Thanks to Sheldon and BikeForums….
Fellow tandemists may recall my post a while back telling the tale of the drivetrain issue I was having with my bike – the drive train would lock up intermittently while standing and climbing – very disconcerting. First time it happened, I dropped the bike and my stoker while tying to cope with the unplanned stop on a steep upgrade.
I thought it was chain suck, as did the guys at the shop. I replaced the drive chain – did not fix it. I replaced the middle and outer chain rings with non-ramped rings – did not fix it.
I emailed the shop again, and they said to bring it in (2 hours drive round trip). A day or so prior to taking it in, I post the problem on BikeForums – Bicycle Mechanics. After several posts, Sheldon Brown writes that it could be the timing chain swinging under the stoker’s crankarm.
And…low and behold that’s what the problem was! I witnessed it with my own eyes. Seems the timing chain was a bit loose at the tight spot and very loose at the loose spot. Being and old motorcyclist, I always erred on the side of loose with chains....
So… to fix it up right I am going to relocate the timing chainrings from the outside to the inside of the cranks and replace the chain.
If anybody else experiences this problem during spirited out-of-the-saddle climbing – look to see if the timing chain is under the stoker’s crank!
Thanks Sheldon and BikeForums!