Join Date: Jan 2008
Location: Beaverton, OR
Bikes: 2013 Kona Jake, 2015 Kona Jake the Snake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2001 LeMond Buenos Aires, 1984 Pinarello Gran Turismo, 1982 Trek 614, 1978 Austro-Daimler, 1987 Pinarello Montello
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Yes, that was not helpful.
I tried Google before posting here and didn't find anything, but with a little more perseverance afterward I did find my answer. For the curious, the bottom of the derailleur cage rubs the middle ring when the big ring is in use (at least potentially).
An interesting artifact of this seems to be that you can't use anything above an Alivio-level derailleur on a crankset with 48-38-28 rings (which, of course, is probably not above Alivio level itself), and on the road side of things, you apparently can't use anything above a 105-level triple front derailleur on a 50-39-30 crankset (which is also something I was contemplating -- Ultegra 6603 FD with my FSA Gossamer triple).