I have to take issue with the claim that the timing "mid" chainring will compress or flex and "pop" out of the timing chain. It doesn't seem likely that both runs of the timing chain could simultaneously be under tension, causing a compressive force on the mid ring. My experience with this "popping" out (and I do have experience with it) is that there is enough flex in the frame to misalign the front and rear rings and allow the mid ring to "walk" out.
<jameshmerz@gmail.com>; "Peter Brueggeman" <4peebee@peterbrueggeman.com> Sent: Monday, January 10, 2011 6:41 PM Subject: [CR] unattached chainring, was Re: GP Wilson.... tandem photo
> Once upon a time, I converted a tandem from this arrangement to a
> conventional eccentric for my buddy the great frame painter Les Lunas.
> Les had a lovely tandem built by one of the Portland (OR) frame builders
> of back then, with the "floating" chainring to take up chain slack. It was
> an elegant solution: you just moved the ring closer to a BB when the chain
> stretched, and it looked just great.
>
> But one day Les and his wife Sue were competing in the NJ state time
> trial, when Les noticed as chainring rolling down the road, pretty well
> matching their pace. It seems that if you jump hard, you can compress
> things and on the rebound the ring can pop out. After that, we put in an
> eccentric, and sacrificed the lovely visuals.
>
> harvey sachs
> mcLean VA, where we still have a fabulous tri-color fade Les did in Imron,
> so subtle you can't find the fade areas.
> ++++++++++++++++++++++++++++++++++++++
> Jim Merz wrote:
> One can do this to adjust the chain tension without an eccentric BB.
>
> after Peter Brueggeman pointed out:
> In Dale's GP Wilson photos, there's a tandem photo showing an unattached
> chainring in the tandem chainline located between the captain and stoker
> chainrings. That's a new one to me. It looks like it is a couple of teeth
> larger than the front and rear chainrings. Is this something others have
> seen done, and does it accomplish anything other than looks?
> http://www.flickr.com/
> _______________________________________________
Andrew R Stewart
Rochester, NY