Is it time to upgrade or replace your rear derailleur? Are you plagued by sleepless nights contemplating the intricacies of “tooth capacity”? Have you ever harbored a burning desire to acquire a comprehensive understanding of purchasing a rear derailleur or pondered endlessly, “which rear derailleur do I need?” If you find yourself relating to any of these questions, then you have arrived at the optimal destination.
While we advise against engaging in deep derailleur discussions at social gatherings, the information we are about to divulge is indispensable if you are in the market for a new rear derailleur or seeking an upgrade.
Within this comprehensive guide, we shall navigate the realm of rear derailleur compatibility for both mechanical and electronic groupsets. Furthermore, we shall elucidate the essential specifications and intricacies associated with tooth capacity.
Please note that this particular article focuses solely on rear derailleurs. For a dedicated guide exploring the realm of front derailleurs, we have a separate resource available.
Which brand Rears should you consider?
Dura-Ace R9200 shines as Shimano’s unparalleled road groupset, occupying the position of utmost prestige. Russell Burton / Immediate Media
In the realm of road cycling, Red eTap AXS stands tall as SRAM’s premier choice. Steve Sayers / Our Media
Lastly, Campagnolo’s Super Record EPS 12 emerges as the epitome of excellence within their premium road groupset collection.
Drivetrain Market
Shimano, SRAM, and Campagnolo dominate the drivetrain market. But there are other players like Microshift, Box, Rotor, FSA, and more. However, in this article, we’ll focus on the three main players.
Mixing and Matching Components
It’s generally advised not to mix and match drivetrain components from different brands. While cranksets, chains, and cassettes are usually inter-compatible between brands, the same cannot be said for shifters and derailleurs.
Why is that? Well, it all comes down to cable pull ratios. Shimano, SRAM, and Campagnolo each have their own unique cable pull ratios. So if you mix parts with different ratios, your shifting performance will suffer greatly.
- Road and mountain bike groupset components, in most cases, are not compatible with each other.
- But, as with many things in cycling, it’s more complex than a simple yes or no.
The Cable Pull Ratio Conundrum
Even when you compare mountain bike groupsets and road groupsets from the same brand, you’ll find that they usually have different cable pull ratios. This adds another layer of complexity to the mix.
So, when it comes to choosing and upgrading drivetrain components, it’s crucial to consider the compatibility of cable pull ratios within the same brand. It’s a delicate balancing act between finding the right components that work together harmoniously.
Furthermore, cable pull ratios also vary between different ‘speed’ groupsets. For instance, a 11-speed Shimano road bike groupset and a 10-speed Shimano road bike groupset have distinct cable ratios.
There may even be pull ratio differences between various generations of groupsets with the same number of gears.
Of course, there are exceptions – numerous Sramshimpagnolo mash-ups that can be coerced into functioning.
However, it is typically easiest to stick with the same brand as your shifters when purchasing a rear derailleur.
How many gears does my bike have?
To determine how many ‘gears’ your bike has, simply count the number of sprockets on your cassette.
Jack Luke / Immediate Media
Once you have settled on the brand, you must now ascertain the number of gears in your drivetrain.
If you are replacing an existing derailleur on your bike, just count the number of cogs on your cassette and you’re good to go.
If the speed of your drivetrain is unknown, you can count the number of steps your shifter goes through and add ‘one’ to figure out the number of gears in your drivetrain.
Derailleur compatibility explained
Compatibility across Shimano’s road and mountain bike groupsets is a little intricate.
Steve Behr / Immediate Media
As a general rule, it is advisable not to mix and match groupset parts from different generations or combine mountain bike and road components. However, there are a few exceptions. Below, we have provided compatibility breakdown by manufacturer.
For clarity, the following information is relevant for both derailleurs and shifters.
