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SRAM XX1 mountain bike drivetrain

By:
James Huang
Published:
May 31, 2012, 14:58 BST,
Updated:
May 31, 2012, 18:28 BST
Naturally, the new SRAM XX1 rear derailleur will feature the new Type 2 pulley cage clutch internals to help control the chain movement.

Naturally, the new SRAM XX1 rear derailleur will feature the new Type 2 pulley cage clutch internals to help control the chain movement.

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This article originally published on BikeRadar

We'd long heard rumblings of a new 11-speed mountain bike group coming from SRAM and we expected it to be an updated XX group with a wider range that more closely replicates a conventional triple. Instead, SRAM has developed a boldly progressive dedicated single-ring 11-speed group called XX1 (say, "ex ex one") that the company hopes will change mainstream thinking in mountain bike drivetrains. We happened to score an in-depth look at the prototype during our recent visit to SRAM's European headquarters where we not only got but also a lengthy sit-down with drivetrain product manager Chris Hilton but also got to pedal the bits around a little, too.

What it is

SRAM isn't pitching XX1 as having eleven speeds – rather as a 1x ("one by") drivetrain that just happens to have eleven cogs out back. The heart of the XX1 concept is its ultra-wide 10-42T cassette, which provides a generous 420-percent range. That still falls short of XX's roughly 470-percent but it's a big improvement over current 1x10 drivetrains that many riders already run. In essence, the XX1 cassette adds a gear on either end of a current XX cassette while still retaining reasonable jumps in between.

"We're not trying to start an 11-speed war," Hilton stated. "An 1x11 war? Fine, but a 2x11 war isn't beneficial to anybody."

SRAM also doesn't intend for XX1 to replace current 2x10 groups, instead aiming XX1 at trail and enduro riders who oftentimes are already strong enough to power a single-ring drivetrain and also place a major emphasis on chain retention and impact protection with dedicated guides and bashguards. That being said, SRAM is nonetheless fully expecting a smaller faction of forward-thinking cross-country riders to jump on board as well. In addition to being simpler and more secure, it's also lighter with target production groups shedding 200-300g (0.44-0.66lb) relative to the already ultralight XX.

"Almost all of that is from omission, not from anything getting lighter," Hilton told BikeRadar, so there shouldn't be any drop in overall durability, either.

Final pricing is still to be determined but SRAM plans to officially launch XX1 later this summer.

A new cassette, a new freehub body

The XX1 cassette construction is very similar to that of XX, being mostly machined from a single block of steel to form a hollow domed structure. That dome is then capped with an aluminum innermost cog – in this case, one that's much more dished than the one on XX – which also transfers all of the drive torque to the splined freehub body. Instead of having the smallest cog as a separate bit, though, XX1's 10-tooth cog is now integrated into the rest of the steel structure.

Squeezing on a 10-tooth cog required some doing, however, as it's too small in diameter to fit on a conventional freehub body. As such, SRAM created a new 'XD' driver body that is essentially just a slight modification on what's currently in use. The end where a conventional lockring would normally thread into is lopped off and nearly all of the splines on the outside of the body are shaved smooth to leave a mostly bare cylinder.

Replacing the conventional lockring is now a Delrin-lined aluminum locking 'tube' that snaps into the inside of the 10-tooth cog but is otherwise free to rotate. Threads at the inner edge then match up to new threads that are added just outboard of the remaining freehub body splines and the Delrin lining is sized such as to provide a light press-fit on the freehub cylinder. When it's all secured, the locking tube keeps everything firmly concentric on the freehub body (with no chance of cogs digging into its surface) and in essence, it's not much different from how a non-driveside SRAM GXP crankarm attaches to a bottom bracket spindle.

Because all of the freehub body modifications are restricted to the exterior, that also means an XD body can be adapted to current hub designs with no changes to other bits like axles, bearings, and spoke flange spacing. For now, wheels will be available from just SRAM and DT Swiss and hub spacing will remain unchanged at either 135mm or 142mm. Licenses to other makes are under discussion though given the prevalence of DT Swiss drivers in other companies' wheels, that should also open things up to the likes of Specialized, Reynolds, Enve Composites, and others right away.

XX1's 11-speed cog spacing will be unique to the group, however, and isn't shared with 11-speed offerings from Shimano (more on that soon) or Campagnolo. Naturally, there's a dedicated 11-speed chain as well with a slightly narrower external width. Inner plate spacing and roller width are unchanged from SRAM's 10-speed chains, however, so wear supposedly isn't adversely affected.

As with the current XX cassette, the XX1 is mostly machined from a single block of steel to create a hollow, 'domed' structure - only now, the 10T cog is included in the one-piece construction, too

The end of the slant parallelogram

XX1's rear derailleur will signal a radical departure from current designs. Instead of a conventional slant parallelogram (where derailleur body simultaneously moves the cage in-and-out as well as up-and-down), XX1 will use a so-called "straight P" layout where the body only moves along the horizontal plane – an idea Hilton admits has been floating around within SRAM for the past eight years but is only now seeing the light of day with the rise of 1x drivetrains in the mainstream.

Hilton acknowledges that a straight parallelogram is counterintuitive – one would assume the derailleur body should follow the contours of the cassette – but the XX1 cassette's extreme gear range and tighter cog spacing necessitated a change in thinking.

