Idea #1: Like a lot of semi-production builders, standardization can keep you out of trouble and 116mm is a 'good' standard for most tandem buyers who aren't sensitive to crank tread width (aka, Q-factor). Again 116mm is actually narrow compared to what you'll find on other stock tandems. So, I believe this is where the folks at R&E are coming from, particularly if your original quote from them was accurate, i.e., they find that 116mm works 'best'. Two of our tandems -- the Erickson's -- were fabricated by R&E's current master builder, Dennis. Erickson and Rodriquez tandems share the same pedigree, hence the R&E name on the shop. Therefore, Dennis would be about the only one who would be able to know with any certainty if there are any significant differences in the chainstay diameter or perhaps the length of the stays that would limit the size of the spindle that could be used with the daVinci cranks. daVinci's cranks have the benefit of using 34t timing rings which also factors into making them more compatible with a narrow spindle vs. the more common 39t and 42t timing rings used on other tandem cranksets.
Originally Posted by tiggermaxcocoa
Idea #2: Not knowing how you've made your request for a BB spindle configuration that diverges from what your builder recommends, if you haven't done so you may want to approach your desire's by asking these questions:
1. What's the most narrow width spindle that I can use for the rear bottom bracket without having any granny ring or daVinci 34t timing ring interference?
2. If the answer remains 116mm or isn't as narrow as you'd like, follow-up with, "What is the source of the interference: insufficient drive-side granny ring clearance to the chain stay, insufficient timing ring clearance to the chain stay or possible heel clearance issues?
Note: When I was running the 108mm rear spindle on our Ericksons I had about 2mm of clearance between the granny ring and the stay, which is 'tight' by builders standards, particularly with square taper since over-torqing the fixing bolts when installing your rear crank could easily consume that clearance.
3. Not knowing what type of BB's they are using, "Would using something like a Phil Wood BB that can easily be off-set to adjust chain line solve any of the potential intereference issues?"
Bottom Line: A good builder will try to steer customers towards "good" configurations using reliable components that work well for the vast majority of average consumers. Unique requirements provide unique opportunities for future problems and uphappy customers that the builder must address as well as the potential bad rap they may get for delivering a product that didn't "work right". When you start to move out of that box, you have to recognize that you will encounter some resistance and further from the center of the box you move, the more resistance you get until you enter the realm of a true-custom buyer... and that'll cost extra and you get what you ask for. It's also at that point that -- as someone else already mentioned -- you assume the risk for stuff that doesn't work. If you're OK with that, and are willing to take the "hit" for the stuff that doesn't work (that is, not blaming your builder or dealer), life is good. But it clearly is a quid pro quo. You get what you want, but you give up the right to assign blame to your builder or dealer. Yes, I've paid to have things I got wrong fixed and ate crow on a couple "I told you so" things over the years. I'm OK with that.