Originally Posted by
oldacura
The torque placed on the stokers bottom bracket by the timing belt (basically the force you are applying to your pedals) will attempt to twist the stoker's bottom bracket clockwise (when viewed from above).
The tension caused by the drive chain trying to rotate the rear wheel is also attempting to twist the stoker's bottom bracket clockwise (when viewed from above).
The combination of these 2 torques could twist the chainrings quite a bit out of plane with the cassette (the chainline?).
If you can utilize some stout string on your frame to gauge frame flex, this may help solve the issue. Or at least identify or rule out flex. If you have ever used the string method to check frame alignment, then you'll have an idea of what I'm talking about. If not, check out this YouTube video:
However, this is usually done with the rear wheel removed and no one actually riding the bike. But I'm sure there might be a way to route a string from the head tube to the rear triangle so your stoker can see if s/he can observe the string and frame changing position relative to each other. Focus on the seat tube area to observe flex. Perhaps attach some rudimentary measuring cards next to the string so if flex moves the string's position, it will be easy to see. Or perhaps use a laser pointer/laser level? There must be some way to assess frame flex issues while riding. And I realize tandem frames flex in more axes than just side-to-side, so perhaps there's a more rigorous method to assess flex issues. You mentioned you're an engineer, so I'm sure you can appreciate this.
That being said, have you confirmed that your frame is indeed in alignment? I ask because if it were out to one side, this may also be the cause. It could be out of alignment an amount that doesn't cause any problems, but under frame flex, it is near and quickly passes a threshold that exceeds acceptable chainline and/or chain/cog mesh.
Also, no issues with a loose derailleur hanger or dropout somehow?
And this is the same rear wheel/hub used previously? Axles can flex under high loads, so this could be a cause as well. Have your stoker watch the rear wheel between the chainstays under high pedal loads to see if there's any movement.
Boy, this is obviously frustrating for you. Good luck with resolving this; I'm sure many of the people here are eager to see you solve it and learn from your experience.