Home Cycling Complete guide to rear derailleurs: everything you need to know

Complete guide to rear derailleurs: everything you need to know

If you’ve ever tried to change a rear derailleur, you already know how difficult it can be. The problem is that these derailleurs have evolved into such a specialized piece of engineering that it’s not enough to just look at the thing. Rather, you need to actually know what it is and how it works. This article covers everything you need to know about rear derailleurs, including what they do, and how to use them.

In the world of cycling, the rear derailleur is a device that connects the chain on your bike to the cassette on your rear wheel. It does this by moving the chain in and out of the hub.

If you’re looking for a new road bike, you’ll be interested in knowing that there are a huge variety of available options. Choosing the right one is a matter of deciding what type of riding you want to do, and then finding out which components will suit you best. The classic road bike options are: road racing, mountain biking, and what’s called ‘cross, which is a combination of road and mountain biking.

Is your rear derailleur in need of an update or replacement? Do you toss and turn at night, trying to figure out what “tooth capacity” is? Have you ever wished to know all there is to know about purchasing a rear derailleur, or have you ever wondered which rear derailleur you require? If that’s the case, you’ve arrived to the correct spot.

While we don’t suggest bringing up this hot derailleur topic at your next social gathering, it is unquestionably valuable knowledge if you’re in the market for a new rear derailleur.

It’s important to note that this post just covers rear derailleurs; adding front derailleurs would make this guide much too long. Otherwise, here’s everything there is to know about the back derailleur.

Which derailleur should I purchase?

Rear derailleur

Shimano’s top-of-the-line road groupset is the Dura-Ace 9100. Immediate Media / Matthew Loveridge

SRAM RED eTAP AXS, 12-speed rear mech and SRAM RED XG-1290 cassette on road bike

On the road, the Red eTap AXS takes SRAM’s top spot. Wragg, Tom


Finally, Campagnolo’s top road groupset is the Record EPS 12. Immediate Publication

I’m not sure which rear derailleur I’ll need. The three major companies in the drivetrain industry are Shimano, SRAM, and Campagnolo.

Microshift, Box, Rotor, FSA, and others all produce groupset components, but covering all of them would be beyond the scope of this article. 

It’s recommended not to mix and match drivetrain components from various brands as a general rule. While parts like cranks, chains, and cassettes are usually interchangeable across manufacturers, shifters and derailleurs aren’t.

In a nutshell, Shimano, SRAM, and Campagnolo utilize different cable pull ratios (the amount of movement a derailleur makes for every centimetre of cable pushed through by the shifter), and combining components will result in extremely poor shifting. 

Collage of black and silver bicycle groupset parts on a white background

In general, components from road and mountain bike groupsets will not function together. Various

But it’s a little more complex than that. Even though they’re made by the same company, mountain bike and road cycle groupsets have distinct cable pull ratios.

Ratios may vary as the number of gears increases or decreases, and there may even be variations in pull ratios across generations of the same gear type. 

There are exceptions, and there are plenty of bodged Sramshimpagnolo mashups that can be coaxed into functioning, but for the sake of convenience, we recommend purchasing a rear derailleur from the same manufacturer as your shifters.

What is the number of gears on my bike?

Shimano 105 R7000 groupset review, cassette

Count the number of sprockets on your cassette to figure out how many “gears” your bike has. Immediate Media / Jack Luke

After you’ve decided on a brand, you’ll need to figure out how many gears your groupset contains.

If you’re replacing an existing derailleur on a bike, all you have to do is count the number of gears on your cassette.

If the speed of your drivetrain is unknown, you may count the number of steps your shifter takes and multiply by one to get the number of gears your drivetrain has.

Compatibility with derailleurs is discussed.

A complete guide to rear derailleurs

Shimano’s road and mountain bike groupset compatibility is a little more complex. Immediate Media / Steve Behr

There are a few exceptions to the rule of not mixing and matching groupset parts from various generations or mountain bike and road components. Compatibility has been split down by manufacturer in the table below.

The following information applies to both derailleurs and shifters for clarity.

Compatible with Shimano derailleurs

Because they both utilize the same cable pull ratio, most Shimano mountain bike and road equipment is interchangeable. This implies you may use a road shifter with an 8- or 9-speed mountain bike derailleur, or vice versa. 

The lone exception is Dura-Ace gearing from before 1997, which won’t work with anything since it utilizes a different cable pull ratio. 

