Originally Posted by
HillRider
There is a limit to this too. Current front derailleur's have the inner cage plate significantly deeper than the outer so a minimum chainring difference must be maintained or the inner plate will hit the smaller chainring and keep the derailleur from shifting to the larger chainring unless the derailleur is set too high above the big ring. Older fd's had the two cage plates about the same depth which is why half-step gearing was practical but newer designs don't.
Good point! I'm more used to "road" stuff up to the 8-speed era, but I did notice this when I tried to set up a 48/42 double (not half-step, but an experiment with what I had) on my road bike with a Deore LX triple FD. Because the FD was designed around a 48/38/28 triple, it didn't "like" less than a 9T difference between rings.