, your post is very interesting, especially for me: I am slightly older an I am building a four-wheel-bike, i. e. adding two stabilizer wheels to a regular mountain bike. I haven't rode since I was 15-16, re-started after 60 and, to my surprise, could ride again! Not as well as in my teen years (I cannot do figure 8 nor even figure 1 no-hands, yet I do not own a car and use my bike almost all year long. My balance is not good now, so starting and finishing, as well as riding at slow speed in general is not easy.
I just started reading your post (I am still on Chapter 1) and have a question already (I have some work to do here and will return to your post time permitting): a bike made of 2x4 will probably be heavy and aesthetically unappealing for sure. Bamboo would be estetically acceptable, yet joining the pieces is problematic now (I still did not read that Chapter) but certainly doable - I know that it was done
Since I have a regular bike, I don't have to build the frame, Schwinn already did it. I only have to build a "framelette", to support the stabiliser wheels (I use standard front forkes). Actually, I alredy made one, of welded steel. But it has needed minor modifications, which meant cutting the original one and welding modified pieces again. I do not weld myself, so it is expensive (and cumbersome too as I have to walk the bicycle to the welder). My next door neighbor is an amateur plumber (he said "It's my hobby") and he suggested two alternatives: steel pipes, say ~1" OD and PVC pipes. The former are very easy to join together with elbows and T's, but the final product will be heavy, even if I need just to support 2 wheels+forks. PVS pipe is even easier to join, is incredibly cheap and, to me, is more aesthetically acceptable. So far I could not find anything re: PVC pipe mechanical strength: the only thing I found so far is it resistance to internal pressure - after all that's what PVC pipe is made for. What is your opinion? To me, the pipe (~2" OD) appears strong enough, especially short pieces. I even wonder why PVC pipes are not used to make bicycle frames, at least for kids. The bicycles will be very lightweight.