A Comprehensive Guide to Rear Derailleurs
Thinking about upgrading or replacing your rear derailleur but not sure which specifications to choose or what suits you best? Don’t worry—this guide has got you covered. These details are essential and worth your time to understand.
Here, we’ll walk you through the compatibility of mechanical and electronic shifting systems, along with key specifications and gear ratio pairing tips, to help you find the perfect fit for your needs.
Which Brand of Rear Derailleur Should I Buy?
The three biggest drivers are Shimano, SRAM, and Campagnolo. But keep in mind Campagnolo provides nothing for mountain bikes since they solely concentrate on road and dirt settings. Though we cannot cover everything here, other brands such as Microshift, Box, Rotor, and FSA also manufacture drivetrain components.
Generally speaking, mixing and matching components from several brands is not a good idea. Most cranksets, chains, and cassettes are essentially cross-compatible; shifters and derailleurs usually aren't. For mechanical groupsets, Shimano, SRAM, and Campagnolo all employ various cable pull ratios; electronic shifting systems run on their own special communication protocols. Later in the book, we will get into these specifics.
For every millimeter of cable the shifter pulls, cable pull ratio describes how much the derailleur moves. Combining parts with varying cable pull ratios could cause horrible shifting performance and possibly cause you to go insane.
The matter is more complex than it first seems. For instance, whilst SRAM's mountain and road components have the same ratio, Shimano's mountain and road groupsets employ distinct cable pull ratios.
The cable pull ratio also differs among groupsets at varying speeds. Shimano's 11-speed road groups, for example, have a different ratio than their 10-speed road groupsets. Variations exist even within the same speed category across generations of groupsets. Having said that, with the correct configuration, you can occasionally mix and match; third-party components can help close compatibility issues.
To be honest, though, following components from the same brand and groupings is the easiest approach to prevent all these problems. It is the most hassle-free fix available.
Finding the number of speeds in your drivetrain comes second once you have selected a manufacturer.
Just count the cogs on your cassette if you are replacing a current rear derailleur.
If you only have the shifter and it is not installed yet, just move the shifter and count the gear changes it lets, then add 1—that is your speed count.
Rear Derailleur Compatibility Explained
In general, it’s not a good idea to mix components from different generations or for different purposes, but there are exceptions. Below, we’ve broken down compatibility by manufacturer.
The following information applies to shifters and rear derailleurs.
Shimano Compatibility Explained
Shimano Road and Gravel Mechanical Shifting Compatibility
Complementing the 2x12-speed Shimano GRX RX820 is the 12-speed Shimano 105. Shimano GRX RX820 shifters, for instance, can be coupled without problems with a Shimano 105 R8100 rear derailleur.
Every 11-speed Shimano road group set fits perfectly with the others. You can thus combine a Dura-Ace R9100 rear derailleur with a pair of 105 R7000 shifters. The same compatibility holds for Shimano's 11-speed GRX gravel setups.
Apart from 10-speed GRX and Tiagra 4700, 10-speed road groupsets are also compatible with each other. One might combine an Ultegra 6700 rear derailleur with 105 5700 shifters, for example.
Given their comparable cable pull ratio, most 8-speed and 9-speed Shimano mountain and road groupsets are also compatible. This means that vice versa, you may use 8-speed or 9-speed mountain shifters with road rear derailleurs.
Important Note: The pre-1997 Dura-Ace is the lone exception for 9-speed systems since it cannot be properly coupled with any other component and employs a somewhat different cable pull ratio.
Shimano Mountain Bike Mechanical Shifting Compatibility
All 12-speed Shimano mountain bike groupsets are fully compatible with each other. For example, you can pair an SLX M7120 rear derailleur with an XTR M9100 shifter without any issues.
Similarly, 11-speed Shimano mountain bike groupsets are also cross-compatible. For instance, you can mix an XTR M9000 rear derailleur with an SLX M7000 shifter seamlessly.
As for 10-speed Shimano mountain bike groupsets, they are all compatible as well.
Currently, 9-speed Shimano mountain bike groupsets remain compatible with older 9-speed road and mountain bike groupsets, except for the exceptions mentioned earlier.
However, Shimano’s 10-speed, 11-speed, and 12-speed road/gravel and mountain bike groupsets are not compatible with each other, as they use different cable pull ratios. This means you cannot pair a road shifter with a mountain bike rear derailleur.
