The Shimano Di2 SM-JC41 is an internal junction box (commonly called Junction B) designed for use with Shimano’s Di2 electronic shifting systems. It serves as a central hub inside the bike frame, connecting the front and rear derailleurs, cockpit controls, and battery via SD50 E-tube wires. With four female connectors, the SM-JC41 is ideal for cyclists who want a clean, internal wiring setup—typically being housed in the downtube or seat tube, sometimes secured with double-sided tape for stability. This box is lightweight and compact, measuring 30.5mm long, 14.5mm wide, and 8mm high, making it suitable for tight spaces, though users should account for the extra length added by installed E-Tube wires.

The SM-JC41 is compatible with most Di2-equipped bikes that support internal wiring and uses the standard SD50 cable system for connections among derailleurs, cockpit components, and the battery. It is recommended for riders who prioritize a streamlined appearance and protection from external elements, as it routes cables within the frame. This product is not suitable for externally wired frames; for those setups, Shimano offers the SM-JC40 external junction box. The SM-JC41 is particularly useful for time trial, triathlon, and road cyclists who seek reliable, clutter-free electronic shifting solutions.