Building a Di2 disc brake bike used to mean ordering the lever from one place, the caliper from another, the hose kit from a third, and hoping everything showed up ready to bleed. Shimano's Ultegra ST-R8170 Dual Control Lever with Caliper collapses that parts list into a single J-kit assembly that ships pre-bled with the BR-R8170 flat mount caliper, the SM-BH90-JK-SSR hose already connected, L03A finned resin pads installed, and the mounting hardware you need to bolt it to the frame. Order the front and rear sides separately depending on how your bike is plumbed, and you have a complete braking and shifting system in two boxes.

The R8170 lever runs Shimano's semi-wireless Di2 architecture, which means the shifters talk to the derailleurs over a wireless signal while the derailleurs and central battery stay wired together. A CR1632 coin cell tucked inside each hood powers the shift signal for roughly 1.5 to 2 years of typical use, and swapping it is a job you can do at the kitchen table without a charger, cable, or app. The hood shape carries a raised peak that shifts your hand slightly forward and up compared to the previous generation, and the brake lever itself is aluminum with a carbon-reinforced body to keep pair weight around 391 grams for the levers alone.

On the braking side, the BR-R8170 is a two-piece flat mount caliper with two resin pistons and roughly 10 percent more pad clearance than the previous R8070 generation. That extra clearance is the difference between a wheel change that just drops in and one that needs a pad spread with a tire lever. Servo Wave lever action pulls the pads to the rotor quickly at the start of the stroke, then slows down through the power phase so you get modulation where you actually want it. The caliper works with 140mm or 160mm Center Lock rotors, which are sold separately.... Read More