Brands like Ratio Components, JTek, and WolfTooth Components have developed cable pull converters, allowing you to mix and match drivetrains more freely. Additionally, Microshift offers grip shifters that are compatible with Shimano mountain bike derailleurs.
Recognizing the inconvenience of multiple incompatible systems for both riders and bike shops stocking spare parts, Shimano introduced the Cues system. Cues replaces all Shimano 9-speed, 10-speed, and 11-speed groupsets with a unified system that uses the same cable pull ratio. The first products are designed for flat-bar bikes, and drop-bar Cues groupsets are currently in development.
Shimano Electronic Shifting Compatibility Explained
Every 12-speed Shimano Di2 road and gravel group set fits perfectly.
Cross-compatibility exists also among all 11-speed Shimano Di2 road, gravel, and mountain bike groupsets. For example, there are no problems matching an XT M8050 rear derailleur with Ultegra R8070 shifters.
Regular bicycles cannot be used on the Shimano XT Di2 M8150 12-speed rear derailleur, though; it is intended especially for electric mountain bikes.
Different wiring systems make older 10-speed Dura-Ace 7970 and Ultegra 6770 incompatible with one another. SD50 cables—also carried over into the 11-speed Di2 era—are used in the Ultegra groupset.
Apart from the special wiring of Dura-Ace Di2, the second-generation 11-speed Di2 groupsets are totally interchangeable across road, gravel, and mountain bike configurations. Their electronic components are the same, and they all use SD50 cables.
Although you can mix and match groupsets from several categories, keep in mind that Shimano's groupsets vary in their type-dependent chainlines—that is, the distance between the chainrings, derailleur, and cassette centerline.
All 12-speed Shimano electronic groupsets today use the newer SD300 cables; the wiring connections and electronic components are compatible with one another. For a reasonably priced solution, say your Dura-Ace Di2 R9250 rear derailleur is damaged, and you may replace it with the more reasonably priced Ultegra Di2 R8150 or even the 105 Di2 R7150.
Shimano changed the wiring and connectors for their newest 12-speed road groupsets; hence, 11-speed Di2 groupsets are not backward-compatible. The 11-speed Di2 time trial shifter is the one exception; the EW-AD305 adaptor allows one to attach it.
SRAM Compatibility Explained
SRAM Road and Gravel Mechanical Shifting Compatibility
Currently the only mechanical SRAM road/gravel groupset available, the 12-speed SRAM Apex Eagle fits SRAM Eagle mechanical mountain bike groupsets.
Cross-compatible are 10-speed and 11-speed SRAM road groupsets. For instance, there are no problems pairing a 10-speed SRAM Red rear derailleur with SRAM 22 shifters.
Road or mountain, 10-speed SRAM groupsets are also compatible with each other. This allows you to mix mountain bike rear derailleurs with road shifters and still have appropriate functioning.
Likewise, road and mountain bike configurations allow 7-speed, 8-speed, and 9-speed SRAM groupsets to be totally interchangeable.
Wider gear ranges have been advocated by SRAM as their newest cassettes have more ratios. To guarantee seamless performance, though, it's crucial to make sure a rear derailleur you're changing fits your cassette size.
Different cable pull ratios make the SRAM Force CX1 11-speed groupset incompatible even if it resembles the 11-speed mountain bike groupset. With 10-speed road rear derailleurs, the Force CX1 may nevertheless be used in good harmony.
SRAM Mountain Bike Shifting Compatibility
Offering great flexibility, SRAM's newest 12-speed Eagle series is completely compatible both inside the lineup and with the SRAM Apex Eagle and SRAM Transmission groupsets.
But 10-speed and 11-speed SRAM mountain bike groupsets cannot be combined as they are incompatible.
For 10-speed SRAM groupsets—road or mountain bike—they are totally cross-compatible. For a mountain bike rear derailleur, for instance, you could find no problems using a road shifter.
Likewise, 7-speed, 8-speed, and 9-speed SRAM groupsets are interoperable across road and mountain configurations, enabling diverse combinations.
SRAM Electronic Shifting Compatibility
While the eTap batteries are an exception and run with all 12-speed AXS and 11-speed configurations, 12-speed SRAM groupsets are not backward-compatible with 11-speed groupsets.
