Shimano's Dura-Ace Di2 RD-9070 Eleven-Speed Rear Derailleur is a pretty amazing derailleur even before you realize it has a motor and works electronically. Shimano started with the knowledge base that comes with designing some of the best mechanical shifting mechanisms around. The links on the parallelogram are wide for easier shifting. The pulley plate is made from carbon fiber to save weight while still retaining all the strength of aluminum. The pulleys run on sealed bearing and are 11-teeth in circumference to minimize drag. And the cage design allows for cogs up to 28 teeth. That it's Di2 adds another layer of impressive engineering. The derailleur makes the same exact shift regardless of where it is in the derailleur's travel, unlike mechanical shifters that pull a bit differently depending on position. And the derailleur slightly over shifts and comes back into perfect position each time. Even with the mini-motor added, the derailleur is a fairly svelte 217g, meaning almost no weight penalty for the electronics. There's a "saver" feature built in. This means that if the derailleur experiences a hit of sufficient force, the motor disengages from the derailleur, thus protecting both. When you get up to ride again, you hold the button on Junction Box A (SM-EW90) down, pedal, and the motor will re-engage with the body. The derailleur comes with nothing other than the derailleur itself. You'll need to purchase the wires, shifters, and battery separately. Because of all the options available, Shimano sells this as an ala carte system. The Shimano Di2 RD-9070 Rear Derailleur brings 11-speed shifting to the electronic world.
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