Should you be changing or upgrading your back derailleur? Technical words like tooth capacity sometimes cause you uncertainty. Are you keen to know every detail on selecting a rear derailleur or feel bewildered during the process? If so, you have found the correct site.
Although we advise against mentioning derailleur trivia at your next party, anyone wishing to buy or modify a rear derailleur should find this knowledge quite vital. We will go over in great detail in this tutorial the compatibility of rear derailleurs for mechanical and electrical groupsets, together with discussing important derailleur specs and tooth capability.
Which Brand of Derailleur should I Buy?
Three main drivers in the market are Shimano, SRAM, and Campagnolo. But Campagnolo only provides groupsets for road and dirt bikes; mountain bikes are not catered for. Though discussing all of these would be outside the purview of this page, other brands, including Microshift, Box, Rotor, and FSA, also produce drivetrain components.
Mixing and matching drivetrain components from several companies is generally advised against. While brands allow mainly interchangeable cranksets, chains, and cassettes, shifters and derailleurs are usually not. This is so because Shimano, SRAM, and Campagnolo apply various cable-pull ratios for mechanical groupsets and distinct electronic protocols for their electronic groupsets—which we will go into great depth later. A cable-pull ratio is the distance a derailleur moves for every millimeter of cable pushed by the shifter. Components mixed with varying cable-pull ratios will produce unsatisfactory riding feel and poor shifting performance.
Compatibility, though, is far more nuanced than that. Whereas SRAM's mountain and road bike sets have the same cable-pull ratio, Shimano's mountain bike and road bike sets use varying cable-pull ratios. Moreover, varying drivetrain system speeds—such as 10-speed, 11-speed, or 12-speed—have varied cable-pull ratios. Shimano's 11-speed road groupings, for instance, have a different cable-pull ratio than their 10-speed groupset. Derailleurs having the same number of gears could have varying cable-pull ratios even within the same generation.
Naturally, there are some exceptions as well. For instance, various combinations of SRAM, Shimano, and Campagnolo components can be created to function together with some tweaks; there are also several third-party component suppliers who provide adapters to enable these mixed systems to run. When shopping for a rear derailleur, though, it is typically simplest to keep to the same manufacturer as your shifters.
How many gears does my bike have?
Finding out the gear count of your drivetrain comes next once you have chosen a brand. Counting the cogs on your cassette will help you to ascertain the gear count if you are only replacing an existing derailleur on your bike. Count the places on your shifter and add one to ascertain the gear count of your drivetrain if you are not sure its speed.
Derailleur compatibility explained
Generally speaking, it is not recommended to mix and match components from different generations of groupsets, nor to mix mountain bike and road bike components, although there are some exceptions. Below, we break down compatibility by manufacturer.
1x and 2x derailleurs
Though there are rare exceptions, generally speaking, it is not advised to combine elements from various generations of groupsets or blend mountain bike and road bike components. We break compatibility by manufacturer below.
The kind of derailleur arrangement adds still another degree of difficulty. Certain derailleurs are intended especially for single-chainring (1x) or double-chainring (2x) cranksets. Nearly all SRAM road groupsets, Shimano's GRX gravel sets, and certain SRAM MTB groupsets have both choices available. Generally speaking, you will require a 1x derailleur for a wider-range 1x cassette or a 2x derailleur for a narrower-range 2x cassette, even if there may be some overlap in specific circumstances. The material below relates to shifters as well as derailleurs.
Shimano derailleur compatibility explained
Shimano road and gravel bike mechanical derailleur compatibility
●Shimano 12-speed 105 fits 2x12-speed Shimano GRX RX820, so Shimano GRX RX820 shifters might be used with a Shimano 105 R8100 rear derailleur.
●All 11-speed Shimano road components are inter-compatible; hence, you could utilize a Dura-Ace R9100 derailleur with 105 R7000 shifters. This compatibility also relates to the 11-speed GRX parts of the brand.
