Originally posted by brad
Dang Mark, if you are not in the industry, you should be. You've got more know how pertaining to tandems than any bike shop I've ever visited (I'm in the industry and I've been to hundreds of bike shops). When ever I have a question, I just post it here in hopes you'll check the site and tell me what I want to know. On second thought dont open a shop, because then you'd charge for your knowledge.
I'm humbled and not worthy. And to prove it let me point out that I had a senior moment when I wrote my note (I have a problem with keeping numbers straight - dyslexia limited to numbers, if you will) Anyway, the number that I should have used is .65 (65%), not .64.
I don't think it will change the answer too much but I would point out that some bike fitters / calculators use a factor as high as .67 (Colorado Cyclist is one of them I believe). Again, they're just starting points -- they key is finding a bicycle that fits you well and then writing down those numbers for future reference. My bikes are all funky when it comes to size -- but so's my body.
Getting back to the factors, .65 assumes you're using a Center-to-Center measurement: that would be center of the bottom bracket to the center of the junction of the seat tube with the top tube. I'm showing my my age. Center-to-Center works fine with conventional frame designs but is nearly meaningless in the age of slopping top tubes and compact frames. The other factor I mentioned, .67 is used as a reference for bikes that are sized based on the Center-to-Top of the Seat Tube which is what is more common these days.