Shimano Di2 offers the most precise and reliable shifting imaginable, at the push of a button. Adjust the derailleurs once, and the intelligent, auto-trimming system returns derailleurs to the same position every time. The intuitive and user-friendly design can be set up at home and accommodates any frame; new or old.
The Shimano Di2 SM-JC4 junction box connects the electric wires from the rear derailleur, front derailleur, battery, and shift levers. There are two types, external (mounted under the bottom bracket) and internal (for Di2 compatible frames.)