Leonard Bulger wrote: <snip> The seat tube is curved right where the derailleur clamps, and = if you try to lower the derailleur, the back of the cage comes up.
Beloved Spouse and I rode Schwinn Town & Country tandems with curved rear tube for decades (really). The most successful front shifters were based on an idea I stole from Bill Boston. For some reason, the cage and parallelogram assemblies of the Shimano Titlist were two independent assemblies, joined by spotwelds on the inner face of the cage assembly. It was possible to drill out the spotwelds, then braze the two together again with the attachment rotated and moved a cm or so forward. This exactly emulated the position of the cage on a standard bike, and didn't take but a few hours of fiddling around. Of course, the Titlist had a short cage, so the mods had to include a cage extension. By this point, the chrome was a pretty uniform dull grey, of course. Did I mention that the one at hand was also modified to accept a top cable run? This was nuts.
Desperate times call forth desperate measures. I finally got fed up, moved the derailleur and drive to the front, and lived happily afterwards -- after finding a rear derailleur that would support all that chain!
Your mileage may vary
harvey sachs
mcLean va