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Of all the bikes we rode at this year's test, we were most excited about the . Why? It was the only bike spec'd with the new electronic  drivetrain. This shifting system, which borrows from the company’s two road Di2 component groups, will officially launch in early February. And it would have been almost impossible to test except that Pivot was the bike partner in the group’s development and was therefore able to get us a system. 

Our Mach 4c, with 27.5-inch wheels, 115mm of rear travel, and a 120mm Fox fork, came equipped with a 2×11 drivetrain—a departure from the SRAM offering, which offers either 1×11 or 2×10, including a 36-26 configuration in front and the new 11×40 rear cassette. 

With two rings installed, Shimano’s Synchro Shifting moves the front and rear derailleur using input from only the rear shifter. When set in an auto mode, the system selects the most efficient gear ratio for you, based on chain line and the smallest steps between gears. If necessary, it shifts both your rear and front derailleurs simultaneously. This sounds as if it could be disastrous, but the shifting is so fast and efficient that in practice we never even noticed it, other than an audible, dual-tone beep to alert us when both derailleurs shifted at once. 

The XTR Di2 also has all the benefits of Shimano’s Ìý²¹²Ô»åÌý kits on the road: lightning fast shifting, incredible accuracy, the capacity to shift even while standing and exerting heavy load on the pedals, and the ability to dump gears with a single button push. And the new shifters, the buttons of which can be programmed to handle any task, have a crisp, tactile feel.

The only downside is the cost. A complete groupo, with cables, battery, and a single chain ring up front, will run around $3,500, which is about $1,500 more than a comparable XTR M9000 setup. But that's the price of first-adoption technology. If you’re unwilling to pay the premium for XTR Di2, just wait a few years. For now, though, the Mach 4c with XTR Di2 costs a significant $8,200, though the frame is available in less expensive builds for half that price.

Though Pivot’s integration of XTR Di2 is flawless, including a hidden port in the down tube for the battery and slick internal cabling throughout, the Mach 4c is more than it’s high-tech drivetrain. The DWLink-powered rear suspension provides the pedaling efficiency of a race bike but still feels supple and linear throughout the travel. The steering is spunky but not nervous, the handling is agile, and the very low stand-over height makes it easy to push the bike around.

It’s the exemplar of the new standard of cross-country race bikes, with smaller wheels and more travel that make the bike feel capable beyond race day. Testers exclaimed how fast and proficient the Mach 4c felt, though several lamented the smaller wheels. While 27.5ers continue to catch on, most riders continue to prefer the 29er for short-travel machines. As if to preempt that criticism, Pivot just launched a revamped , a big-wheel version with weight savings and all the XTR Di2 integration of the 4c, meaning there’s now an option for everyone.