A complete guide to rear derailleurs

Derailleur compatibility, tooth capacity and much more explained

By Jack Luke

Published: Friday, 24 February 2023 at 12:00 am

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

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

In this guide, we’ll take you through rear derailleur compatibility for both mechanical and electronic groupsets. We also explain key rear derailleur specs and tooth capacity.

This article only covers rear derailleurs. We have a separate guide dedicated to front derailleurs.

Which brand of derailleur should I buy?

"Merida
Dura-Ace R9200 is Shimano’s top-flight road groupset. 
Russell Burton / Immediate Media
"SRAM
Red eTap AXS takes SRAM’s top slot on the road. 
Steve Sayers / Our Media
"Campagnolo
And finally, Super Record EPS 12 is Campagnolo’s premium road groupset. 
Shimano, SRAM and Campagnolo are the three main players in the drivetrain market.

Microshift, Box, Rotor, FSA and others 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 cranksets, chains and cassettes are mostly inter-compatible between brands, generally speaking, shifters and derailleurs aren’t.

This is because Shimano, SRAM and Campagnolo all use different cable pull ratios.

Cable pull ratio is the amount a derailleur moves for every millimetre of cable pulled through by the shifter. Mixing parts with different cable pull ratios will result in very poor shifting.

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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.

Furthermore, cable pull ratios also differ between different ‘speed’ groupsets. For example, an 11-speed Shimano road bike groupset has a different cable ratio from a 10-speed Shimano road bike groupset.

There may even be pull ratio differences between different generations of groupsets with the same number of gears.

There are, of course, exceptions – many Sramshimpagnolo mash-ups that can be cajoled into working.

However, it’s usually easiest to stick to the same brand as your shifters when buying a rear derailleur.

How many gears does my bike have?

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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 your drivetrain 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 your shifter runs through and add ‘one’ to determine the number of gears your drivetrain has.

Derailleur compatibility explained

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Compatibility 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 explained

Shimano road and gravel bike mechanical derailleur compatibility

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Shimano’s road and gravel mechanical shifting tops out at 11-speed.
Oscar Huckle / Our Media
  • 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 you can use an 8- or 9-speed mountain bike derailleur with road shifters, or vice versa
  • 10-speed road components (except and 10-speed GRX gravel and Tiagra 4700 – notes below) are inter-compatible. You could, for example, use an old Ultegra 6700 derailleur with old 105 5700 shifters
  • All 11-speed Shimano road components are inter-compatible – you could use a Dura-Ace R9100 derailleur with a pair of 105 R7000 shifters. This is also true of the brand’s 11-speed GRX components

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

10-speed Tiagra 4700 (and the associated RS405 hydraulic shifters) and GRX400 gravel derailleurs also use the same cable pull ratio as 11-speed road components. This means you could, for example, use an 11-speed derailleur with 10-speed Tiagra 4700 shifters

Shimano mountain bike mechanical derailleur compatibility

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Things are more simple on the mountain bike front.
Oscar Huckle / Our Media
  • 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
  • This is also the case with Shimano’s 11-speed mountain bike components – for example, you could use an XTR M9000 derailleur with a pair of SLX M7000 shifters
  • All 10-speed Shimano mountain bike components are inter-compatible
  • Current 9-speed Shimano mountain bike components are compatible with older 9-speed road and mountain bike components, excluding the aforementioned exception

For clarity, 10-, 11- and 12-speed Shimano road/gravel and mountain bike components are not inter-compatible with each other. This is because they use different cable pull ratios

For example, you couldn’t use a set of road shifters with a mountain bike rear derailleur.

Cable pull converters from brands such as JTek and Wolf Tooth Components exist and will enable you to mess about with your drivetrain configuration to your heart’s content. Microshift also produces drop-bar shifters that will work with mountain bike derailleurs.

Shimano electronic derailleur compatibility explained

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Shimano’s Dura-Ace Di2 RD-R9250 rear derailleur.
Simon von Bromley / Our Media
  • All 12-speed Shimano Di2 road components are compatible with one another
  • All 11-speed Shimano Di2 road, gravel and mountain bike Di2 components are inter-compatible – you could, for example, use an XT M8050 rear derailleur with a pair of Ultegra R8070 shifters
  • Shimano’s latest XT Di2 M8150 12-speed rear derailleurs are designed for electric mountain bikes and are not compatible with non-assisted bikes
  • Dura-Ace 7970 and Ultegra 6770 Di2 10-speed components are not compatible with each other. This is because they use different wires, with the Ultegra variant using the SD50 type that would go onto be used on 11-speed.