Although it may not seem so at first, the situation is a little clearer with Shimano’s latest generation of road and mountain bike components. Please bear with us…

Compatible Shimano road bike derailleurs

  • All Shimano 11-speed road components are interchangeable, so you could use a Dura-Ace 9100 derailleur with a set of 105 7000 shifters, for example.
  • Shimano’s 11-speed GRX gravel groupsets are also compatible with each other.
  • All 10-speed road components are interchangeable (excluding Tiagra 4700 and 10-speed GRX gravel, see below) — for example, you could use an old Ultegra 6700 derailleur with old 105 5700 shifters.
  • With the exception of the aforementioned limitation, current 9-speed road components are backwards compatible with previous 9-speed road and mountain bike components.

The Tiagra 4700 (and accompanying RS405 hydraulic shifters) and GRX400 dirt mechs are the sole exceptions to the above criteria, since they utilize the same cable pull ratio as current generation 11-speed road equipment. 

This means you could, for example, combine a 10-speed Tiagra 4700 shifter with an 11-speed derailleur.

Compatible with Shimano mountain bike derailleurs

  • All Shimano 12-speed mountain bike components are interchangeable, so you could use an SLX M7120 derailleur with an XTR M9100 trigger shifter, for example.
  • All Shimano mountain bike components are interchangeable, so you could use an XTR M9000 derailleur with a set of SLX M7000 shifters, for example.
  • All Shimano 10-speed mountain bike components are also interchangeable, so you could use an old 10-speed XTR M986 rear derailleur with modern Deore M610 shifters, for example.
  • With the exception of the aforementioned limitation, current Shimano mountain bike components are compatible with older 9-speed Shimano road and mountain bike components.

To be clear, contemporary Shimano road and mountain bike components (10-, 11-, and 12-speed) are incompatible with each other since they utilize different cable pull ratios. A set of road shifters, for example, would not work with a mountain bike’s rear derailleur.

Cable pull converters from companies like JTek and Lindarets/Wolf Tooth Components exist, and they’ll let you play about with your drivetrain arrangement to your heart’s content.

Compatible with SRAM derailleurs

SRAM GX Eagle mountain bike drivetrain

SRAM’s new 12-speed GX rear derailleur for mountain bikes. MBUK magazine/Russell Burton

With SRAM, the situation is a bit clearer, and we’ve outlined the key aspects below.

SRAM rear derailleurs are compatible with both road and mountain bikes. 

  • SRAM’s 7-, 8-, and 9-speed components are completely interchangeable, whether they’re for a road or mountain bike.
  • SRAM 10-speed components are interchangeable across road and mountain bike applications; for example, road shifters may be used with a mountain bike rear derailleur.
  • Components from SRAM’s 10- and 11-speed mountain bikes are not interchangeable.
  • SRAM road components for 10- and 11-speeds are interchangeable, so you could use a 10-speed SRAM Red rear derailleur with a set of SRAM 22 shifters.
  • SRAM components for 12-speed are not backwards compatible with 11-speed. eTap batteries, on the other hand, are compatible with all 12-speed AXS and 11-speed components.

SRAM has also been at the forefront of the drive for ever-larger gear ranges, with its latest rear derailleurs built to accommodate cassettes with a wider range. As a result, you should think about whether a new mech can accommodate all of the cassette sprocket sizes you may wish to use.

SRAM’s Force 1 11-speed groupsets, despite their apparent similarity, are not compatible with 11-speed mountain bike components because they utilize different cable pull ratios (though it will work with 10-speed road shifters).

Compatibility with Campagnolo derailleurs

Campagnolo Centaur groupset

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

Because there are no mountain bike groupsets to deal with (yes, we know it made Euclid and a bunch of other off-road parts long back), cross-compatibility across various generations of groupsets is pretty simple to comprehend. 

However, there is an unusual flourish of awkwardness, as is characteristic of Campag.

Compatibility with Campagnolo rear derailleurs 

  • Before mid-2001, all Campagnolo 8- and 9-speed groupsets had the same pull ratio and were interchangeable. This generation of components is dubbed “Campy old” by many.
  • Campagnolo began utilizing a revised pull ratio for their newer 9-speed kit after mid-2001, and these, as well as any 10-speed (and 11-speed) groupsets from this time period, are interchangeable – for example, you could use an Athena derailleur with Record shifters.

