Lew,
Basic design looks really good.
Couple of thoughts, mainly about rear triangle.
On my bamboo built the rear triangle caused one of the few “aw hell” moments,
when it looked like only a skinny tire was going to fit between the chain stays.
I got lucky, and a 38C tire works. But getting enough spread between the stays is challenge with bamboo.
Your first idea of bending the bamboo – don’t know, but I’m not too confident about that one.
Your second idea re building rear stays out of solid bamboo – cool idea, would work to give the right
space for substantial tires, but think it may have one killer problem. The areas I shaded in diagram
will both be “short bamboo fiber”. While bamboo fibers have great long-fiber strength, not sure about
bamboo strength in this kind of short-fiber situation. Looks like those areas could be vulnerable to breaking.
View attachment 1
Not fair to suggest problem without suggesting possible solution, so here is a thought.
Fabricate your own custom bottom bracket assembly, with a wider shell, like 120mm instead of 68.
Braze on the stubs for chain-stays and down-tube.
Relatively easy to do using just a propane torch and silver solder.
Instead of BBS02, use a BBSHD – empoweredcycles and maybe other vendors has them for up 120mm bottom bracket.
Then use simple straight bamboo poles for chainstays.
Presto, you’ve got an additional 50mm of tire clearance.
Crappily rendered illustration of concept below:
Thinking separately about Chalo's comment re seat tube. He might be on the money re need for more bracing, just not sure.
