Twelve speeds, a small cog of ten teeth and a large cog of 50 teeth. That’s both a lot of cogs and a huge range to cover. SRAM redesigned their 1x rear derailleur to do it all, and do it better. This is the XX1 Eagle version, the lightest of the breed.
The first order of business was accommodating the wide range of cogs. It necessitates not only a long cage, but a different way of pivoting. SRAM decided that the way to shift perfectly and eliminate ghost shifts was to make the shift action of the derailleur body a straight parallelogram. They call this X-Horizon. The body and the cage pivot remain parallel to the ground throughout the shift range. The cage pivots in such a way that the upper jockey wheel remains a consistent, short distance from the cog it is feeding.
The jockey wheels have been designed to better work with the twelve-speed Eagle chain and maintain shifting precision. The upper wheel possesses twelve teeth, runs on sealed stainless steel bearings, and has the X-Sync tall, squared, alternating tooth profile you find on X-Sync chainrings. They hold the chain better and shed mud better and the larger diameter means that the pulley wheel spins more slowly and has a more gradual bend, both of which lower friction. The lower jockey wheel has grown to fourteen-tooth circumference, also with the X-Sync profile for the same reasons.
The spring controlling the cage is more sophisticated than you’ll find on 1x eleven-speed derailleurs. They’ve moved on to Type-3 Roller Bearing Clutch technology. Basically, it’s a better way of working with a more consistent torque. Derailleur bounce and chain slap go away, the ride is quieter, while you still get easy, precise shifts with a light touch.
Because of the Roller Bearing Clutch design, SRAM realized that locking it out would make wheel changes, especially with thru-axles, easier. The earlier 1x rear derailleurs had a cage lock. The Eagle rear derailleur has the feature as well, but the button placement has changed. While it’s still within easy reach of your fingers, it has been moved to be out of the way of sticks, stones, gravel, and assorted trail debris that might be interested in locking out your derailleur while you’re pedaling.
When dealing with twelve cogs on a cassette, space between the rear dropouts is tight. Concerned about both shifting precision and a fear that the mounting bolt could interfere with the ten-tooth cog, SRAM went ahead and re-designed the B-Plate and B-Knuckle. The former is serrated to better hold the derailleur hanger, while the knuckle itself pivots on a bushing to as not to get locked into place.
As the XX1 rear derailleur, this model features all the weight-saving features SRAM could pack in. There’s liberal use of carbon fiber for the pulley cage as well as the outer cage plate for the derailleur body. The spring is titanium. Weight for this derailleur is 264g.
The SRAM Eagle XX1 rear derailleur is designed to work with SRAM’s Eagle twelve-speed 10-50 cassette and function with a full SRAM Eagle group.