No happy medium with front derailleur on this bike!
This is driving me crazy.
I have a dj style bike with only the bottom and middle front ring in use. The top ring is a chainring guard. I had to replace the bottom bracket and the crankset. The old BB was an Octalink 118 mm with matching crankset. The new setup is a square taper crank 113mm with matching crankset. The chainline seems fine, the middle of the rear 8 speed cassette lines up with the middle chainring, plus I did a bunch of calculations to make sure it was correct.
Anyways, I have tried 2 different dérailleurs and I cannot get the chain to not rub on the last 3cogs of the cassette when in the small ring in the front, or vice versa. I would expect some rub on the small/small but it does this on the 6/7 gear as well. If I adjust it any more, it will rub on the top cogs. The current dérailleur is a 9 speed front. There is just no way I can set it up any nicer, it is perfectly in line and as low as it can go without smashing the chainring guard.