Apart from first-generation Dura-Ace Di2, which uses a different wiring harness, the second (11-speed) generations are inter-compatible, across road, gravel and mountain bike groupsets as they all use the SD50 wires and share the same electronics.

While the derailleurs will work, chainlines differ between GRX, Shimano’s road groupsets and its mountain bike groupsets.

The current 12-speed generation of Shimano electronic groupsets have inter-compatible wiring and electronics and use the newer SD300 wire variant.

So, if you smash up your £700 Dura-Ace Di2 R9250 rear derailleur, you can replace it with a £380 Ultegra Di2 R8150 derailleur.

As Shimano revised the wiring on its latest 12-speed road groupsets, they are not backwards-compatible with its 11-speed counterparts. The only exception to this is the 11-speed Di2 time trial shifters, which need to be wired using an EW-AD305 adaptor.

SRAM derailleur compatibility explained

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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 gravel 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 road components are inter-compatible – meaning you could run a 10-speed SRAM Red rear derailleur with a pair of SRAM 22 shifters

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 derailleur can handle all the cassette sprocket sizes you might want to run.

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 they will work with 10-speed road shifters).

SRAM mountain bike derailleur compatability

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A 10-52t cassette from SRAM gives no excuses on the climbs.
Russell Burton / Immediate Media
  • 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
  • SRAM’s latest 12-speed Eagle drivetrains are all cross-compatible with one another

SRAM electronic derailleur compatability

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SRAM’s electronic systems are fully wireless.
Jack Luke / Immediate Media
  • 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’s current 12-speed eTap AXS rear derailleurs are all compatible across road, gravel and mountain biking. For example, you can run an Eagle rear derailleur on a road or gravel bike. If you’re doing this, you’ll need to run a compatible chain (you cannot run a road FlatTop chain with an Eagle rear derailleur), chainring and cassette

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

However, SRAM’s Force AXS Wide groupset means you may not have to create your own mullet builds. Bridging the gap between road-going AXS groupsets and mullet builds, the groupset 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 specific rear derailleur designed to handle 10-36-tooth cassettes. The original eTap AXS Red and Force rear derailleurs are only rated up to 33-tooth cassettes. The older 11-speed Red eTap shifters and rear derailleur aren’t compatible with 12-speed eTap AXS.

Campagnolo derailleur compatibility explained

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Although there are quirks to be aware of on Campagnolo’s legacy groupsets, the latest 12-speed systems are inter-compatible with one another.
Oscar Huckle / Our Media

Campagnolo road and gravel derailleur compatability

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.

  • 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. These and all 10- and 11-speed groupsets from this period are inter-compatible – for example, you could run an Athena derailleur with Record shifters
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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

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.

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).

Compatible parts are now 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 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.

However, things get much easier with its latest 12-speed road groupsets. You can mix and match components between Chorus, Record and Super-Record.

The brand’s Ekar gravel groupset is currently its only 13-speed offering and you can’t run it with the brand’s 12-speed components.

Campagnolo road electronic derailleur compatability

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Campagnolo’s Super Record EPS electronic groupset is now onto its fourth generation.
Matthew Loveridge / Immediate Media

Campagnolo’s Super Record EPS electronic groupset is now onto its fourth generation.

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 derailleur.

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 there’s some reverse compatibility between version three 11-speed EPS components and older generations.

What cage length derailleur should I buy?

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

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

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 (eg, 10-42t or larger) or when there is a large difference between chainring sizes (eg, 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 (ie, 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 (ie, 11-28t or smaller), you can use a short cage derailleur.

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Downhill-specific groupsets use exceptionally short derailleur cages.
Alex Evans

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

You will notice there are lots of ifs and buts in this guide – 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

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 11-speed road derailleur here.

The total capacity of an 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. 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. Shimano, however, wouldn’t condone this.

Clutch derailleurs explained

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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, and making for a far quieter and more reliable drivetrain.

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

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

With clutch derailleurs offering a quieter and more reliable drivetrain, there are few compelling reasons 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 derailleur? To keep the topic simple, we’ve broken things down into weight, longevity and finish:

Weight

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More expensive rear derailleurs will, typically, weigh less.
Tom Marvin / Immediate Media

A more expensive derailleur will usually weigh less than a cheaper one.

This is achieved by using more exotic materials (eg, 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.

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

Finish

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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 nice anodising.

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. If you want to be the coolest person in the bunch, we won’t judge if you splash out on some shiny loveliness.