Two SKUs, two cassette options: 8100Di2KIT11-30 pairs the R8170 Di2 kit with an 11-30T cassette for riders who spend most of their time on rolling terrain, while 8100Di2KIT11-34 swaps in the 11-34T for climbers and anyone building up a bike for varied gradients. Everything else in the kit is identical — ST-R8170 shift/brake levers, BR-R8170 flat-mount calipers, FD-R8150 front derailleur, RD-R8150 rear derailleur, CN-M8100 12-speed chain, BT-DN300 internal battery, EW-EC300 charging cable. Pick the gearing that matches the terrain you actually ride, then bring your own crankset, bottom bracket, and rotors to finish the build.

The R8100 series is the generation where Shimano moved to semi-wireless Di2, so the ST-R8170 levers run on CR1632 coin cells and talk to the rear derailleur wirelessly. The derailleurs still wire back to a central BT-DN300 battery mounted inside the seatpost or downtube, and a single EW-SD300 wire runs between the two derailleurs. That's the reason there's no wire running through the bars or stem on current-generation builds — a big deal on modern frames with fully integrated cockpits, where every wire you don't have to route is a headache avoided.

Shifting speed jumped notably over the previous R8050 generation, with Shimano quoting 58% faster rear shifts and 45% faster front shifts thanks to redesigned servo motors. Hyperglide+ tooth profiles on the CS-R8100 cassette smooth out the transitions across all 12 sprockets, particularly under load, and the CN-M8100 chain — the same 12-speed chain Shimano uses on XTR and Dura-Ace — handles the actuation cleanly. Front shifting under power, historically the weak spot on electronic groupsets, is genuinely quiet now.