Trail Tested: SRAM’s XX Group

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Editor’s Note: Check out part-two of the SRAM review — Best of the rest of XX: shifters, derailleurs, brakes.

It’s been almost a year since the world started catching glimpses of SRAM’s first complete mountain bike parts group. At last year’s Sea Otter, never-seen-before brake rotors were impossible to hide on Christophe Sauser’s S-Works Epic. New brake levers and calipers lurked nearby, hiding under masks of black electrical tape.

Asked about a fork lockout button that looked nothing like any known predecessors, Specialized team mechanics only grinned and said, “We’re not supposed to talk about that.”

Then in May of 2009, spy photos from European World Cup races revealed a little more. A wildly excavated cassette with 10 cogs. A carbon crankset with only two chainrings. And again, a push-button fork lockout, seamlessly integrated with the brake lever clamp.

In late May SRAM finally pulled back the curtain, and what we saw was the first-ever complete mountain bike component group from this Chicago-based corporation. SRAM had pulled from its four off-road brands (Avid brakes, RockShox suspension, Truvative cranks, and of course SRAM’s own derailleurs and shifters) to bring together the best of what each had to offer.

Dubbed XX, it promised lightweight, precise shifting, and powerful braking in a cross-country, race-oriented package. Former VeloNews editor Matt Pacocha gave it an early “thumbs up” after a weekend of riding at the product launch in Italy.

But now that it’s available to the rest of us, on 2010 bikes and in aftermarket catalogues, does XX deliver?

After almost two months of trail time, I’m here to say, “yes – absolutely.” I’m a cross-country and light trail rider with a road background, and this group meets my needs — and more.

I’m still planning more punishment, but if my first 40 hours are any indication, XX is the race group to own for 2010. The drivetrain is so smooth and so efficient, I find myself wondering why we’ve had to wait this long for a viable two-chainring/10-cog combination.

And if SRAM wants this group back any time soon, I won’t hesitate in offering my highest praise: I would pull out my checkbook to keep this XX group right where it belongs, which is bolted to my bike. I like it so much, I’d buy the friggin’ group.

2×10=20, but that’s half the story

When the news of XX broke, the single most defining characteristic of the group was (and is) the fact that it’s based on a 2×10 drivetrain. Two chainrings up front and ten cogs out back.

The evolution to 2×10 didn’t present any dramatic surprises, as road bikes have been fitted with ten cogs for years now. And elite cross-country racers have been tinkering with their cranksets for years, trying to lose the granny ring and find a viable two-chainring combination, or even experimenting with single-ring setups. Riders and race teams have long clamored for a more efficient drivetrain. All that remained to be seen was exactly how it could be pulled off.

It turns out SRAM’s first effort on a complete 2×10 drivetrain is so well thought out, it could be mistaken for a third- or fourth-generation revision, rather than a starting line first stab. Engineers rightly paid extremely close attention to the chainrings and front shifting. The result is the best front shifting I’ve ever felt from a SRAM or Truvativ crank and front derailleur combo.

Every design feature contributes to exceptional smoothness and accuracy up front, starting with the chainrings, which are built for impressive stiffness. They’re CNC machined from 7075-T6 aluminum billet, which permits a stout 6mm thickness at the bolt circle and a smooth taper to precisely placed shift gates, ramps and pins out by the teeth. By comparison, Truvativ’s Noir crank is fitted with chainrings that are only 4mm thick. This taper in thickness on the big ring also allows the chain to slide smoothly down to the small ring in a downshift, rather than popping off and dropping in abruptly.

Further boosting stiffness (a key to fast, accurate gear shifts) is a new bolt circle diameter of 80mm for the inner ring, and 120mm for the outer. It places the ring attachment points as close to the outer edge of the ring as possible, by the chainring teeth. And, there are no chainring nuts – the four outer bolts thread directly into the big chainring, and the small ring is bolted directly to the crank spider arms.

X-Glide: It’s not a deodorant

But probably the most important development is X-Glide. Truvativ, SRAM’s crank division, built X-Glide around a technology patented 12 years ago but not leveraged until now. Its core element is size-paired chainrings, in which the large ring is 1.5 times the size of the small ring. It allows the ring combinations (26/39 and 28/42) to be partitioned into four equal segments with precisely located chain pick up ramps and release points. The chainring teeth are then precisely clocked so that at each of these four locations, the chain can stretch from one ring to the other, yet the chain rollers remain firmly engaged on the teeth of both rings.

