Ingezonden

The XX1 rear derailleur will feature the new Type 2 pulley cage clutch internals to help control chain movement (James Huang/Future Publishing)

We’d long heard
rumblings of a new 11-speed mountain bike group coming from
SRAM. We expected it to be an updated XX with a wider range that more closely replicated a
conventional triple. Instead, SRAM have developed a progressive, dedicated,
single-ring 11-speed group called XX1. The company hope it will change
mainstream thinking in terms of 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 got a lengthy sit-down with
drivetrain product manager Chris Hilton. We also got to pedal the bits around a
little, too.

What is the
XX1?

SRAM aren’t
pitching XX1 as having 11 speeds. Instead, they’re describing it as a 1x
drivetrain that just happens to have 11 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 the XX’s rough 470 percent but it’s a
big improvement over current 1×10 drivetrains that many riders already run. In
essence, the XX1 cassette adds a gear on either end of a current XX cassette
while retaining reasonable jumps in between.

“We’re not trying
to start an 11-speed war,” Hilton says. “A 1×11 war? Fine, but a 2×11 war isn’t
beneficial to anybody.”

SRAM also don’t
intend for the XX1 to replace current 2×10 groups. Instead, they’re aiming it
at trail and enduro riders who are often already strong enough to power a
single-ring drivetrain. The target group often also place a major emphasis on
chain retention and impact protection with dedicated guides and bashguards.

Nonetheless, SRAM
are fully expecting a smaller faction of forward-thinking cross-country riders
to jump on board as well. As well as being simpler and more secure than the XX,
the XX1 is also lighter. Target production groups are 200-300g relative to the
ultra-light XX.

“Almost all of
that is from omission, not from anything getting lighter,” Hilton tells BikeRadar.
So there shouldn’t be any drop in overall durability, either.

Final pricing is
still to be determined but SRAM plan to launch the XX1 later this summer.

Video: SRAM’s prototype XX1 drivetrain

New cassette,
new freehub body

The XX1 cassette
construction is very similar to that of XX, 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. This also transfers all the drive torque to the splined freehub body.
Instead of having the smallest cog as a separate bit, XX1’s 10-tooth cog is now
integrated into the rest of the steel structure.

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

A Delrin-lined
aluminum locking ‘tube’ replaces the conventional lockring. The ‘tube’ 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 added just outboard of the
remaining freehub body splines. The Delrin lining is sized 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. It’s not very different to how a non-driveside SRAM
GXP crank arm attaches to a bottom bracket spindle.

Because all the
freehub body modifications are restricted to the exterior, an XD body can be
adapted to current hub designs with no changes to things like axles, bearings
and spoke flange spacings.

The XX1 cassette fits onto a modified XD cassette body

For now, wheels
will be available from SRAM and DT Swiss only. Hub spacing will remain
unchanged at 135mm or 142mm. Licenses to other makes are under discussion, but
given the prevalence of DT Swiss drivers in other companies’ wheels that should
open things up to the likes of Specialized, Reynolds, Enve Composites 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. Of course, 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,
though. So wear supposedly isn’t adversely affected.

Straight P derailleurs

The XX1’s rear
derailleur will signal a radical departure from current designs. With a
conventional slant parallelogram the derailleur body simultaneously moves the
cage in and out as well as up and down. But the XX1 will use a so-called “straight
P” layout where the body only moves along the horizontal plane. It’s 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.

Hilton
acknowledges that a straight parallelogram is counterintuitive. One would
assume that 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 downhill bike,” Hilton says. “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.”

Because 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, pulling
the cage fore and aft and the upper pulley up and down.

XX1 will only be
offered with a single 10-42T cassette ratio, so 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,” explains Hilton.

The XX1’s “straight
P” design also means it will only work with a single-ring crank. Likewise, suspension
designs with lots of chainstay 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. This decreases cable friction. We expect
production units to be built with cold-forged parallelogram plates and a
carbon-fiber pulley cage.

The 1×11 concept
doesn’t require a huge re-engineering of shifters. Therefore, XX1 models will
essentially be the same as current trigger and Grip Shift offerings, albeit with different badging, an extra
click and specific internal spacing.

No chainguide?

XX1 was conceived
as a single-ring drivetrain, allowing engineers to rethink the chainrings as
well. Conventional 2x or 3x chainrings are designed with elaborately shaped
teeth to improve shift performance. But this also affects 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,” says 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 those
found on dedicated singlespeed rings. However, they’re also built with
alternating tooth thicknesses that are syncronized with the gaps in the chain –
slightly narrower to fit 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 that you won’t need 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 XX1 drivetrain fits in well with the ‘new school’ of trail bike kit, which includes wide bars and short stems 

The synchronized
design of the chainring teeth will limit the XX1’s chainring choices to even
numbers. But the overall range is admirably broad – all the way from 28-38T.
That variation 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, as for current
XX cranks).

1×11
drivetrains: the bonuses

Why go to all this
trouble just to eliminate a chainring, though? Don’t current 2×10 systems
already work well enough? That all depends on who you ask.

It’s true that
modern two-ring drivetrains work well. But they still can’t match the security
of single-ring setups, which 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 chainguide models in recent years.

Moreover,
single-ring drivetrains are simpler and lighter, as well as less confusing to
newer riders. Hilton doesn’t refute the idea of eventually bringing the 1×11
concept to much lower price points. And XX1’s gearing range sounds generous
enough to be useful for a wide swathe of riders.

“As long as you
choose your range properly, this type of system is applicable to a majority of
people,” Hilton says. “It’s not intended to replace 2×10. Some people need a
bigger range, and that’s fine – we still have 2×10. But if you’re Ross Schnell
or various other people, this is a radically improved 1×10.”

Hilton also points
to the difficulties an OEM company can face 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,” Hilton says. “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 use a
2×11 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.

“2×11 is certainly
feasible – it’s not impossible,” Hilton says, though he also adds 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 11 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 brief test ride inside SRAM’s Schweinfurt facility, in
Germany, it certainly seems to work. We’ve been promised parts for testing in
the near future so we’ll know for sure soon.

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