A complete guide to rear derailleurs

Derailleur compatibility, tooth capacity and much more explained

Is it time to upgrade or replace your rear derailleur? Do you sit awake at night worrying about what a 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.

While we certainly don’t recommend you break out this hot derailleur chat at your next party, 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. Plus, if you’re to believe SRAM, the front derailleur is dead anyway.

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

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

There are of course exceptions and there are lots of bodged, Sramshimpagnolo mashups on the ‘net, 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?

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

You can determine the speed of your groupset by counting the number of cogs on your cassette
You can determine the speed of your groupset by counting the number of cogs on your cassette

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

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. We’ve broken down compatibility by manufacturer.

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

Shimano derailleur compatibility

Older generation 8- and 9-speed Shimano mountain bike and road kit is mostly inter-compatible since until the introduction of Dyna-sys on 10-speed mountain bike groupsets, they both used 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.

This lowly 9-speed, Deore mech is compatible with nearly two decades worth of drivetrains
This lowly 9-speed, Deore mech is compatible with nearly two decades worth of drivetrains

The only exception is pre-1997, 9-speed Dura-Ace gear which won’t play nicely with anything as 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 9000 derailleur with a pair of 105 5800 shifters
  • All 10-speed road components (except Tiagra 4700, see below) are inter-compatible — you could for example use an old Ultegra 6600 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 exception to the above rules is Tiagra 4700 (and the associated RS405 hydraulic shifters) which uses 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, 4700 shifters. 

Shimano mountain bike derailleur compatibility

  • 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

And just for clarity, current generation Shimano road and mountain bike components are not inter-compatible with each other as they use different cable pull ratios. For example, you couldn’t use a set of road shifters with a mountain bike rear derailleur.

If you really must fettle, JTek almost certainly has a solution for you
If you really must fettle, JTek almost certainly has a solution for you

It bears mentioning that cable pull converters from brands such as JTek, Lindarets and Problem Solvers do exist and will allow you to futz about with your drivetrain configuration to your heart’s content.

Di2 — a whole other kettle of fish
Di2 — a whole other kettle of fish

We have purposefully excluded Di2 derailleurs from this guide as the situation is more complicated and warrants a separate guide entirely.

SRAM derailleur compatibility

The situation is a little simpler with the American component giant and we’ve summarised the main points below.

  • 7-/8-/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

10- and 11-speed SRAM road components are intercompatible
10- and 11-speed SRAM road components are intercompatible

Combining 10-speed road shifters with a mountain bike derailleur is popular among those looking for a true, 1X set up on a 'cross or road bike at a lower cost than CX1.

Despite visual similarities, CX1 derailleurs are not compatible with mountain bike shifters
Despite visual similarities, CX1 derailleurs are not compatible with mountain bike shifters

For clarity, despite visual similarities, SRAM's Force CX1 11-speed groupsets are not compatible with 11-speed mountain bike components as they each use different cable pull ratios (though it will work with 10-speed, road shifters).

SRAM Eagle is the only 12-speed drivetrain currently on the market
SRAM Eagle is the only 12-speed drivetrain currently on the market

As it's currently the only 12-speed option on the market, SRAM Eagle components are only compatible with each other.

Campagnolo derailleur compatibility

Campagnolo Super Record — a thing of carbon beauty
Campagnolo Super Record — a thing of carbon beauty

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

But as is typical of Campag', there is 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, and 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

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.

Campag has introduced many changes to its groupsets, resulting in reduced intercompatability
Campag has introduced many changes to its groupsets, resulting in reduced intercompatability

In short, these changes have affected most of its drivetrain parts (different width bottom bracket cups result in altered chain lines, different cable pull ratios, etc) 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 'A' or 'B' 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.

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.

Simply put, 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

A long cage (SGS), Shimano XT rear derailleur designed for super wide-range cassettes
A long cage (SGS), Shimano XT rear derailleur designed for super wide-range cassettes

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.

A medium cage, SRAM Red WiFli rear mech designed for large road cassettes
A medium cage, SRAM Red WiFli rear mech designed for large road cassettes

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.

A short cage, Shimano M820 Saint rear derailleur designed for closely spaced, downhill cassettes
A short cage, Shimano M820 Saint rear derailleur designed for closely spaced, downhill cassettes

If you have a traditional road double drivetrain with a regular cassette (ie, 11-28t or smaller), you will will require a short cage derailleur. Some downhill specific drivetrains also use short cage derailleurs (e.g. Shimano, Saint and SRAM X0 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:

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

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

(36 – 11) + (38 – 26) = 37t capacity

If you're using a triple chainset like this one, you'll need a long cage mech
If you're using a triple chainset like this one, you'll need a long cage mech

For the sake of example, let’s assume you are looking at a Shimano derailleur here. The total capacity of a GS (medium cage) Shimano derailleur is 33t and 45t for a SGS (long cage) derailleur. As such, in this circumstance you would require a long 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 medium length cage derailleur in this circumstance, so long as you avoided extreme (e.g. big and big) gear combinations.

Clutch derailleurs explained

Clutch mechs are almost ubiquitous in the mountain bike world and rightly so
Clutch mechs are almost ubiquitous in the mountain bike world and rightly so

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 tension of this pivot, 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 differs slightly, they both aim to achieve the same thing.

Almost all modern mountain bike derailleurs (and SRAM’s CX1 'cross groupsets) include a clutch mechanism, particularly as the move towards single ring drivetrains becomes more universal.

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 in 2016.

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:

Weight

Simply put, a more expensive derailleur will usually weigh less than it’s cheaper brethren. This is achieved by using more exotic materials (e.g. carbon cages, titanium pivots) in the derailleurs 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

£150, ceramic jockey wheels? Sure
£150, ceramic jockey wheels? Sure

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

This is particularly apparent with jockey wheels, where cheaper ones 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

The Shimano Dura-Ace 9000 rear derailleur is a thing of beauty
The Shimano Dura-Ace 9000 rear derailleur is a thing of beauty

Since everything up to this point has been exhaustingly 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. 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 dude in the bunch, splash out on some shiny loveliness.

Jack Luke

Staff Writer, UK
Jack has been riding and fettling with bikes for his whole life. Always in search of the hippest new niche in cycling, Jack is a self-confessed gravel dork and thinks nothing of bivouacking on a beach after work. Also fond of cup and cone bearings, skids and tan wall tyres.
  • Discipline: Long days in the saddle by either road or mountain bike
  • Preferred Terrain: Happiest when on a rural road by the coast or crossing a remote mountain pass. Also partial to a cheeky gravel adventure or an arduous hike-a-bike.
  • Current Bikes: Custom Genesis Croix de Fer all road adventure wagon, Niner EMD 9.
  • Dream Bike: A rigid 44 Bikes Marauder, all black please.
  • Beer of Choice: Caesar Augustus
  • Location: Bristol, UK

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