Campagnolo compatibility squares letter A

Many modifications have been made to Campagnolo’s groupsets, resulting in less compatibility across generations. As a result, boxed letters (showing ranges) can help you figure out whether your components are compatible. Campagnolo

However, minor changes to all of Campagnolo’s groupsets have resulted in decreased inter-compatibility across groupsets, making things more complex (or easy, depending on how you look at it).

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

Campagnolo offers a far more detailed explanation of the modifications here, and we strongly advise you to read it well before purchasing any new components from the Italian manufacturer.

There are also variations in the pull ratios employed by various groupsets: mechanical Super Record, Record, and Chorus 11-speed Ergopower shifters are incompatible with Potenza, which is incompatible with both Ergopower and Centaur Power-Shift systems.

compatibility with electronic groupsets

SRAM Force eTap AXS wide gearing

Electronic groupsets aren’t the standard, but they’re becoming more popular every year. Immediate Media / Jack Luke

If your bike has an electronic groupset, or you’re considering about getting one, you’ll need to stick to one brand since the groupsets from various manufacturers utilize different systems. 

With the exception of the first-generation Dura-Ace Di2, which has a separate wire harness, subsequent Shimano electronic groupsets — road, MTB, and GRX – feature inter-compatible wiring and electronics.

You may replace your £550 Dura-Ace Di2 R9150 rear mech with a £250 Ultegra Di2 R8050 derailleur if you smash it up. While the mechs will function, the chainlines for GRX, Shimano’s road groupsets, and Shimano’s mountain bike groupsets vary. 

SRAM’s new 12-speed eTap AXS road and MTB groupsets may be combined, allowing for mullet designs, which combine road shifters with an Eagle AXS rear mech and a 10-50 cassette for an ultra-wide dirt bike construction. 

That was fantastic until SRAM released the Force AXS Wide groupset. It bridges the gap between road-going AXS groupsets and mullet builds by providing a lower bottom gear for road riding and a decent range for gravel riding with cassettes up to 36 teeth, but by changing the drivetrain orientation. 

This implies there’s a new rear mech that can handle cassettes with 10-36 teeth. Only 33-tooth cassettes are supported with the original eTap AXS Red and Force rear mechs. The rear mech and shifters for the previous 11-speed Red eTap shifters aren’t compatible with the 12-speed eTap AXS.

Road bike against wall

The fourth version of Campagnolo’s Super Record EPS electronic groupset is now available. Immediate Media / Matthew Loveridge

The Super Record EPS electronic groupset from Campagnolo is currently in version four and has been upgraded to 12-speed. It’s a self-contained system with its own electronics for communication between the shifters and the rear mech.

Furthermore, components for the disc and rim braking systems are incompatible. There is no backward compatibility with previous 11-speed EPS models, nor is there any forward compatibility when upgrading from 11-speed to 12-speed.

The good news is that earlier versions of EPS components are backwards compatible with version three 11-speed EPS components.

What derailleur cage length should I get?

Now that we know the speed, manufacturer, and compatibility of your derailleur, we need to figure out if your drivetrain needs a long, short, or medium cage.

The range, or spread, of gears you may have on your bike is determined by the length of your derailleur’s cage; the longer the cage, the more slack in the chain the derailleur can take up.

We’ve provided a short guide below for convenience, but if you’re still unsure, read on to discover how we came to these findings.

Quick guide to derailleur cage length

When utilizing a super-wide range cassette (e.g. 10-42t or bigger) or when there is a significant gap between chainring sizes on your bike, there may be a large range, or spread, of gears on your bike (e.g. when using a triple chainset). You’ll need a long cage derailleur in these situations.

You may wish to utilize a medium cage derailleur if you’re using a 1x drivetrain with a regular-sized cassette (i.e. 11-36t or smaller) or certain 2x mountain bike drivetrains with a similar-sized cassette.

A short cage derailleur may be used with a conventional road double-drivetrain with a normal cassette (11-28t or smaller). 

Scott Gambler 900 Tuned downhill mountain bike

Short derailleur cages are used in downhill-specific groupsets. Alex Evans is a writer who lives in the United

Short cage derailleurs are also used in certain downhill-specific drivetrains (e.g. Shimano, Saint and SRAM X01 DH). 

There are a lot of ifs and buts in this tutorial since there are just too many variables to provide a definitive answer in every case.

The tooth capacity of a derailleur is explained.

