Shimano's Ultegra Rear Derailleur benefits from trickle-down technology from Shimano's top-line Dura-Ace model, which means you get awesome shifting at a more affordable price point. This choice derailleur boasts a light and tough cold-forged-aluminum body with a wide pivot design that boosts rigidity to speed and smooth shifts. Plus, the fluorine-coated pivots minimize friction for nearly effortless shifting and the oversize pulleys keep the chain running more smoothly, too. The pulleys also last longer than smaller ones and are sealed to keep out water and dirt.
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.