●Except from 10-speed GRX and Tiagra 4700, 10-speed Shimano road components are inter-compatible. You might utilize an Ultegra 6700 derailleur with 105 5700 shifters, for instance.
● Given their identical cable-pull ratio, most 8- and 9-speed Shimano mountain bike and road components are inter-compatible. You can thus utilize an 8- or 9-speed mountain bike derailleur with road shifters or vice versa.
* Pre-1997 Dura-Ace gearing, which employs a completely different cable-pull ratio and won't operate with anything else, is the sole exception for 9-speed groupsets.
The same cable-pull ratio as 11-speed road components is also used in 10-speed Tiagra 4700 and GRX400 dirt derailleurs together with the related RS405 hydraulic shifters. You might thus utilize an 11-speed derailleur with 10-speed Tiagra 4700 shifters.
Shimano mountain bike mechanical derailleur compatibility
Every 12-speed Shimano mountain bike component is inter-compatible; you could, for instance, combine an XTR M9100 trigger shifter with an SLX M7120 derailleur.
Shimano uses distinct cable-pull ratios; hence, his 10-, 11-, and 12-speed road/gravel and mountain bike components are not inter-compatible with one another. For a mountain bike rear derailleur, for instance, you could use a pair of road shifters.
Shimano electronic derailleur compatibility explained
● Every 12-speed Shimano Di2 road and gravel component fits exactly the others.
● Every 11-speed Shimano Di2 road, gravel, and mountain bike component is inter-compatible; you might utilize an XT M8050 rear derailleur with Ultegra R8070 shifters.
Furthermore, unfit for non-assisted bikes, Shimano's XT Di2 M8150 12-speed rear derailleurs are made especially for electric mountain bikes. Different wiring causes the older Dura-Ace 7970 and Ultegra 6770 Di2 10-speed components to not be compatible with each other; the Ultegra model uses the SD50 type that subsequently became standard for 11-speed systems. Shimano's 11- and 12-speed Di2 components are mostly interchangeable despite these variations as long as the proper wiring configuration is followed.
SRAM derailleur compatibility explained
SRAM road and gravel mechanical derailleur compatibility
● Currently the only mechanical SRAM road/gravel available, 12-speed SRAM Apex Eagle cross-fits with SRAM Eagle mechanical MTB components.
● Inter-compatible 10- and 11-speed SRAM road components let you utilize a 10-speed SRAM Red rear derailleur with SRAM 22 shifters.
● Regardless of their intended use—road or mountain bike—all 7-, 8-, and 9-speed SRAM components are inter-compatible.
SRAM's newest 12-speed rear derailleurs are made to support wider-range cassettes, hence when changing a derailleur, you should carefully assess if it can manage all the possible cassette sprocket sizes you could wish to use. Though visually comparable, SRAM's Force 1 11-speed groupsets have different cable-pull ratios, so they are incompatible with 11-speed mountain bike components.
SRAM mountain bike derailleur compatibility
● SRAM's most recent 12-speed Eagle drivetrains are all cross-compatible with SRAM Transmission (T-type) components and SRAM Apex Eagle.
● Not inter-compatible are 10- and 11-speed SRAM mountain bike components.
● Whether they are road or mountain bike components, 7-, 8-, and 9-speed SRAM components are all inter-compatible.
SRAM electronic derailleur compatibility
● 12-speed SRAM components are not 11-speed backwards-compatible. ETap batteries are the exception; they run with all 12-speed AXS and 11-speed components.
● The present 12-speed AXS rear derailleurs of SRAM fit road, gravel, and mountain bikes. An Eagle rear derailleur, for instance, can be run on a road or gravel bike. Should you be doing this, you must run a compatible chain, chainring, and cassette.
Together, SRAM's 12-speed AXS road and MTB groupsets create the possibility for "mullet builds," whereby road shifters are matched with an Eagle AXS rear derailleur and a 10-50t or 10-52t cassette, therefore enabling an ultra-wide dirt bike construction.
