Shimano Dura-Ace Di2 FD-7970 Front Derailleur
A self-trimming front derailleur; a dream that disappeared with the Suntour Cyclone Mark II down tube shifter. Until now: the Shimano Dura-Ace Di2 FD-7970 front derailleur trims itself. That cool. Frickin' cool. Jaw-dropping cool. Something so amazing you do have to see it to truly appreciate the marvel it is. There's an integrated central processing unit (CPU) that knows which gear your bike is in and thus knows where the front derailleur needs to be in relation to the chain. The first time you see it, you'll want to see it again. And again. And again, just like the day you first saw a derailleur in action. Do it again, please!That's the coolest feature and probably the main reason they call their electronic shifting Di2, Digital Integrated Intelligence. A second feature of the shifting that is much harder to see, but equally significant is the "programmed motion" of the derailleur. As with the rear derailleur, the front derailleur cage moves back and forth via a motor-driven worm drive. With programmed motion, the derailleur, once the low and high limits are set (on the main body of the derailleur, not the cage or pivots), for the small ring and the big ring you don't have to do anything else; that's obvious. What is most impressive is the two-stage motion of the derailleur. It derails the chain onto the big ring or small, the cage pushing the chain off and on. But a half second after the shift, the derailleur will re-center. This re-centering reduces the chance of over shifting over the big ring or over shifting past the small ring and dropping the chain on the bottom bracket shell.Lennard Zinn, Velo News' tech writer, claims the motion of shifting from the small ring to the big is so easy, smooth, and fast that it amounts to a "virtual hand sling." When he tested it, he found he could gain three or more bike lengths on riding companions after cresting a hill and shifting into the big meat.