SRAM teased upcoming their XX1 group in May, showing not only the first 11-speed cassette for mountain bikes, but also the first dedicated complete group designed around a single front chainring.

Now, they’ve let loose the specs and details. Available in October 2012, the XX1 group introduces an entirely new Type2 X-HORIZON rear derailleur that has an interesting movement range to accommodate the massive 10-42T cassette range. The crankset uses new carbon arms designed specifically for a single chainring, and the chainring and cassette pulleys are designed with narrow and wide teeth to fit inside the inner and outer chain links to keep the chain stable. The cassette rides on a new XD Driver Body freehub body that’s (for now) only available for SRAM and DT Swiss wheels.

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The ring uses X-SYNC tooth profiles, an alternating pattern of narrow and wide tooth profiles. This not only helps manage the chain (like a built-in guide), but SRAM says it improves durability and stability. It’ll be available in even number tooth counts only (obviously): 28-30-32-34-36-38. It’ll be offered in both BB30 and GXP formats. All chainrings use the same BCD, so you can swap them in and out, and you can do so without removing the crankset…a big bonus. There’s a chance you’d have to shorten/lengthen the chain if you went from one extreme to the other.

Claimed weight with BB is 650g, about 50g lighter than the standard double BB30 crankset.

The cassette started out as a 9-36 during initial testing, but ended up at the larger 10-42. The 10t cog does not double as the lock ring as expected, and the entire piece comes in at a claimed 260g (versus 208g for an 11-36 XX 10-speed). Tooth range is listed as 10-12-14-16-18-21-24-28-32-36-42.

UPDATE: There is a 21T cog, which we originally thought might be a typo since it’s the only odd number in the system (derailleur pulleys are 12T each) and that it might throw off the alternating chain/chainring tooth profiles. SRAM’s MTB PR guy says because cassette’s cog tooth profiles aren’t alternated like the chainring, they don’t throw off the synchronization of the chains inner and outer plates with the chainring. There’s not really a technical explanation of why it doesn’t, it just doesn’t.

The cassette uses their single-piece machined X-Dome design. The new XD Driver Body steps down as it moves away from the hub, providing a smaller diameter section for the 10t cog. DT Swiss sorta let the cat out of the bag at Sea Otter when they announced 11-speed ready mountain bikes wheels, so we new something was coming, but they didn’t say anything about a proprietary freehub body. The idea isn’t new in and of itself, Hope and Kappius and shown similar systems, albeit slightly more integrated, but it’s the first from two major component manufacturers. We’re seriously hoping it becomes an open standard that other hub and wheel brands can offer.

UPDATE: It’ll work on all of DT Swiss’ Star Ratchet equipped wheels (most of their high-end stuff) and on SRAM’s Rise 60 wheels. It won’t work on the Rise 40 wheels because they use a different driver body. Lalonde says nothing changed inside the driver body compared to a freehub body, just the outer shape was changed to accommodate the 10T cog. Theoretically, it pops on and off a hub the same way a standard freehub body would (depending on hub model, of course), which means it’s not a stretch that we’ll see it offered as an option on other brands. The design is open for other wheel manufacturers to use without licensing.

SRAM says it saves a 6-8g over a traditional freehub body, but that it offers better stable connection between the cassette and hub. They’re being intentionally light on the details until the Crankworx events this summer, but we’ll be seeing them before then and press for info.

UPDATE: While it looks like the 10t cog is the lock ring, it’s not. The “tube” on the inside of the XX1 cassette is the “lock ring”. There are still splines on the Driver Body, but they’re all the same so you don’t have to line anything up. You slide the cassette on and then use a standard cassette lock ring tool, which threads the entire cassette on rather than just a lock ring on the end.

Generally, much of a chain’s plate design is to facilitate front shifting. This chain is a 1×11 specific model that SRAM says is “optimized for use without front shifting.” They’re claiming it’s stronger, with a new finish to reduce wear. At face value, this means you won’t be able to run this system with a double up front, and the alternating tooth profiles mean you can’t sub in a Shimano chain (unless, perhaps, you were to replace the pulleys on the rear derailleur…but then you’d likely need a really, really long chain to accommodate that range. Hmmm…)

One of the biggest changes comes at the rear derailleur. Rather than a parallelogram that’s set at an angle to move the upper pulley diagonally along the lower half of the cassette, the X-HORIZON rear derailleur moves in a perfectly horizontal (get it, horizon?) plane. In order to keep the upper pulley moving up or down across the cassette, it’s offset far back from the cage’s upper pivot. Once you see it, it’s a pretty darn simple solution. Another benefit is that the horizontal plane is perpendicular to the up and down forces your bike will see as it goes over bumps and drops, meaning even the worst terrain won’t affect shifting performance. To minimize chainslap and noise, it uses their Type2 clutch mechanism to hold the chain tight and Cage Lock to ease wheel removal. Cable routing also looks to be drastically improved, similar to the way it’s routed on the new Red road group.

This diagram shows the difference in movement patterns for the X-HORIZON rear derailleur versus a traditional mech. Claimed weight is about 220g for the rear derailleur (compared to 181g for the lightest XX model).

You’ll have the option of running it with the new Grip Shift or their trigger shifters, both 11-speed specific.

Overall, weight savings from the crankset is countered by the cassette and rear derailleur, but as a system it should come in reasonably lighter by eliminating the front shifter and derailleur.