Originally Posted by
Brad Bedell
I seem to think that it rubs a bit in the last 3-4 'low' gears. I'm considering going to a double setup. With 110bcd, I think I can find a 54t front ring and run a 34-36ish small ring and be done with these issues. Of course, gear spread would be 'tricky' to shift, but with a 12-29 rear cassette, I doubt we'd ever need more.
Again, let me be the camel with his nose in the tent...
I'm not sure what you mean by the "low" gears, but if you mean the three smallest rear sprockets, e.g., 12, 13, 14... and what you've got is chain rub on the derailleur when you're in the granny ring and sometimes even with the chain in the middle ring, that's not all that uncommon for tandems with their non-standard rear chainlines on 145mm rear-spaced frames and, worse yet on 160mm rear spaced frames. You can also find that what initially sounds like chain rub on a front derailleur cage is, in fact, the chain bushing up against the middle (when you're in granny) or big (when you're in the middle) chain rings when you're cross chained in the aforementioned smallest rear cogs. Again, its usually the combination of a rear crank / bottom bracket that's designed to have the correct chain line for a 130mm rear spaced frame with a 145mm (or 160mm) rear spaced tandem that creates these interference issue. Moreover, the bigger the gap between the big - medium - small chain rings the more pronounced the chain ring rub can be.
The latter is mentioned in regard to your proposed 54/34 "compact" (or standard, if you prefer) 110 BCD crank configuration. Unless you get really fancy by using a superwide rear bottom bracket and 'far-out' front derailleur mount from Santana or one of FSA's tandem FD mounts, you'll most likely have all kinds of chain rub on the big chain ring when you're in any of the smaller cogs.
Again, I'm not exactly clear on what interference issues you're having, so I could be way off base here. But, as mentioned before, when it comes to tandems and shifting systems sometimes the best performance will come from the less complex solutions... and even then you'll be hard pressed to get a perfect shifting solution if you're not using a tandem with narrow rear spacing (e.g., 130mm or 135mm) that doesn't have the same chain line issues that the more standard-sized production tandem frames use, e.g., 145mm and 160mm.
Feel free to ignore and keep us posted on your science project.