To the casual observer, the SRAM XX Rear Derailleur might look just like a SRAM X.0 rear derailleur. Far from it. While it does share a great number of details, the XX Derailleur is quite different.For one, the full carbon fiber cage is built specifically for its role in the XX group to accommodate the narrower 10-speed chain and 36t cog. The XX Rear Derailleur is also the first SRAM mtb derailleur to use their Exact Actuation Technology along with the matching XX shifters. While their 9-speed groups have long relied on a 1:1 pull ratio, designed to be more tolerant of the muck and grit associated with mountain biking, the Exact Actuation in the 10-speed XX group takes the performance to another level by ensuring a perfect and consistent shift distance between each gear, all the way across the cassette. And like the rest of their rear derailleurs, the XX model is designed for direct cable routing, eliminating the typical loop of excess housing necessary with other derailleurs.The XX Rear Derailleur is feathery light, mostly due to the use of high tech materials. We mentioned the full carbon fiber cage, but just as impressive is the combination of the forged magnesium and 6061-T6 aluminum inner and outer links, the forged 7075-T6 aluminum B-knuckle, and the use of aluminum and titanium for the hardware and cage spring respectively. It’s apparent that no material was worth overlooking to create what we feel like has been their finest offering to date. As evidence of their quest for perfection, they even used their Blackbox ceramic bearings in the pulleys to eliminate excess drag when you’re trying to go as fast as possible.The SRAM XX Rear Derailleur is designed to work with SRAM’s 10-speed 11-32 and 11-36 cassettes paired with a double crankset. It’s available with two cage length options — Mid and Long. The mid length cage is best suited for use with hardtails and single-pivot suspension designs with minimal chain growth throughout the range of travel. The long cage model can handle the greater chain growth potential that many full suspension trail bikes exhibit. If in doubt, go with the long cage version. It was once the only option anyway.Click here to save when you purchase an XX build kit.