Campagnolo mechanical derailleur compatibility explained
Since Campagnolo primarily generates road and gravel bike groupsets and lacks mountain bike groupsets, his derailleur compatibility is rather simple to understand. Although there is some cross-compatibility between several generations of Campagnolo groupsets, there are also some alterations to be alert for. Older Campagnolo groupsets before 2001, for instance, 8-speed and 9-speed ones had the same pull ratio and were interchangeable. Later generations did, however, see some changes in pull ratios, which would impact fit.
Campagnolo road electronic derailleur compatibility
Top-tier Super Record WLR groupset from Campagnolo is a stand-alone system with wireless protocol for component communication. Though it makes connected connections, the Super Record EPS electronic groupings from the previous generation are also 12-speed. Older 11-speed EPS components are not backward compatible nor forward compatible for upgrading from 11-speed to 12-speed. Fortunately, Campagnolo offers a thorough compatibility guide to enable consumers to make wise choices prior to any component acquisition.
What cage-length derailleur should I buy?
Choosing the derailleur cage length that fits your drivetrain comes next once you have decided on the speed, manufacturer, and compatibility of your derailleur: long, medium, or small. The range of gears you can utilize on your bike depends on the length of the derailleur cage; the more chain slack it can absorb, the more gears you can use.
This is a basic guide: You will require a long-cage derailleur or one especially made for a 1x group if you are using a super-wide range cassette (e.g., 10-42t or greater) or if chainring sizes vary greatly (e.g., when utilizing a triple chainset). A short-cage derailleur is best for conventional road double drives using standard cassettes (e.g., 11-28t or less).
How to calculate tooth capacity
Calculating the derailleur's tooth capacity with the following calculation will help you to find a clear response:
● (biggest cog – smallest cog) + (largest chainring – smallest chainring) = needed capacity.
For a modern, double-chainring road bike drivewheel, for instance, we would have something like:
● (32 - 11) + (52 - 36) = 37 t capacity
If you are looking at a Shimano 11-speed road derailleur, an SS (short cage) Shimano derailleur has an overall capacity of 35 t and a GS (medium cage) derailleur of 39 t. Under this situation, you would want a medium-cage derailleur.
Clutch Derailleurs Explained
Regular rear derailleurs hold the chain in place by depending just on the tension generated by the main pivot. Either using a spring-loaded clutch or more sophisticated hydraulic systems, a clutch derailleur increases the resistance of this pivot to minimize the fore and aft movement of the derailleur cage, so producing a quieter and more dependable drivetrain. A clutch mechanism is almost always found in modern mountain bike derailleurs (along with some gravel and road groupsets).
What do I get with a more expensive derailleur?
Let's start with selecting the appropriate price point after we have covered how to match a rear derailleur with your drivetrain. But in a rear derailleur, what should you be looking for? We'll split it down into weight, lifespan, and finish to make things easier.
Weight
Usually speaking, a more costly derailleur will weigh less. More polished machining and higher-grade materials—such as titanium pivots and carbon fiber cages—help to accomplish this. Should weight be your main concern, get ready to spend extra.
Longevity
More expensive derailleurs survive longer than less expensive models because they are designed to stricter tolerances and typically feature more durable components. With jockey wheels, where less expensive derailleurs usually employ simple steel bushings that don't last as long as high-end counterparts spun on cartridge bearings or ceramic bushing, this is particularly evident.
Finish
Given everything up to now has been somewhat technical, let's pause to discuss aesthetics. Like jewelry, high-end derailleurs have exquisitely polished surfaces and superb anodizing. Although many people advise not to judge a book by its cover, bike aficionados have a habit of assessing a bike by its rear derailleur. Investing in some bright, premium gear is not a terrible decision if you wish to stand out in the group.
To sum up, selecting the correct rear derailleur requires weighing the brand, speed, compatibility, and tooth capacity of your bike. With the thorough explanations in this article, perhaps you now have a thorough awareness of how to choose or update your rear derailleur.
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