Shimano's Dura-Ace Rear Derailleur boasts a light and reliable cold-forged-aluminum body with a carbon cage and a wide pivot design that boosts rigidity for smooth, speedy shifts. It has fluorine-coated pivots that minimize friction for nearly effortless shifting. The oversize sealed-bearing pulleys are silky smooth, ultra quiet, and keep out dirt and water. Plus, the 9000 accepts up to a 28-tooth cassette cog for even greater gearing possibilities.
An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 44/29 chainrings and a 11-28 cassette: (44 - 29 = 15) + (28 - 11 = 17) = 32-tooth chain-wrap capacity.