Unsung hero.

With the XX1 X-Horizon Rear Derailleur, SRAM essentially reinvented the way rear derailleurs work. While this impressive feat tends to be overshadowed by the innovated one-by drivetrain that it's part of, the XX1 Rear Derailleur's reviews from Competitive customers are indicative of the impact that it's enjoyed on the industry. For the XX1 2.1 Rear Derailleur, SRAM stuck to that same script, making nominal changes for almost imperceptible improvements in shifting.

XX1 2.1's horizontal parallelogram is the same design that set XX1 2.0 apart from its competitors. Unlike a typical derailleur, the X-Horizon derailleur knuckle only moves side-to-side — not up-and-down. This limits the movement to the horizontal axis, making ghost shifting nonexistent. Looking at the derailleur pulley wheels, you'll notice the offset of the upper pulley. Unusual in design, the upper pulley provides a constant chain gap through all of the gear ratios.

Through the combined efforts of the X-sync chainring, pulley wheels, and a Roller Bearing Clutch, a chain guide isn't needed with XX1. Your chain is tensioned with the derailleur and locked in place with the pulleys and chainring. Also, combining the Roller Bearing Clutch and parallelogram design means that you'll never get chain slap, so your chainstay won't look marmot-gnawed. The derailleur is also made to withstand repeated hits while you're navigating through a rock garden in order to eliminate the chance of throwing the chain.

We'd be remiss if we didn't sing the praises of the XX1 2.1's Cage Lock technology. It locks in the open position, so you don't have to gorilla-fist your rear wheel out to address a mid-ride flat. Please note that the SRAM XX1 2.1 Rear Derailleur isn't compatible with a double or triple crank. In fact, it is only compatible with the XX1 shifter and cassette.

• Carbon fiber
• Parallelogram design
• Offset chain pulleys
• Compatible with the SRAM XX1 shifter and cassette