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!

Yikes... That usually takes a lot of slack and bike throwning to get far enough off course to hook on the crank arm. I'm surprised y'all hadn't dropped the timing chain before the hooked crank episode. I can only recall seeing something like that happening once before but the chain was still wrapped around the stoker's pedal when the tandem came to a stop.

Good to hear Sheldon was able to break the code.

We have gotten into the habit - for better or worse - of rocking the bike while standing. I think most of the problem stems from the chain being stretched unevenly. I adjust the chain when it is rotated to the tightest spot and set it with about half an inch of vertical play. But, if I rotate the cranks to the most stretched portion of the chain, I have an inch or more of play. Also, even though the chain is not technically 'worn out' by it's how much it has stretched, it may have lost some lateral stiffness with use.

Or...my super-human power output is deflecting the entire frame causing excessive slack on the bottom half of the timing chain.....!!! :eek:

There might be more of that coming into play on supersteep climbs than you realize, particularly if you two are tossin' the bike side to side.

Also, with regard to the cam effect you're seeing on your sync chain, before moving your rings to the inside of the spider you might want to read Sheldon's article on centering timing rings: http://sheldonbrown.com/synchain.html