Tuesday, 30 June 2009

Chassis construction

Possibly the biggest single challenge is going to be the chassis. OK, it’s only a bracket that connects all the important bits together. Bit it is a particularly large bit and will take a big slice of the effort we put into the project.

The most obvious approach would be a welded steel frame, but I don’t have the facilities to weld. And, even if I did, I want the design to allow my sons to have a hands-on involvement in the build. That would be tricky with welded construction as they are still a bit young to play with hot flames and high–voltage arcs.

Thinking about cold forming processes, I have considered several options including a stressed-skin, glass-fibre monocoque. This would be a very elegant solution with the aerodynamic outer skin also comprising part of the main structure. However, once I started figuring out the process including bucks and moulds, it all started to look very labour intensive and expensive. Did I mention that bit? It needed to be cheap.

I then considered a bonded plywood tub. Having been a longtime fan of the Mosquito aircraft, the idea of using thin ply and balsawood end grain to create a structure was very appealing. Again, after close consideration, the process started to look very complex and, with the attention span of youngsters today, I would pretty soon end up doing it alone. The devil is so often in the details.

Thinking quick, cheap, and easy, I looked around the garage to see what I had available and came across an almost forgotten bundle of Dexion perforated steel angle. Suddenly it started to look like grown-up sized Meccano and all sorts of possibilities presented themselves.

What if we are to create a perimeter frame in Dexion, then panel it in plywood sheet to triangulate the structure. It would be simple, quick, cheap and my sons can be engaged in the process. This is starting to look like a plan. It might not be the lightest form of construction and may well end up being substantially over-engineered. But gravity racing usually specifies a maximum weight limit, in the case of our event 100kg, so we had a pretty large margin to work with. Yes some of the weight would end up higher up in the car than is ideal for optimum centre of gravity, but the penalty in performance would be relatively minor. Also, if my sons are going to compete in it, then added strength can only be welcomed.

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