Explanation of Shimano Derailleur Compatibility
Shimano’s mechanical shifting system for road and gravel bikes is limited to 11-speed. Oscar Huckle / Our Media
- Most 8- and 9-speed Shimano mountain bike and road equipment are compatible with each other due to the same cable pull ratio. This means that you can use an 8- or 9-speed mountain bike derailleur with road shifters, and vice versa.
- 10-speed road components (excluding 10-speed GRX gravel and Tiagra 4700 – see notes below) are also interchangeable. For instance, it is possible to use an old Ultegra 6700 derailleur with old 105 5700 shifters.
- All 11-speed Shimano road components, including GRX, are compatible with each other. So, you could use a Dura-Ace R9100 derailleur with a pair of 105 R7000 shifters.
*One exception for 9-speed groupsets is the pre-1997 Dura-Ace gearing, which is not compatible with anything due to its unique cable pull ratio.
Furthermore, 10-speed Tiagra 4700 (and the associated RS405 hydraulic shifters) and GRX400 gravel derailleurs have the same cable pull ratio as 11-speed road components. This means that you could use an 11-speed derailleur with 10-speed Tiagra 4700 shifters.
Shimano’s mountain bike mechanical derailleur compatibility is simpler:
- All 12-speed Shimano mountain bike components are compatible with each other. For example, you can use an SLX M7120 derailleur with an XTR M9100 trigger shifter.
Shimano Mountain Bike Component Compatibility
Introduction
In the world of mountain biking, compatibility between components is crucial. This is also true for Shimano’s 11-speed mountain bike components. For instance, you have the option to combine an XTR M9000 derailleur with a pair of SLX M7000 shifters.
10-Speed Shimano Inter-Compatibility
All 10-speed Shimano mountain bike components are designed to work together seamlessly.
9-Speed Compatibility
The current 9-speed Shimano mountain bike components are compatible with older 9-speed road and mountain bike components, except for the aforementioned exception.
However, there is an important distinction to be made. The 10-, 11-, and 12-speed Shimano road/gravel and mountain bike components are not inter-compatible with each other due to their different cable pull ratios.
Distinct Cable Pull Ratios
For example, you cannot use a set of road shifters with a mountain bike rear derailleur. The cable pull ratios do not align.
Fortunately, there are cable pull converters available from reputable brands like JTek and Wolf Tooth Components. These converters enable you to experiment freely with your drivetrain configuration. Additionally, Microshift produces drop-bar shifters that are compatible with mountain bike derailleurs.
Shimano Electronic Derailleur Compatibility
Overview
Shimano’s electronic derailleur compatibility may seem complex, but here’s a breakdown:
12-Speed Di2 Components
All 12-speed Shimano Di2 road components are designed to be compatible with one another, ensuring a seamless integration.
11-Speed Di2 Inter-Compatibility
All 11-speed Shimano Di2 road, gravel, and mountain bike components can work harmoniously together. For example, you have the freedom to pair an XT M8050 rear derailleur with a set of Ultegra R8070 shifters.
Specific Compatibility for Electric Mountain Bikes
Shimano’s latest XT Di2 M8150 12-speed rear derailleurs are specifically designed for electric mountain bikes, so they are not compatible with non-assisted bikes.
Incompatibility Between 10-Speed Di2 Components
Take note that Dura-Ace 7970 and Ultegra 6770 Di2 10-speed components are not compatible with each other. This is due to the use of different wires, with the Ultegra variant adopting the SD50 type, which would later be utilized in 11-speed systems.
Inter-Compatibility of Second (11-Speed) Generations
With the exception of the first-generation Dura-Ace Di2, which utilizes a distinct wiring harness, the second-generation (11-speed) Shimano Di2 components are inter-compatible across road, gravel, and mountain bike groupsets. They all employ the SD50 wires and share the same electronics.
Differences in Chainlines
While the inter-compatible derailleurs will function properly, it’s essential to note that chainlines differ between GRX, Shimano’s road groupsets, and its mountain bike groupsets.
