It's been four years since SRAM's groundbreaking wireless electronic Eagle AXS drivetrains came out. If you've been following mountain bike news and peeking at spy shots, then you already know that SRAM's been cooking up something new with a direct mount rear derailleur.
Well it's finally here! Today, SRAM introduces its next generation of mountain bike groupsets: the Eagle Transmission. That’s right, it’s being called a “transmission” rather than “drivetrain,” but more on that in a bit.
Over the last decade, SRAM has pushed the evolution of mountain bike drivetrains forward by leaps and bounds, and the new Eagle Transmission promises to do so again. It's a monumental shift (haha) in drivetrain performance, so let’s take a closer look to understand why it's one of the most exciting new pieces of MTB tech to come out this year.
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SRAM Eagle Transmission Highlights
- Integrated design and “cassette mapping”
- Direct mount rear derailleurs (compatible with UDH frames)
- Overload clutch
- Replaceable derailleur parts
- Magic Wheel lower derailleur pulley
- New AXS Pod controller
- New spider and spindle-based power meter options
- Redesigned 10-52t cassettes
- Flattop chains
SRAM Eagle Transmission details
Eagle Transmissions have already scored big wins, including a world championship under Nino Schurter.
First off, SRAM is referring to these new Eagle groupsets as “Transmissions” rather than drivetrains. Why the new and confusing name? The reason is the integrated design, which SRAM explains is designed around the “universal constant of the wheel axle itself” to optimize shifting. Unlike drivetrains of years past, the direct mount rear derailleur clamps directly to the frame, creating a stronger, stiffer, and more stable interface.
This direct mount interface also physically touches the cassette. This removes any variation in rear derailleur's position, even on different bikes, eliminating the need for limit or b-gap adjustment screws. Once set up, the derailleur just works because it knows the exact position of every cog on the cassette. Not only that, but the system uses "cassette shift mapping" to provide seamless shifts. The cassette teeth have a slightly narrow-wide profile (like a narrow wide chainring) to retain the chain on the desired gear and shifts are synchronized with the shift ramps on the cassette, which means it will always shift crisply and easily under maximum pedal load, even with e-bikes.
To achieve this level of precision, all the components in the Eagle Transmissions are designed to work together as one cohesive unit. Components within it, are referred to as “T-Type.” So you have T-Type derailleurs, cassettes, chainrings, chains, etc. Unfortunately, this means there is little to no backwards compatibility with older Eagle and Eagle AXS groups, so you can’t mix and match parts. Only T-Type chainrings are backwards compatible with older Eagle chains, so can be used on the current Eagle drivetrains. The only non-T-Type chainrings that SRAM says will work with the new T-Type chains are 1x AXS Road chainrings, which is good news for riders interested in a mullet build for a road or gravel bike.
Otherwise, the new Eagle Transmissions still use SRAM’s wireless AXS technology. The AXS batteries, chargers, and the app remain unchanged. And Eagle Transmissions are compatible with SRAM AXS road shifters to create a mullet drivetrain. SRAM’s current Eagle mechanical and Eagle AXS products will also remain in the SRAM line-up for riders without a UDH frame.
XX SL vs. XX vs. X0
Eagle Transmissions will be offered in three spec levels. The top-of-the-line XX SL Eagle is the lightest, highest-end groupset. XX Eagle sits just below it. And below that is the slightly more affordable X0 Eagle group. Very much like the previous generation XX1 and X01 Eagle groups, XX SL and XX are very similar, but the lighter XX SL is geared more toward the cross-country racing while XX is intended for more rugged trail and enduro use.
