Shimano redesigned their Di2 wiring when they launched 12-speed, and the EW-SD300 is the result. The cable diameter dropped to 2.1mm with a 3.2mm plug—noticeably smaller than the SD50 wires used in 11-speed systems. This reduction makes internal routing easier through tight frame passages and allows for more compact junction points throughout the system. The smaller connectors also enabled Shimano to shrink the components themselves, which is part of how 12-speed Di2 derailleurs ended up lighter than their predecessors despite gaining a gear.

With 12-speed road Di2, the wiring architecture simplified dramatically. The shifters communicate wirelessly with the system, so wires only need to run from the internal battery to each derailleur. A typical R9200 or R8100 build uses two SD300 wires—one to the front derailleur and one to the rear—with no junction boxes required. The rear derailleur handles all the system communication that previously required a separate wireless unit. For most builds, a 750mm wire reaches the front derailleur and a 1200mm wire handles the rear, though frame geometry and battery placement affect actual requirements.

Shimano offers the SD300 in lengths from 150mm up to 1600mm to accommodate different frame sizes and routing paths. Measure your routing before ordering since these wires cannot be cut or shortened—the connectors are molded onto each end and trimming would destroy them. When connecting SD300 wires, use Shimano's TL-EW300 plug tool to seat the connectors without damaging the smaller pins. The old TL-EW02 tool designed for SD50 wires doesn't fit the SD300 connector diameter.... Read More