"Originally, the first prototype of this was built for a DH bike," Hilton said. "We built it because when bikes hit bumps, the derailleur absorbs the shock of that chain. So you hit a bump today and you've got your chain mass and your derailleur mass – it can actually activate the parallelogram and cause ghost shifting. If you add a clutch to that and you've really significantly stopped that force, you've actually added to the potential for ghost shifting. We've compensated for that [in current Type 2 rear derailleurs] by not decreasing our spring tension in derailleurs."

Since XX1 doesn't rely on a slant parallelogram to control chain gap, the upper pulley is now hugely offset from the lower derailleur body knuckle. As the chain shifts across the range, more or less chain is wrapped around the cassette, thus pulling the cage fore and aft and the upper pulley up and down. Since XX1 will only be offered with a single 10-42T cassette ratio, that upper pulley pivot offset could be precisely calculated to keep chain gap consistent across the entire range.

"The chain gap is exactly the same in every single cog no matter what cog you're in, even if you were to change the cassette because it's driven by the amount of free chain length," explained Hilton.

Consequentially, though, the XX1's "straight-P" design also means it will work only with a single-ring crank. Likewise, suspension designs with lots of chain stay growth could be problematic.

Other details include an integrated cable pulley at the rear of the derailleur – just like on Avid's long defunct Rollamajig – to decrease cable friction. We expect production units to be built with cold-forged parallelogram plates and a carbon fiber pulley cage.

The 1x11 concept doesn't require a huge re-engineering of shifters so XX1 models will be essentially the same as current trigger and Grip Shift offerings – albeit with different badging, an extra click, and specific internal spacing.

No chain guide required?

Since XX1 was conceived from the outset as a single-ring drivetrain, that allowed engineers to also rethink the chainrings. Conventional 2x or 3x chainrings are necessarily designed with elaborately shaped teeth to improve shift performance. However, those same features also affect the ability to retain the chain on bumpy terrain.

"As we make chainrings shift faster and smoother, we're taking material away, making them more expensive, limiting their lifespan, and potentially affecting chain retention capability," said Hilton. "There's no question that making a chain shift makes a chain fall off."

As a result, XX1's chainring teeth are unusually tall and quite squared-off, similar to what's found on dedicated singlespeed rings. However, in this case they're also built with alternating tooth thicknesses that are synchronized with the gaps in the chain – slightly narrower to fit in between inner chain plates and wider to take advantage of the extra space between outer chain plates.

Combined with XX1's improved chainline, Hilton claims this improves chain retention to the point of not needing any sort of guide in most applications while also slowing down wear and reducing drivetrain noise. Hilton admits that aggressive trail and enduro riders might still choose to run some sort of minimal upper guide if only for peace of mind.

"I like to say that chain retention is sort of like birth control – there are various levels of safety," he quipped. "You could choose to use a full-on X0 DH guide with bash protection and a lower roller in addition to this whole system, but that would be like abstinence."

The synchronized design of the chainring teeth will limit XX1's chainring choices to even numbers but the overall range is admirably broad – all the way from 28-38T. That wide range will require a dedicated bolt circle diameter but the spider will be shaped so that users won't have to remove the cranks to swap rings. The production crank will feature hollow carbon fiber arms and be offered in both narrow and wide stance widths – we're guessing 156mm and 166mm like current XX cranks.

The argument for 1x11 drivetrains

Why go through all this trouble just to eliminate a chainring, though? Don't current 2x10 systems already work well enough? That all depends on who you ask.

True, modern two-ring drivetrains work well but they still can't match the security of single-ring setups that are increasingly finding favor in the mainstream marketplace, especially with more aggressive riders. If you need proof of that, just note the explosion of two-ring chain guide models in recent years.

Moreover, single-ring drivetrains are simpler and lighter as well as less confusing to newer riders – and Hilton doesn't refute the idea of eventually bringing the 1x11 concept to much lower price points. More to the point, XX1's gearing range sounds generous to be legitimately useful for a wide swath of riders.

"As long as you choose your range properly, this type is system is applicable to a majority of people," he said. "It's not intended to replace 2x10. Some people need a bigger range and that's fine – we still have 2x10. But if you're Ross Schnell or various other people, this is a radically improved 1x10."

Hilton also points to the difficulties from an OEM company's perspective in terms of getting front derailleurs to play nicely with the huge range of rear suspension designs.

"Front derailleurs are limiting because there are so many choices. It's limiting because of where you can move the wheel to, where the suspension pivots are placed, where the cable routing comes from – all those things are limitations to a drivetrain. Front derailleurs are one of the most expensive engineering and tooling costs on a bicycle yet it's the first place manufacturers go to cut money."

So why not a 2x11 setup? Combined with that 10-42T cassette, such a drivetrain could easily replicate the full range of a traditional triple but with the advantages of a double.

"2x11 is certainly feasible – it's not impossible," Hilton said, though he also added that the cassette's added width presents problems with chainline if more than one chainring is used. "11-speed is a by-product of wanting to make a wider-range cassette without funky steps in there. We want to sell it as a 1x drivetrain solution that just happens to have eleven gears. The eleventh gear becomes problematic because the overall spacing is now wider."

Could the XX1 concept be further expanded with a revamped HammerSchmidt or high-performance internally geared setup, though? Hilton doesn't rule out that possibility but says it's not imminent.

Either way, XX1 sounds awfully appealing on paper and after a (very brief) test ride inside SRAM's Schweinfurt, Germany facility, it certainly seems to work. We've been promised parts for test in the near future so we'll know for sure soon enough.

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