Frame Design

I really know very little about bamboo as a building material. Exactly how thick a pole of what species do I need? My cursory research didn't reveal "how strong is bamboo" so I did what Idle Dads do, which is to get some and see. I need to keep costs down, but I figured that, depending on what I found, I could probably adapt the design. I ordered 1" dia. tonkin poles from http://bamboofencer.com/, which doesn't have surcharges for small orders and ships from nearby Massachussets.





Here are the three basic frame designs:

The Single Pole, or cross design, is what to use if the bamboo proves very strong and sufficiently rigid. In the picture above, the rear wheel is on the long end of the cross, the front wheels on the ends of the crossbars, and the crank and pedals on the bent tip (the crank needs to be above the seat, for various reasons).

The Double Pole design should be much stronger, and stiffer in the vertical dimension. Not shown in this sketch are short lengths of bamboo connecting the two principal poles. The weakness of this design is that there is no stiffening in the horizontal direction, nor from twisting. This means loss of some of the shock absorbency of bamboo, and some wiggle in steering. It is possible to put the two poles side-by-side instead of over-and-under, which would restore the shock absorbency and stiffen the steering, but the trade of would be loss of strength in the vertical direction. Since this is the direction of stress from a pothole hit at 30mph with 180lbs of Idle Dad aboard, it would seem to be the primary need for extra reinforcement.

The Triple Pole design is stronger in all directions. I can't imagine that it won't be strong enough. It loses some shock absorbency, and it is significantly more complex to build than the others, but is the assurance that my 1" poles will be adequate. The picture above may be tricky to see, but I am finishing a model that I will post soon.