets from this period are inter-compatible – for example, you could run an Athena derailleur with Record shifters

Campagnolo compatibility squares letter A
Campagnolo has introduced many changes to its groupsets, resulting in reduced compatibility between generations. So boxed letters (indicating the ranges) will aid you in identifying if your parts are compatible.
Campagnolo

But now things get more complicated (or simple, depending on which way you look at it) with slight tweaks to all of Campagnolo’s groupsets resulting in reduced inter-compatibility between groupsets.

In short, these changes have affected most of its drivetrain parts (different width bottom bracket cups result in altered chainlines, different cable pull ratios, and so on) and compatible parts are marked by a letter surrounded by a square box – simply put, if all of your components have the same stamped letter on them, they’ll work together.

Campagnolo provides a much more thorough description of the changes here and we highly recommend you read through this guide carefully before committing to any new parts from the Italian marque.

There are also differences in the pull ratios used between different groupsets: the Ergopower shifters used by mechanical Super Record, Record and Chorus 11-speed are not compatible with Potenza, which itself is incompatible with both Ergopower and Centaur Power-Shift systems.

Electronic groupset compatibility

SRAM Force eTap AXS wide gearing
They certainly aren’t the norm, but electronic groupsets are getting more popular each year.
Jack Luke / Immediate Media

If your bike has an electronic groupset, or if you’re thinking about upgrading to electronic shifting, you’ll need to keep within one brand because the makers’ groupsets use different systems. 

Apart from first-generation Dura-Ace Di2, which uses a different wiring harness, newer Shimano electronic groupsets – both road, MTB and GRX – have inter-compatible wiring and electronics.

So, if you smash up your £550 Dura-Ace Di2 R9150 rear mech (ouch), you can replace it with a £250 Ultegra Di2 R8050 derailleur. Note that while the mechs will work, chainlines do differ between GRX, Shimano’s road groupsets and its mountain bike groupsets. 

SRAM’s latest 12-speed eTap AXS road and MTB groupsets can be used together, setting up the option for ‘mullet builds’, which pairs up road shifters with an Eagle AXS rear mech and a 10-50 cassette, for an ultra-wide gravel bike build. 

That was great until SRAM introduced its Force AXS Wide groupset. Bridging the gap between road-going AXS groupsets and mullet builds, it offers a lower bottom gear for road and a good range for gravel riding with cassettes up to 36 teeth, but does this by altering the drivetrain alignment. 

This means there’s a new rear mech designed to handle 10-36-tooth cassettes. The original eTap AXS Red and Force rear mechs are only rated up to 33-tooth cassettes. The older 11-speed Red eTap shifters and rear mech aren’t compatible with 12-speed eTap AXS.

Road bike against wall
Campagnolo’s Super Record EPS electronic groupset is now onto its fourth generation.
Matthew Allen / Immediate Media

Campagnolo’s Super Record EPS electronic groupset is on version four now and has gone 12-speed. Again, it’s a system unto itself, with its own electronics to communicate between the shifters and the rear mech.

Plus, parts for the disc brake and rim brake systems don’t work together properly. There’s also no backward compatibility with older 11-speed EPS or forward compatibility to upgrade from 11-speed to 12-speed.

The good news is that there’s some reverse compatibility between version three 11-speed EPS components and older generations.

What cage length derailleur should I buy?

Now that we’ve determined the speed, brand and compatibility of your derailleur, you must now determine the cage length that your drivetrain requires: long, short or medium cage.

The length of your derailleur’s cage defines the range, or spread, of gears you can have on your bike – the longer the cage, the more slack in the chain the derailleur can take up.

For the sake of simplicity, we have included a quick guide below, but if you’re in any doubt read on to see how we come to these conclusions.

Derailleur cage length quick guide

There are two common situations where there may be a large range, or spread, of gears on your bike; when using a super-wide range cassette (e.g. 10-42t or larger) or when there is a large difference between chainring sizes (e.g. when using a triple chainset). In these circumstances, you will require a long cage derailleur.

If you are running a 1x drivetrain with a regular-sized cassette (i.e. 11-36t or smaller) or some 2x mountain bike drivetrains with a similarly sized cassette, you may want to use a medium cage derailleur.