Inter-Compatibility of 12-Speed Generations
Lastly, the current 12-speed generation of Shimano electronic groupsets features inter-compatible wiring and electronics, utilizing the newer SD300 wire variant for enhanced performance and compatibility.
Dura-Ace Di2 and Ultegra Di2 Rear Derailleur Compatibility
So, if you happen to damage your expensive Dura-Ace Di2 R9250 rear derailleur costing £700, do not fret! You can easily replace it with the more affordable Ultegra Di2 R8150 derailleur, priced at only £380.
Shimano has made significant changes to the wiring in their latest 12-speed road groupsets, rendering them incompatible with their 11-speed counterparts. However, there is one exception to this rule – the 11-speed Di2 time trial shifters. To make them compatible, you will need to connect them using an EW-AD305 adaptor.
Understanding SRAM Derailleur Compatibility
Let’s delve into the compatibility of SRAM derailleurs, particularly with their latest 12-speed GX mountain bike rear derailleur. Allow us to provide a concise summary of the key points:
SRAM road and gravel derailleur compatibility:
– SRAM components that are 7, 8, and 9-speed are fully interchangeable, regardless of whether they are designed for road or mountain bikes.
– Both road and mountain bike 10-speed SRAM components can be mixed and matched. For instance, you can use road shifters with a mountain bike rear derailleur.
– 10-speed SRAM road components are also compatible with 11-speed SRAM road components. This means you can use a 10-speed SRAM Red rear derailleur with a pair of SRAM 22 shifters.
It’s worth noting that SRAM has been a pioneer in developing wider gear ranges. Consequently, their newer rear derailleurs are designed to handle cassettes with a wider range of sprocket sizes. Therefore, it’s crucial to consider whether a replacement derailleur can accommodate all the cassette sprocket sizes you desire.
Despite their visual similarities, SRAM’s Force 1 11-speed groupsets are not compatible with 11-speed mountain bike components due to differences in cable pull ratios. However, they can still function with 10-speed road shifters.
Compatibility of SRAM Mountain Bike Derailleurs
Now, let’s focus on SRAM mountain bike derailleurs. Here is an overview of their compatibility:
– SRAM components that are 7, 8, and 9-speed are fully interchangeable, whether they are meant for road or mountain bikes.
SRAM Derailleur Compatibility
- SRAM’s 10-speed components, whether for road or mountain bikes, are fully inter-compatible. This means you can mix and match road shifters with a mountain bike rear derailleur, for instance.
- However, SRAM’s 10- and 11-speed mountain bike components are not compatible with each other.
- On the other hand, SRAM’s latest 12-speed Eagle drivetrains are completely cross-compatible. This means that all 12-speed Eagle components can be used interchangeably with one another.
SRAM Electronic Derailleur Compatibility
- When it comes to SRAM’s electronic systems, they are entirely wireless. This means that you can enjoy full compatibility without the hassle of wires.
- Although 12-speed SRAM components are generally not backwards-compatible with 11-speed systems, there is an exception for eTap batteries. These batteries are compatible with both 12-speed AXS and 11-speed components.
- In terms of rear derailleurs, SRAM’s current 12-speed eTap AXS models are compatible across road, gravel, and mountain biking. For instance, you can use an Eagle rear derailleur on a road or gravel bike. However, do keep in mind that you’ll need a compatible chain, chainring, and cassette for this setup. You cannot use a road FlatTop chain with an Eagle rear derailleur, for example.
SRAM eTap AXS Road and MTB Compatibility
SRAM’s latest 12-speed eTap AXS road and MTB groupsets offer the option for “mullet builds.” These builds involve pairing road shifters with an Eagle AXS rear derailleur and a 10-50 cassette, resulting in an ultra-wide gravel bike build.
