Was wondering if anyone knows this answer. I think I might have read somewhere that the brains to di2 is located in the front derailleur.
I want to build a flat bar bike for city riding. But instead do it with ultegra di2, I only require the bike to have 10 speed, so no front derailleur. I would use the climbing shifter (see link below).
Shimano Di2 Remote Switch SW-R610 - Competitive Cyclist
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Thread: 10 speed/gears only di2.