It’s worth repeating – the chain remains fully engaged on the teeth of both the big and small chainrings as a shift from one to the other is made. The resulting effect is impressive. And after a few months of riding, I can say emphatically that it works. X-Glide permits full-power upshifts and downshifts. I’ve powered through up- and downshifts, cringing at the prospect of a dropped chain, chainsuck or the momentum-robbing scrabble of a chain trying to claw up the big ring. But every shift has been perfect. It feels like the best shifting available on some road cranks, and that’s damn good.

Is the XX front shifting better than XTR, which is widely regarded as the reference point for front shifting performance on mountain bikes? That’s hard to quantify, but I would say shifting performance between the two XX chainrings is at least as good as XTR. And I would venture farther and argue that in terms of efficiency, XX is actually better than XTR. I perceive the narrow chainline, two-chainring system to be inherently more efficient, both under pedaling load and in gearshifting.

What else do you need to know up front? There’s fitment for almost any bottom bracket shell in existence, including the standard threaded GXP outboard BB, internal press-fit GXP (which fits the Shimano-designed 86mm threadless BB shell), BB30, and SRAM’s new PressFit 30. The standard crank option for those frames that will fit it has a Q-factor of 156mm (about 17mm narrower than a standard Noir). For frames that are too wide, there’s a wider 164mm BB30 option and a 166mm GXP option.

A perfect ‘10′ out back

SRAM’s new 10-speed cassette is available in two combinations, 11-32 and 11-36. Shared between the two cassettes is the X-Dome construction, in which eight of the 10 cogs are machined as one piece from a single block of 4140 chromoly steel billet. The largest cog (made from 7075-T6 aluminum) and the smallest cog (4140 chro-mo) are made separately. The largest cog is pinned to the back of the steel cassette, and is replaceable, as is the smallest cog, which is held in place along with the cogset by an aluminum lockring.

By now you’ve probably seen firsthand the extensive machine work on the interior of the cog body. It results in a lightweight and mud-clearing cogset that shifts great. Whether it’s the result of a harder surface treatment, stiffer cogs, or smaller gaps between cog sizes, the rear shifting mimics the front in that it’s among the best you can buy. Shifts are crisp, precise, and accurate, even under load and in adverse conditions.

Even better is the freedom to use every single cog, in either the small or large chainring. Remember that old admonishment to avoid using cross-chain gears? With XX, jus’ fuhgettaboutdit. In fact, this group encourages you to run big to big, small to small, and everything in between. To maximize its potential, cross chaining is required and supported by XX’s design.

XX’s inboard chainring placement leaves both rings lined up closer to the center of the cog spread, which is key to the usability of the whole 10-cog range. The chainline, as measured from the middle of the two rings, is 49.5mm, which is 2mm less than Noir’s. It means that in the big chainring, the chain doesn’t have to angle inward as sharply inward to hit the large cog.

Pedaling in the extreme gear combinations feels little different from pedaling in any other gear. Big to big, there might be a little more noise and maybe a little more friction, but not much. It’s a gear that I find myself using frequently and comfortably. It’s all the more impressive considering that some bikes with triple cranks and wider chain lines don’t do well with the chain even mildly stretched from the middle ring to the largest cog.

Gone Granny Gone

One of the questions about XX’s 2×10 system is, “Will I have enough gears when things get tough?” Especially on the low end, without a granny ring, I’ve heard concern about overgearing on climbs.

In my own experience, riding SRAM’s 11-36 XX cogset and 28-42 chainring combo, there are plenty of gears to go around. Using a gear calculator, I find the 28×36 low gear measures only 3.4 gear-inches higher compared to the 22×34 found on many 3 by 9 drivetrains. It’s almost as low. And the top gear of 42×11 is only 5 gear-inches shy of that on a 44×11. I find this slight narrowing of the overall gearing range to be virtually imperceptible.

But what’s much more apparent with 10 cogs fully accessible from two chainrings is the efficient sensation of having a complete spread of 20 gears available, rather than 27 gears peppered with a few off-limits and overlapped combinations. I find myself staying in one chainring or the other much longer. Most of my shifts are easy rear mech movements, which lends a feeling of absolute efficiency in both movement and momentum. Plus, the gearing spread between rear cogs feels very even and progressive.

I most appreciated SRAM’s 2×10 XX system when I swapped back to a traditional 3×9 drivetrain for a few rides. I was constantly aggravated from feeling overgeared in the big ring and undergeared in the small ring. But the middle ring wasn’t “just right,” either – rather, it was almost an annoying purgatory between the gears I really wanted. By the time I wanted a bigger gear, I was shifting twice: once up front to the big ring, and next in back to a larger cog. Same thing if I wanted a smaller gear. First a downshift off the big ring, then a few clicks down the cogset to even out the jump in cadence. With XX, finding the perfect gear simply feels much easier and more efficient.

XX part-two