Alternatively, you may not need to create your own mullet builds thanks to SRAM’s Force AXS Wide groupset. This groupset bridges the gap between road-going AXS groupsets and mullet builds by offering a lower bottom gear for road riding and a suitable range for gravel riding with cassettes up to 36 teeth. However, it achieves this by altering the drivetrain alignment.
It’s important to note that there is a specific rear derailleur designed for handling 10-36-tooth cassettes. The original eTap AXS Red and Force rear derailleurs are only rated for use with up to 33-tooth cassettes. Additionally, the older 11-speed Red eTap shifters and rear derailleur are not compatible with 12-speed eTap AXS systems.
Campagnolo Derailleur Compatibility Explained
When it comes to Campagnolo’s legacy groupsets, there are some quirks to be aware of. However, the latest 12-speed systems from Campagnolo are fully inter-compatible with one another.
Unlike SRAM, Campagnolo does not have mountain bike groupsets, making cross-compatibility between different generations of groupsets relatively straightforward.
But as is typical of Campagnolo, there’s the occasional touch of awkwardness.
All 8- and 9-speed Campagnolo groupsets prior to mid-2001 shared the same pull ratio and are mutually compatible. This particular generation is often referred to as the ‘Campy old’.
- After mid-2001, Campagnolo adopted a revised pull ratio for its newer 9-speed equipment. These and all 10- and 11-speed groupsets from this period can be interchanged. For example, you could pair an Athena derailleur with Record shifters.
Campagnolo has implemented numerous changes to its groupsets, resulting in diminished compatibility across different generations. Thus, marked letters (indicating the ranges) will assist you in determining whether your components are compatible.
So boxed letters (indicating the ranges) will aid you in identifying if your parts are compatible.
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.
However, now things become more intricate (or simpler, depending on perspective) with slight modifications to all of Campagnolo’s groupsets, leading to decreased interchangeability among them.
In short, these changes have impacted most of its drivetrain parts (different width bottom bracket cups result in altered chainlines, different cable pull ratios, etc.).
Compatible parts are now designated by a letter enclosed in a square box. Simply put, if all your components bear the same stamped letter, they will work together.
Campagnolo provides a more comprehensive description of the changes, and we highly recommend you carefully peruse this guide before committing to any new parts from the Italian brand. Click here for more information.
Furthermore, there are variations in the pull ratios used between different groupsets. The Ergopower shifters employed 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, matters become much simpler with the latest 12-speed road groupsets. You can mix and match components between Chorus, Record, and Super-Record.
It is worth noting that Campagnolo’s Ekar gravel groupset, currently its only 13-speed offering, cannot be combined with the brand’s 12-speed components.
Campagnolo’s Super Record EPS electronic groupset is now in its fourth generation.
Campagnolo’s Super Record EPS electronic groupset is currently in its fourth version and has now been upgraded to a 12-speed system. Once again, it stands as a complete and self-contained unit, boasting its own electronic components that facilitate communication between the shifters and the rear derailleur.
It is worth noting that the components for the disc brake system and the rim brake system are not compatible with one another. Furthermore, if you are using an older 11-speed EPS groupset, there is no backward compatibility to seamlessly integrate the new 12-speed system. Similarly, there is no forward compatibility to upgrade from an 11-speed to a 12-speed system.
However, there is some positive news in terms of reverse compatibility. The third version of the 11-speed EPS components can still be utilized alongside earlier generations of the groupset.
Determining the Appropriate Derailleur Cage Length
Now that we have considered the speed, brand, and compatibility of your derailleur, it is essential to determine the ideal cage length for your drivetrain. There are three options: long, short, or medium.
The length of your derailleur’s cage dictates the range of gears that can be accommodated on your bike. A longer cage allows for more slack in the chain that the derailleur can effectively manage.
While we have included a concise guide below, if you have any uncertainties, continue reading to gain a comprehensive understanding of the selection process.
