Originally Posted by
velosprinter
The BBs is 68mm so I would not want to go over 2.25" (57mm) so that would be 5mm on each side for joints.
You probably won't need to use anything larger than 2". You might need larger tubes on a tandem though, I'm not too sure. :/
I am also planning on hemp for the joints to be as eco as possible. It seems to come in two sizes .5 and 1.0 mm which should I get?
It probably doesn't matter either way. If you get .5 it will probably take a little bit longer to put on because it's smaller, but might look a little bit better because it will probably leave smaller gaps.
Do you have a tandem frame to cut up or at least model your bike after? If you've never made a frame before, I would recommend cutting the tubes out one, or maybe two at a time and replacing them with bamboo.