The biggest difference between the groups (beyond price) is materials and weight. The higher-end groups use more exotic materials (e.g., a carbon pulley cage for XX SL derailleurs) and have been machined more to reduce weight. XX SL is the lightest while X0 will be the heaviest. Here are weight comparisons for some key components:
|
Group |
XX SL |
XX |
X0 |
|
Derailleur Weight |
440g |
465g |
475g |
|
Cassette Weight |
345g |
380g |
380g |
|
Chain Weight |
240g |
247g |
256g |
SRAM didn’t provide claimed weights for the cranksets, so we’ll have to weigh those on our scales when we have them all in the shop, though SRAM does claim that the XX SL crank is “the lightest ISO certified crank on the market.” I did get a chance to weigh an XX SL cassette, and it came in at 347 grams on my scale — pretty much in line with SRAM’s claimed weight.
Eagle Transmission Direct Mount Rear Derailleurs
XX SL, XX, and X0 derailleurs
The biggest change is the new direct mount rear derailleur. A few years ago, SRAM introduced its UDH (universal derailleur hanger). This derailleur hanger design was released as an open standard that any bike manufacturer could use for free. Ostensibly, this was to reduce the need for the countless proprietary hangers and axle thread standards across the industry, thus reducing headaches for consumers. But it was also a clever 4D chess move by SRAM to help pave the way for its new direct mount derailleurs. UDH has experienced fairly widespread adoption, and more UDH bikes and frames will be available in the near future.
T-Type rear derailleurs are only compatible with UDH-equipped bikes and frames. The direct mount interface replaces the hanger and clamps directly to the frame from two sides. Compared to a traditional hanger, the direct mount interface is much stiffer, which gives you more precise and consistent shifts, especially under higher loads. The mount also directly contacts the cassette, keeping the derailleur’s position relative to the cassette constant.
If you’ve ever had a bike that shifts inconsistently under load, it’s likely because of flex somewhere in the system changing the position of the derailleur relative to the cassette. According to SRAM, over half the time, the culprit is the hanger itself. The direct mount eliminates this issue. An additional perk of a consistent derailleur position is that limit and b-gap adjustment screws are no longer needed. Once the derailleur is set up, and it knows where it is in relation to the cassette, there’s no more adjustment needed.
Durability, Overload Clutch, and Replaceable Parts
So what if you crash and damage your derailleur? Traditional derailleur hangers are considered a sacrificial part. When you smash your derailleur on something, the hanger bends, keeping the derailleur itself from getting too damaged. Obviously, with direct mount, that’s no longer the case.
The direct mount interface itself is incredibly strong. It can handle so much weight, you can actually put your bike on its side and stand on the derailleur (not that you should) and it won't bend or move. Then, as an additional line of defense against impacts is the Overload clutch. This is the same clutch first introduced with Eagle AXS in 2019, so there’s nothing new here. Like any clutch, it prevents chain drops, but if it senses an impact, it opens up to allow the derailleur to fold back and absorb the force, and then return to its original position. The derailleur can also absorb force by rotating backward at the mounting point if you strike it against an object. If this happens, you can simply rotate the derailleur back into position by hand, without resetting the mounting bolt.
If the derailleur does get damaged, key parts like the B-knuckle, outer link, and pulley cage are designed to be easily replaceable so you can rebuild the derialleur rather than replace it, which is good considering its cost. The entire pulley cage can actually be removed and replaced by hand in a few seconds. The trail and enduro-oriented XX and X0 derailleurs also come with extra protective skid plates that are also replaceable.
Magic Wheel lower pulley
The lower pulley wheel of Eagle Transmission derailleurs also uses a new “Magic Wheel.” The outer teeth of the pulley wheel are able to spin independently of the main pulley wheel. If a stick or foreign object lodges itself into the pulley wheel and jams it up, it can rip your derailleur off your bike (ask me how I know this can happen). If this happens with the Magic Pulley, the outer teeth will keep spinning to prevent damage.
