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 SSM-JC41 Junction Box B 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.)