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So max chain wrap is 28 on 6400/6401 short cage. Subtract the small ring from the big ring (52-39 = 13) and subtract the small cog from the large (30-11 =19) add the differences together for you max capacity (32).
So if you’re running the original 6400 cranks and rings you’ll be out of recommended range. You could run 48/39 (9t difference) and with 11/30 be in range.
So the extender thing won't work because the cage is too short for the chain slack?