Originally Posted by
merlinextraligh
Having owned a Bike Friday tandem, and a bike with S&S couplers, IMHO it's a less elegant solution.
You have to fuss with it every time you reassemble the bike to get the tension right on the timing chain.
And as you noted, you have to adjust the deraileurs each time you reassemble, which in my experience could take a bit of effort to get dialed in precisely.
To me, it's a lower cost option with some tradeoffs.
Thanks for sharing your experience. It is quite thought provoking. I agree that in area of derailleur adjustment, S&S couplers are most definitely superior.
So, I've been thinking a bit about the assembly process and how to eliminate, or at least minimize, the need to adjust the derailleurs. I can think of three variables that will cause the boom tube length to change:
1) Pressure on the seat prior to tightening the last clamp - It seems to me that with a little practice this could be consistent enough that it won't be an issue.
2) Non-concentric chainrings changing the length depending on orientation - Supposedly Davinci timing rings are perfectly concentric, so this shouldn't be an issue. However, to eliminate any variation, the cranks could be turned to the same orientation at each assembly.
3) Chain wear - This probably won't be an issue from assembly to assembly, but over the long term this is a big issue. My understanding is that you should replace the chain when it reaches 0.5% elongation. For a 31 inch stoker compartment, that equates to almost 4 mm. So over the life of the chain, the derailleur cables will need to get 4 mm longer. That's a lot!! I will probably end up changing out the timing chain much earlier in than I would with an eccentric BB.
What other variables am I missing?
All of these variables could be eliminated by adding an eccentric BB. I'm going to have to think some more about that.