Currently, 12-speed AXS rear derailleurs from SRAM fit road, gravel, and mountain bikes. On a road or dirt bike, for instance, you might utilize an Eagle rear derailleur as long as you match it with comparable chains, chainrings, and cassettes.
Mixed combinations also apply for 12-speed SRAM AXS road and mountain bike setups. To make a gravel bike with an ultra-wide gear range, for example, connect road shifters with an Eagle AXS rear derailleur with a 10-50T or 10-52T cassette. But SRAM has made it simpler to accomplish this without combining elements with the arrival of Force AXS Wide and Rival AXS Wide groupsets. These wide groupsets may be matched with either normal Force and Rival AXS 1x (XPLR) or 2x rear derailleurs and broaden the number of road configurations for gravel riding with smaller chainrings.
While another version supported up to 33T cassettes, on the previous generation of Red/Force AXS, there was a rear derailleur especially made for 10-36T cassettes. The second-generation rear derailleurs—including Rival AXS—support a maximum range of 2x10-36T.
Cassette sizes 10-36T or 10-44T can be accommodated by the Force and Rival XPLR AXS 1x rear derailleurs.
12-speed eTap AXS components are not compatible with older 11-speed Red eTap shifters and rear derailleurs.
Campagnolo Mechanical Shifting Compatibility Explained
Campagnolo Road and Gravel Shifting Compatibility
Campagnolo does not create mountain bike drivetrains; hence, knowing compatibility between several versions of their groupsets is really easy.
Still, Campagnolo has certain eccentricities that might complicate things somewhat.
All eight- and nine-speed Campagnolo groupsets had the same cable pull ratio prior to mid-2001 and were completely compatible with one another. Many times referred to as "Campyold," this generation of components is
Campagnolo changed the cable pull ratio for its new 9-speed groupsets following mid-2001. From that time, this revised ratio also fit all 10- and 11-speed groupsets. For instance, you could mix an Athena shifter with a Record rear derailleur without problems.
Now, depending on your viewpoint, things have grown rather more complicated—or maybe simpler. Campagnolo has made little changes to every one of its groupsets, therefore lowering compatibility between generations. Boxed letters designate compatible components; if your components have the same marking, they will fit together perfectly. If you intend to purchase parts from this renowned Italian manufacturer, it is also strongly advised to thoroughly review the manual to guarantee you have the appropriate components since Campagnolo also offers thorough compatibility and upgrade information.
The cable pull ratios vary amongst groupsets. For instance, Potenza itself is incompatible with Ergopower or Centaur Power-Shift systems; the mechanical Super Record, Record, and Chorus 11-speed Ergopower are not compatible with Potenza either.
Still, the newest 12-speed road groupsets are really straightforward. As they are totally compatible, you are free to mix and match elements from Chorus, Record, and Super Record.
Regarding the Ekar and Ekar GT gravel sets, they are Campagnolo's sole 13-speed models and cannot be combined with the 12-speed groupsets of the brand.
Campagnolo Road Electronic Shifting Compatibility
Using a wireless communication technology to link its components, the top-tier Campagnolo Super Record WLR system is a totally autonomous system.
Though it operates as a stand-alone system, the previous generation Super Record EPS electronic groupset is likewise 12-speed. Its electronic components are much different from those of the WLR, and it depends on wired communication between the shifters and derailleurs.
Furthermore, disc brake and rim braking systems cannot be combined; hence, this grouping is not backwards-compatible with previous 11-speed EPS systems. It also rejects a change from 11-speed to 12-speed.
The good news is that certain backward compatibility with previous versions is provided by the third-generation 11-speed EPS groupset.
What Length Rear Derailleur Should I Buy?
Now that we’ve determined the speed, brand, and compatibility of the rear derailleur, you need to calculate the cage length required for your drivetrain—long cage, medium cage, or short cage.
The length of the derailleur cage determines the cassette range it can support. A longer cage can handle greater chain slack and accommodate a wider gear range.
Quick Guide to Rear Derailleur Cage Lengths
Using a wireless communication technology to link its components, the top-tier Campagnolo Super Record WLR system is a totally autonomous system.
