Originally Posted by
Steve B.
Well, as far as I've been able to figure out, in order to know where to shift, it needs to know rings and cog sizes. Thats how it calculates the shift. It needs to know so when you are in a 13 cog on an 11-34 and in the 31 ring and want to shift up it can determine the 46/19 is next (using Mike Shermans gear chart), but it should need the ring size to determine what combo. I cannot recall if you had control over where the shift occurs, I remember being able to set something, I mostly set it up and have left it alone and I never really use Synchro.
And thanks, I had a typo on the Di2 GRX crank
It really isn't that technical, and the rear derailleur doesn't really matter as long as it has capacity. And 46/30 has the same 16t differential as any compact crank. Di2 isn't making complex choices to shift - it just needs to know where the cogs are laterally.
I don't think there will be a problem.