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Making you a much more efficient pedaling machine.
For all the talk of the XX Cassette, the XX Crankset is really where they're changing the gearing paradigm. Double chain ring cranks have made consistent if periodic appearances in the MTB world for years. The reasons for their appearance have always been the same -- lighter weight, better Q-factor, better chain line, faster shifting, and no chain suck. Ritchey was the last to push 2x9 gearing starting back in 1997, and that was an era of seven and eight-speed cassettes. With nine-speed making its way into mountain bikes, 2x9 made sense for places without steep terrain.
XX isn't Tom's 2x9. For one, with XX you still get the super-wide range you'd otherwise get with 3x9. For another, unlike Ritchey, they're designing the entire system rather than making a few pieces, then hacking designs for the rest.
XX cranks are optimized for the 2x10 system. The only given was using two rings and a through-axle spindle. They went to the drawing board to re-imagine the rest. Looking at it, you know the crank is different -- different in a more profound way than even XTR. Both chain rings are affixed on the inside of the crankarm. This helps keep the q-factor down as well as keeping the chainline narrow, at 49.5mm. The outer ring as a 120mm bolt circle diameter while the inner has a 80mm bcd. These numbers seem odd, except that the outer was designed to work with 39, 42, and 45 tooth chain rings. The inner was designed to work with 26, 28, and 30 tooth chain rings. The exclusivity is a problem if you're trying to match the crank with any other shifting, but it's a benefit if you want XX. The benefit is that with less metal reaching from the chain ring teeth to the spider, there will be less flex, and faster response when pedaling. They also made the support area of the chain rings wider, out to 6mm, wider than other chain rings on the market. More meat means greater stiffness. And with 7075-T6 aluminum as the material, they're also pretty damn light.
Not only did they design their own optimized bolt circle diameters, but they re-engineered the shift ramps, pins, and cut outs on the outer chain ring. On most chain rings these days, there are two sets of ramps and two drop points where the chain can get lifted or dropped quickly and under pressure. This means that twice each pedal revolution you'll hit these spots, so the shifting can be iffy if you shift at any other time. You've probably experienced this with the pause and clunk on some shifts and the smooth movement on others. SRAM's XX outer ring allows perfect shifts just about all the way around the crank. They call this X-Glide. Another improvement stems from having only two rings. From the small ring, the chain can only go up, from the big ring, only down This made designing the rings so they worked perfectly with each other easier than having to think about how the chain goes up from the small to middle ring and down from the big to middle ring.
SRAM also worked the crank to make the Q-factor a low 156mm. Having two rings as opposed to three helped with this. But so did the crank construction, which is foam core inside an aluminum exoskeleton inside a carbon-fiber shell.
There are two different chain ring combinations: 39/26 and 42/28. The big ring bolts thread through the arm into the chain ring and the small ring bolts into the spider. SRAM spent hours with a spreadsheet and gear chart trying to figure out ideal shifting patterns for easy shifts up and down and big to small. They arrived at a 3:2 ratio, something that had already been developed and forgotten. For those wondering how small they should go, we did a little figuring; 26x36 combo is just slightly bigger than a 22x32, a 28x36 the roughly the equivalent of a 22x28. Using the 32 as your tallest cog, it goes as follows, with a 26 ring it's the equivalent of a 22x27, the 28 ring it's a 22x26. On the big ring side, the 39x11 is roughly halfway between a 44x12 and 44x13. A 42x11 is bigger than a 44x12. SRAM recommends their 10-speed road chains, 1090 and 1090R to mate this crank to their XX cassettes.
The XX crankarms are available in 170 or 175mm lengths, and come without SRAM/Truvativ's GXP bottom bracket. You can use your existing GXP BB, go for a standard GXP, or you can reach for the XX GXP BB, which runs smooth-as-silk on BlackBox ceramic bearings. While the cranks are black, the chain rings are Tungsten Grey and Silver. Claimed weight of the SRAM XX 42/28 crankset with the GXP bottom bracket is 754g, and the 39/26 version at 730g.