This one goes to eleven. SRAM's XX1 rear derailleur packs a massive amount of technology into its compact, lightweight body. It uses a “horizontal parallelogram” design that limits all movement to the horizontal axis, reducing the force it takes to shift, and eliminating ghost shifting. The Type 2 clutch is brilliant. This self-lubricating, one-way roller clutch controls your chain tension to eliminate chain slap and give you smooth, consistent shifts over any terrain. For quicker, smoother, more exact shifting, the large upper pulley's offset design maintains a consistent chain gap across all gears. Welcome to the future of mountain bike drivetrains.