In 2009 Shimano brought electronic gear-shifting to road bikes, in the form of its Dura-Ace Di2 group. The slightly-less-pricey Ultegra Di2 system followed, although it was still aimed at roadies. Today, however, the company announced the upcoming release of XTR M9050 Di2, which will allow mountain bikers to swap their derailleur cables for electrical wiring.
As with the road versions of the system, XTR Di2 electronically relays signals from the shifters to motors in the front and rear derailleurs. This means that shifts are consistently quick and smooth, as they aren't affected by the slackening of stretched steel cables or by contaminants within the cable housings. That said, users will have to keep an eye on the battery level, and they'll definitely want to avoid damaging the wires ... sorry, the system isn't wireless.
Because the thumb levers on the new Firebolt shifters don't have to be aligned with a mechanical mechanism, they're free to be rotated around the shifter body in order to best suit the rider. Their default position is also said to be more ergonomic than that of regular shift levers, plus they require less effort to push.
Additionally, using the Shimano Synchronized Shift function, the system can shift both the front and rear derailleurs at once via a single shifter. Along the same lines as IXOW's lower-tech Synchrobox, the system coordinates the two derailleurs with one another, so that they shift together to attain the desired gear ratios without "cross-chaining."
Besides making shifting quicker and simpler, the feature also allows riders to equip their bike with just one shifter if they so desire. If they'd prefer to manage both derailleurs themselves, however, they can do so by simply switching to Manual on the bar-mounted SC-M9050 Display Unit. That unit also indicates battery level and the gear currently selected, plus it sounds a notification tone when automatic front derailleur shifts are about to occur. It additionally houses the system's battery-charging port.
Other features of XTR Di2 include automated trim on the front derailleur (to keep the chain from rubbing on it), a low-profile clutch-style rear derailleur, and compatibility with 1x, 2x and 3x drivetrains. The system is designed to work with Shimano's existing XTR M9000 group of components. According to a report on BikeRumor, the weight of a 2x single-shifter Di2 setup should be about the same as an equivalent M9000 mechanical system.
Pricing has yet to be announced, and availability is scheduled for the fourth quarter of this year.