Tuesday, 14 July 2009

Cardboard model








Here is our scale subject. Note, if you will, the faithful reproduction of the real driver’s physique. No sniggering at the back please.
We started with a simple five-panelled box [four sides and a bottom] cut closely around our model. Materials included cereal packet to represent the plywood and masking tape to represent the Dexion. Yes, I know the tape can do things Dexion can’t, and the same goes for cardboard and plywood, so we had to assign certain rules to their use. That includes not bending or curving the card and making sure the tape can only be used along a 90-degree angle. This ensures, what we make in miniature we can build for real.
I don’t know about educating my sons, but I found it a very enlightening process. The five-sided box is not very rigid, particularly in torsion because of its open top. You can make it more rigid by closing the sixth side but then a driver can’t get in or out. The challenge is to design in as much stiffness as possible while still allowing access.
With said box in our hands and trying different bits, it very soon became clear what was going on and how to capture the desired stiffness. One innovation we tried was a triangular section floor. The theory was that if it was closed off with a third panel under the tub it would create a rigid structure that would lend strength to the whole chassis. It was a failure making no discernable contribution to the overall stiffness and we left it out of the final design.
The crucial piece of knowledge I picked up is that under torsion, that is twisting the ends in opposite directions, the sides of the tub try to move in opposite directions. If you can prevent this you pick up loads of strength. We achieved it by closing off half of the tub top then putting flanges along the top sides to prevent them buckling. Try it and see for yourself.
With hastily assembled wheels and axles you can see the basic concept for our car in three dimensions. Now, doesn’t our chap look proud?

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