This small part is really the heart of the Ultegra Di2 system.
The Shimano SM-JC40/41 Rear Junction, also known as Junction B, connects all the Dura-Ace 9070 and Ultegra 6770 Di2 electronic components to the battery unit. In addition to this important task, it serves as the interface between the Front Wire/Junction (Junction A), which transmits data from the controls, and the Di2 Derailleurs.
This unit available is available in two versions, one for internal and one external wires. The external model, SM-JC40, mounts under the bottom bracket shell. This Junction also provides several alternate wire routing options which allow the user to take up varying amounts of slack (up to 1.2cm) for electronic wires that are slightly too long. The internal model, SM-JC41, mounts inside the down tube just above the bottom bracket shell. All electronic wires (EW-SD50) are sold separately.
Note: Neither the SM-JC40 or SM-JC41 Rear Junction is compatible with older 7970-series Dura-Ace Di2 components.
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