What profile of pole would be torsionally rigid? Answered
Calling all mechanical engineers and physicists. I want to make a pole that can resist twisting forces. The application is a down tube on a bicycle frame. And I want to use bamboo because it's light, very strong, very cheap, and biodegradable. The problem with bamboo is it lacks torsional rigidity because its fibers only run lengthwise. I was thinking of wrapping a bamboo pole with a composite fiber (hemp and glue) at a 45 degree angle like they do with carbon fiber poles but maybe a different shape would be better.
Can anyone think of a specific geometry of pole that is tortionally stiff?
I noticed some bicycles have a triangular down tube. Maybe if I cut bamboo into three strips and glued them together into a tube with a triangular cross section, this might be less twisty. Am I thinking along the right lines here?
My first bamboo bike has a lot of twist and this might be one way to make it stronger. Please let me know what you think.
The issue was many fold. The short answer is Tonkin Bamboo is the species to use for bikes. This is what most people seem to use. Also Boo Bikes use dendrocalamus strictus (aka Iron Bamboo). My frame had some cracks possibly due to harvesting at the wrong season. I've recently gained more cracks and the bike flexes even more. It flexes like a noodle but I can still do an endo no problem. Go figure.
Also hemp sounds like it gets more stretchy after being flexed about 1000 times whereas carbon fiber stays stiff. And squishy wood glue is probably not as stiff as epoxy. All this adds up to a flexy frame. My next one will be tonkin bamboo but I will try hemp and non-toxic wood glue again.
I'm making a three wheeler and my engineering design teacher gave me some advice involving spaceframes and triangles. The plan thus far looks like this upright tadpole trike:
UPDATE #3: I think I've got it! Instructables sent me a box of K'nex and I built up a model of a standard bike's frame. The thing is, when a bike frame twists, the opposing corners of the trapezoidal shape come closer or further apart. So a deep X brace from corner to corner seems to fix most of the problem. I'll try to post a youtube vid of what I mean soon.