Originally Posted by
vision646
Maybe this has been discussed but I'm only on page 10 of this behemoth. Can you run a 53/34 chain ring with di2, preferably with an 11-28? I've heard that the reasons not to do this with mechanical groups is shifting quality tends to suck, but can di2 account fix that? Thanks.
Shimano road FDs are spec'ed to 16t max capacity (# of teeth in big ring - # in small ring). that is part of the reason for the jumps you see like 50-34, 52-36, 53-39 (<16t....but 39T was basically a minimum for 130bcd), etc.
you'll get not-so-great shifting due to mismatched rings (/timing/shift ramps), but beyond that the cage just isn't set up to efficiently handle a 19T drop. if you set it up (vertically) for the 53, then the bottom of the parallelogram is likely to rub in at least some gears.
Shimano is usually conservative with capacity and wrap, at least for RDs (part of that is that one's hanger geometry impacts the max cog size), but my experience is that the FDs are spec'ed properly.
MTB FDs handle greater capacity, but that doesn't help you.
and di2, while great, has nothing to do with physical limits of the FD.