You’ll need to look up your derailleur’s ‘tooth capacity’ to obtain a definite answer. Calculate the necessary tooth capacity of your bike using the following formula:

How to Work Out Your Tooth Capacity

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

So, for a contemporary double chainring road bike transmission, it would look like this:

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

Let’s pretend you’re looking at a Shimano rear derailleur for the purpose of illustration.

A Shimano SS (short cage) derailleur has a total capacity of 35t, whereas a GS (medium cage) derailleur has a total capacity of 39t. As a result, you’ll need a medium cage derailleur in this situation. 

It’s worth noting that derailleur capacity specifications are often conservative, and you could almost definitely get away with using a short length cage derailleur in this situation if you avoided extreme gear combinations (e.g. large and huge).

Clutch derailleurs are described in detail.

GRX rear derailleur

Clutch-equipped rear derailleurs were previously only found on mountain bike groupsets, but Shimano and SRAM now offer them on both gravel and road groupsets. Immediate Media / Matthew Loveridge

To hold the chain in position, regular rear derailleurs depend entirely on the tension supplied by the main pivot. A clutch derailleur simply increases the resistance provided by this pivot – either via a clutch or more sophisticated electro-hydraulic systems, as seen on SRAM AXS – preventing fore and aft movement of the derailleur cage, resulting in a much quieter and more dependable drivetrain.

Although SRAM and Shimano have somewhat different conceptions of a clutch derailleur, they both strive to accomplish the same objective. 

A clutch mechanism is found in almost all contemporary mountain bike derailleurs (as well as Shimano’s GRX gravel groupsets, Ultegra RX rear mech, SRAM’s single-ring road groupsets, and eTap AXS 12-speed road rear mechs).

There’s little reason not to purchase a clutch derailleur for a mountain bike, given that they provide a quieter and more dependable transmission.

What do I get if I spend extra on a derailleur?

Now that we’ve gone through the painstaking process of determining what would work with your powertrain, the fun part begins: settling on a pricing range.

What, on the other hand, should you look for in a rear mech? We’ve split things down into weight, lifespan, and finish to make things simple:


Shimano XTR and SRAM Eagle AXS derailleur

The weight of more costly rear derailleurs is usually lower. Immediate Media / Tom Marvin

A more costly derailleur will typically be lighter than its less priced counterparts. This is accomplished by utilizing more exotic materials in the derailleur’s construction (e.g. carbon cages, titanium pivots) and cutting away more superfluous material.

If losing weight is your first priority, you’ll have to spend a little more money.


More costly derailleurs utilize more durable components and are manufactured to tighter tolerances, so they will last longer than less expensive ones.

This is especially noticeable with jockey wheels, as less expensive derailleurs often spin on simple steel bushings.

These, predictably, don’t last as long as higher-end equivalents that use cartridge bearings or ceramic bushings.


BMC is still running Dura-Ace 9000 series groupsets

Shimano’s R9000-era Dura-Ace groupset created some of the best-looking rear derailleurs of all time, in our opinion. Immediate / Colin Levitch

We can afford to be superficial for a minute since everything up to this point has been tech-heavy.

Higher-end derailleurs may have jewel-like quality, with a variety of gorgeous polished panels and jazzy, brilliant anodizing. 

While it is said that you should never judge a book by its cover, bike geeks have a tendency of evaluating a bike by its rear derailleur, and we won’t hold it against you if you want to be the coolest person in the group.

The rear derailleur is the most important part of the bike, and knowing the basics is the key to becoming a bike mechanic. This guide looks at the rear derailleur, its parts and how they work together. It covers the different types and how to adjust them, as well as maintenance and how to keep them running smoothly.. Read more about rear derailleur hanger and let us know what you think.

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Frequently Asked Questions

How do I choose a rear derailleur?

The rear derailleur is the component that moves the chain from one sprocket to another. It is located at the back of your bike, and it has a cable that runs through it.

How do I choose a derailleur?

The derailleur is the component that connects your chain to the rear wheel of your bike. It allows you to change gears by moving a lever on the side of the bike.

How do I know what size rear derailleur I have?

You can find out what size your rear derailleur is by looking at the chainring and cog on the front of your bike. The chainring should be a multiple of teeth, such as 32 teeth, 34 teeth, 36 teeth. The cog should also be a multiple of teeth, such as 12-27-34.

Related Tags

This article broadly covered the following related topics:

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