Eagle Transmission AXS Pod Controller
The awkward, single paddle on the previous generation AXS Controller was probably the most annoying part of that drivetrain. Now, the controller has been completely redesigned into a “Pod” that is smaller, lighter, and more ergonomic. There are two buttons that provide a nice, tactile click — a huge improvement. The Pod is also ambidextrous, so it can be flipped to be used on both the left and right, giving riders more possibilities for customization, especially with other AXS components like Reverb AXS droppers and Flight Attendant. For super picky riders, the buttons themselves are also interchangeable between concave and convex options.
Eagle Transmission Cranksets and Chainrings
Eagle Transmission cranksets continue to use SRAM’s DUB spindle and are available in 165mm, 170mm, and 175mm lengths. XX SL and XX crank arms are carbon fiber. 
The XX SL crank arms are hollow for maximum weight savings.
XX crank arms are the same as XX SL, but have a foam core for extra strength.
X0 cranks are aluminum and machined to be as light as possible.
Chainrings are all direct mount, using SRAM’s 8-bolt standard. This replaces the old 3-bolt standard, but it’s not totally new, since it's been used on SRAM’s Road and XX1 Eagle AXS cranksets. 8-bolt is required to run a Quarq AXS spider-based power meter, which is compatible with any Eagle Transmission crankset.
The chainrings have a new look, but continue to use the X-Sync 2 tooth profile. They’re available in 30-38t sizes and 0mm and 3mm offsets. XX and X0 chainrings also have integrated and replaceable bash guards. Beyond the standard 8-bolt direct mount options, there is also a new “Thread Mount” chainring designed specifically for the new integrated XX SL power meter (more on that below). There are also 104 BCD, Bosch, Brose, and Steps e-bike specific chainrings.
Eagle Transmission Power Meters
XX SL has the option for a new integrated spider-based power meter. Race fans will already recognize this as the same power meter already being used by top SRAM XC athletes. It is a dual-sided power meter which has +/- 1.5% accuracy and nifty Quarq features like Magic Zero which automatically recalibrates the power meter as you ride.
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To minimize weight, the power meter uses a proprietary “Thread Mount” chainring which, as the name implies, threads directly onto the spider without chainring bolts. Removing and swapping chainrings will require a removal tool that looks similar to a chain whip.
XX and X0 have the option to run a spindle-based power meter called DUB-PWR. This is the same technology that debuted on Rival AXS and is also available on the revised Force AXS group. It’s neatly hidden inside the DUB spindle and unlike a spider-based power meter, it's single-sided and only measures left-side power (which is more than good enough for the vast majority of riders). It provides +/- 3% accuracy and can be purchased with a crank or as an upgrade kit.
Eagle Transmission Cassettes and Chains
XX SL Cassette
The last generation of Eagle cassettes already provided a great combo of wide range and low weight, but the design has been further refined with the Eagle Transmission cassettes. They still mount to XD Driver freehubs and offer a 10-52t range, but the 36t and 42t gears have been bumped up to 38t and 42t to create smoother jumps between the three easiest gears. You may also notice a "setup cog" marked in red. You simply shift onto this cog to set up the derailleur with the AXS app.
XX (left) and X0 cassettes
The construction has also been revised. Previous generation Eagle cassettes were made from a one-piece X-Dome cluster on 11 cogs machined from hardened steel. This cluster was then attached to a single aluminum big cog. SRAM realized this design could be made lighter by using a pinned design for the largest cogs. For Eagle Transmission cassettes, the three largest cogs are now constructed from two stamped steel cogs pinned to an aluminum big cog. The remaining smaller cogs a still use the machined one-piece X-Dome design. For XX SL, the three largest cogs are all aluminum, reducing weight even more. It is 345 grams, which is impressively light for a cassette this large.
Eagle Transmissions have also switched to Flattop chains. The Flattop design increases strength and durability, and it’s similar to the Flattop chains used on road AXS groups, though they are not interchangeable. However, Eagle Flattop chains are compatible with X-Sync road chainrings, so it’s possible to run a road crank on mullet drivetrains.
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Photos courtesy of SRAM.
