Originally Posted by
dvs cycles
"Since the eccentric has only a limited range of adjustability, in some cases, it may not be possible to use even-size synch chains: if you replace a pair of 39 tooth rings with a pair of 40s, you need to adjust the eccentric 1/4" closer to the fixed bottom bracket, or you can add a link to the synch chain and move the eccentric 3/4" farther. Some eccentrics may not permit this amount of adjustment."
I believe most tandem eccentrics will 'cover the spread'. In a worst case scenario, you could find that as offered by your friends that your bottom bracket may end up being rotated well forward or well back in the eccentric shell. It's actually the length of the boom tube that really determines how it will all play out.
However, if you found yourself with the bottom bracket axle sitting too far forward or backward to be practical you can install a 1/2 link in the sync chain that will move the bottom bracket axle into a more central position where you'll then need to decide if you want it sitting high or low.
The point is, there are always lots of alternatives and options.
FWIW, our Calfee would probably be better-off with a set of odd-numbered timing rings because I spec'd the boom tube at 30" on the dot without running the numbers. Therefore, running even numbered timing rings causes my bottom bracket axle to end up biased well forward or well backward... a less than ideal situation for me that I addressed with a a 1/2 link.