Quick Guide for Derailleur Cage Length
There are two scenarios where you may encounter a wide range or spread of gears on your bike. The first is when you are using a super-wide range cassette, such as 10-42t or larger. The second is when there is a significant difference in size between your chainrings, particularly with a triple chainset.
In these situations, a long cage derailleur is necessary.
If you are running a 1x drivetrain with a regular-sized cassette (11-36t or smaller) or certain 2x mountain bike drivetrains that feature a similarly sized cassette, a medium cage derailleur may be more suitable.
For those with a traditional road double-drivetrain and a regular cassette (11-28t or smaller), a short cage derailleur can be utilized.
It is important to highlight that downhill-specific groupsets often employ exceptionally short derailleur cages, as can be seen in examples like Shimano, Saint, and SRAM X01 DH.
As you can see, this guide comes with several caveats, as there are numerous variables to consider in each unique situation.
Understanding Derailleur Tooth Capacity
To obtain a definitive answer, you must refer to the “tooth capacity” of your derailleur. This can be calculated by following the guidelines below:
How to Calculate Tooth Capacity:
To determine the tooth capacity, you can use the following formula:
(largest cog – smallest cog) + (largest chainring – smallest chainring) = required capacity
For example, in a modern, double chainring road bike drivetrain, the calculation would be:
(32 – 11) + (52 – 36) = 37t capacity
Let’s assume we’re examining a Shimano 11-speed road derailleur for demonstration purposes.
The short cage version of a Shimano derailleur has a total capacity of 35t, while the medium cage version has a capacity of 39t. In this scenario, you would need a medium cage derailleur.
It’s worth mentioning that the quoted capacity of derailleurs is often conservative. In practical terms, you could most likely use a short length cage derailleur in this situation, as long as you avoid extreme gear combinations. However, Shimano does not endorse this approach.
Clutch Derailleurs Explained:
Clutch-equipped rear derailleurs were originally found only on mountain bikes, but they are now also seen on gravel and road groupsets from both Shimano and SRAM.
Regular rear derailleurs rely solely on the tension provided by the main pivot to keep the chain in place. Clutch derailleurs, on the other hand, enhance this by increasing the resistance of the pivot through a clutch mechanism or more advanced electro-hydraulic systems found in SRAM AXS.
The increased resistance prevents excessive movement of the derailleur cage, resulting in a quieter and more reliable drivetrain. While SRAM, Shimano, and Campagnolo have slightly different interpretations of clutch derailleurs, they all aim to achieve the same objective.
Most modern mountain bike derailleurs and various road groupsets, including 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, incorporate a clutch mechanism.
Given the benefits of a quieter and more reliable drivetrain, there are few reasons not to opt for a clutch derailleur for a mountain bike.
What Do I Get with a More Expensive Derailleur?
Now that we’ve covered the technical aspects of determining compatibility with your drivetrain, it’s time to decide on the price point.
But what factors should you consider when selecting a rear derailleur? To simplify things, let’s focus on weight, longevity, and finish:
Weight
Typically, higher-end rear derailleurs will weigh less than their cheaper counterparts. This weight reduction is achieved through the use of exotic materials such as carbon cages and titanium pivots, as well as meticulous machining to remove excess material. If weight is of utmost importance to you, be prepared to invest more.
Longevity
More expensive derailleurs often incorporate durable components and are built to tighter tolerances. As a result, they tend to last longer than cheaper models. This is particularly noticeable with jockey wheels, where cheaper derailleurs often feature basic steel bushings that wear out faster. Higher-end alternatives utilize cartridge bearings or ceramic bushings for improved longevity.
Finish
Finally, aesthetics. While it’s true that looks aren’t everything, higher-end derailleurs often boast exquisite quality and polished panels with appealing anodizing. Shimano’s R9000-era Dura-Ace groupset, for instance, produced rear derailleurs that are considered some of the most visually attractive in cycling history. So, if you want to stand out in the crowd, treating yourself to some sleek elegance might be worth considering.
