We've all been there, grinding slowly up a never-ending climb as our cadence slows to a high-torque, barely moving crawl, fruitlessly clicking the shift lever in hopes that an ethereal body will drop out of the sky and deliver an extra cog to lighten the pressure. All jokes aside, SRAM swoops in with its top-spec XX1 Eagle 12-Speed Rear Derailleur. It's situated as the lightest mechanical rear derailleur in the line, and utilizes top-tier materials for an impressive strength to weight ratio. The titanium spring and carbon fiber outer cage deliver precise gear changing across the entire cassette. The Roller Bearing Clutch, long revered for its nearly unbeatable chain stabilization, gets an update with a repositioned upper pulley and larger lower pulley to accommodate the extra chain length needed to run such a wide range of cogs. With SRAM's Exact Actuation technology, each gear shift requires the same amount of cable pull, resulting in a predictable click and shift no matter where the chain sits in the cassette. SRAM's nifty lock pin design also makes it easy to lock the derailleur in place with the chain slacked out to make it easier to pop your rear wheel on and off.