If you have a traditional road double-drivetrain with a regular cassette (i.e. 11-28t or smaller), you can use a short cage derailleur. 

Scott Gambler 900 Tuned downhill mountain bike
Downhill-specific groupsets use exceptionally short derailleur cages.
Alex Evans

Some downhill-specific drivetrains also use short cage derailleurs (e.g. Shimano, Saint and SRAM X01 DH). 

You will notice that there are lots of ifs and buts in this guide and that’s because there are too many variables to give a conclusive answer in any situation.

Derailleur tooth capacity explained

To get a definitive answer, you must refer to the ‘tooth capacity’ of your derailleur. You can work out the required tooth capacity of your bike by calculating the following:

How to calculate tooth capacity

  • (largest cog – smallest cog) + (largest chainring – smallest chainring) = Required capacity

So, for a modern, double chainring road bike drivetrain we would have something like:

  • (32 – 11) + (52 – 36) = 37t capacity 

For the sake of example, let’s assume you are looking at a Shimano road derailleur here.

The total capacity of a SS (short cage) Shimano derailleur is 35t and 39t for a GS (medium cage) derailleur. As such, in this circumstance, you would require a medium cage derailleur. 

It bears mentioning that the quoted capacity of derailleurs tends to be pretty conservative and, in practical terms, you could almost certainly get away with using a short length cage derailleur in this circumstance, so long as you avoided extreme (e.g. big and big) gear combinations.

Clutch derailleurs explained

GRX rear derailleur
Clutch-equipped rear derailleurs were once the sole preserve of mountain bike offerings, but are now seen on both gravel and road groupsets from Shimano and SRAM.
Matthew Loveridge / Immediate Media

Regular rear derailleurs rely solely on the tension provided by the main pivot to keep the chain in place. A clutch derailleur essentially increases the resistance this pivot provides – either through a clutch or more complex electro-hydraulic systems as seen on SRAM AXS – resisting fore and aft movement of the derailleur cage, making for a far quieter and more reliable drivetrain.

Although SRAM and Shimano’s interpretations of a clutch derailleur differ slightly, they both aim to achieve the same thing. 

Almost all modern mountain bike derailleurs (and Shimano’s GRX gravel groupsets, Ultegra RX rear mech, SRAM’s single-ring road groupsets and eTap AXS 12-speed road rear mechs) include a clutch mechanism.

With clutch derailleurs offering a quieter and more reliable drivetrain, there’s little compelling reason not to buy a clutch derailleur for a mountain bike.

What do I get with a more expensive derailleur?

Now that we’ve gone through the tortuous process of working out exactly what will work with your drivetrain, you have the fun of deciding which price point to go for.

But what should you look for in a rear mech? To keep the topic simple, we’ve broken things down into weight, longevity and finish:

Weight

Shimano XTR and SRAM Eagle AXS derailleur
More expensive rear derailleurs will, typically, weigh less.
Tom Marvin / Immediate Media

A more expensive derailleur will usually weigh less than its cheaper brethren. This is achieved by using more exotic materials (e.g. carbon cages, titanium pivots) in the derailleur’s construction and machining away more excess material.

If weight is the utmost concern to you, you’ll have to shell out some more cash.

Longevity

More expensive derailleurs often use harder wearing components and are built to closer tolerances, so will last longer than cheaper models.

This is particularly apparent with jockey wheels, where cheaper derailleurs will often spin on basic steel bushings.

Unsurprisingly, these don’t last nearly as long as higher-end alternatives that spin on cartridge bearings or ceramic bushings.

Finish

BMC is still running Dura-Ace 9000 series groupsets
We still maintain that Shimano’s R9000-era Dura-Ace groupset produced some of the best-looking rear derailleurs of all time.
Colin Levitch / Immediate

Since everything up to this point has been tech-heavy, we can afford to be superficial for a moment.

Higher-end derailleurs can be jewel-like in their quality, featuring all sorts of beautiful, polished panels and jazzy, bright anodising. 

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While people will tell you never to judge a book by its cover, bike nerds have a habit of judging a bike by its rear derailleur and if you want to be the coolest person in the bunch, we won’t judge if you splash out on some shiny loveliness.