Originally Posted by
HillRider
Yes, you can use a short cage rear derailleur with a triple crank set and a 12x27 cassette (total teeth = (53-30) + (27-12) = 38) BUT you will have to avoid the granny ring with several of the smallest cogs. Shimano claims the short cage RD-7800 will wrap 28 total teeth so you would have to limit the granny to the 21 tooth or larger cogs.
Yes, one would have to pay a bit of attention to the gears in such case. Anyway, it seems to be a long cage, which settles the case.