if you fit a standard bb, the 2mm thickness of the mounting plate will effectively make the spindle even longer on that side and this could be contributing to the problem (despite what fmj erroneously suggests above).
Sorry, but your math doesn't work. Because of the arcs of the parallelogram levers in a derailleur the cage moves out more initially than up, reversing (up more than out) once it's past the 45 deg. point of the radius. The closer you move the chainring towards the centreline of the frame, the less upward travel in the cage you have, and the more likely it is to contact a larger ring. I'm at work now, and I don't think my boss would be too happy with me spending a couple or 3 hours mocking up a 3D model of a front derailleur in MC-X6, so I printed up a diagram off the net and hacked it up with scissors in 5 minutes to try
and show you why.
Interesting, very much so and clears a lot up regards the mechanics
Though makes everything more confusing on a practical level
Does this make e type mechs a pain in the backside to install? Is a dedicated Bb required? And dedicated cranks and rings ?
So basically id be better off with a clamped front mech to be on the safe side ?