Though it operates as a stand-alone system, the previous generation Super Record EPS electronic groupset is likewise 12-speed. Its electronic components are much different from those of the WLR, and it depends on wired communication between the shifters and derailleurs.
Furthermore, disc brake and rim braking systems cannot be combined; hence, this grouping is not backwards-compatible with previous 11-speed EPS systems. It also rejects a change from 11-speed to 12-speed.
The good news is that certain backward compatibility with previous versions is provided by the third-generation 11-speed EPS groupset.
Actually, Shimano and SRAM have optimized their most recent 2x12-speed road rear derailleurs, so usually just one derailleur length is offered. Usually tailored for a limited cassette range, most 1x drives also include a single derailleur length. Like the SRAM Red XPLR 13-speed, occasionally it is even confined to a particular cassette ratio.
Still, one notable exception is the 12-speed Shimano GRX 1x. It provides two choices for rear derailleur: RX822-GS and RX822-SGS. Though their cage lengths vary, these types have the same mechanical body. Whereas the RX822-SGS can manage 10-51T cassettes, the RX822-GS supports 10-45T cassettes. Furthermore, if necessary, you can swap the cage yourself to alternate between these two variants.
Short-cage derailleurs and small-range cassettes, geared to the challenges of downhill riding, are used in some drivetrains made especially for downhill riding, like Shimano Saint and SRAM X01 DH.
Our tutorial will feature numerous "ifs" and "buts," simply because there are so many variables. Giving a firm response is difficult without particular facts.
Shifting Gear Capacity Explained
You can calculate the required gear capacity using the following formula:
(Maximum cassette teeth - Minimum cassette teeth) + (Maximum chainring teeth - Minimum chainring teeth) = Required capacity
For example, in a modern double-chainring road system, we get the following:
(32 - 11) + (52 - 36) = 37T capacity
Now, let’s assume you’re using a Shimano 11-speed road drivetrain. The SS (short-cage) Shimano derailleur has a capacity of 35T, while the GS (medium-cage) derailleur has a capacity of 39T. In this case, you would need a medium-cage derailleur.
It’s worth noting that the listed gear capacity for derailleurs is often conservative. In practice, you can usually use a short-cage derailleur as long as you avoid extreme gear combinations, but Shimano chooses to stay on the safe side with its recommendations.
Rear Derailleur Clutch Explained
A conventional rear derailleur uses the tension the main pivot produces to maintain chain stability.
By means of either a spring-loaded mechanism or a more sophisticated hydraulic system, rear derailleur clutches create resistance to the movement of the derailleur cage. This lessens back-and-forth motion, therefore improving the dependability and quietness of the drivetrain.
Though their clutch designs differ somewhat, SRAM, Shimano, and Campagnolo all aim to improve shifting stability.
Almost all new mountain bike derailleurs now incorporate a clutch system. This also holds true for Shimano GRX gravel groupsets, the earlier 11-speed Ultegra RX mechanical derailleur, Campagnolo Ekar, SRAM's 1x road groupings, and eTap AXS 12-speed road derailleurs.
Mountain bikes make clear decisions for a quieter, more dependable transmission offered by a clutch derailleur.
A more expensive derailleur offers what?
The next decision is to choose which price range fits you and which features you want most after you have found the best derailleur and drivetrain system for your needs. Three main elements define these characteristics: weight, durability, and finish. Think about them while deciding on the derailleur that fits your needs.
Weight
Generally speaking, more expensive derailleurs are lighter than less costly models.
This is accomplished by cutting away more surplus material and using more specialist materials, such as carbon fiber and titanium, in the manufacture of the derailleur.
Should weight be your major concern, you will have to pay more to reach it.
Durability
More expensive derailleurs typically use more wear-resistant components and have tighter tolerances, making them last longer than cheaper models.
This is especially evident in the jockey wheels. Cheaper derailleurs often have jockey wheels that rotate directly on steel bushings, lacking bearings, and their lifespan is significantly shorter compared to high-end products that use ball bearings or ceramic bushings.
Finish
With their superb anodized finishes and elegant polishing methods, high-end derailleurs' quality matches that of fine jewelry.
Although many people advise never to judge a book by its cover, cyclists are well known to evaluate a bike based on its drivetrain. Nobody will criticize you for indulging in some glittering gear if you wish to be the hippest rider in the bunch.