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.

Tuesday, 23 June 2009

Some thoughts on design

Before starting the project, I feel it would be helpful to outline what our objectives are. That way we don’t get distracted and end up going down a blind alley building a machine that can’t be used anywhere. Or, for that matter starting a project that will never get finished.

It needs to be cheap, partly because our budget is very limited, but also cheap materials allow my sons to get involved without the danger of making expensive mistakes. The car needs to be finished for May next year, but that includes being tested and sorted so it needs to be complete at least a couple of months before that. Also, allowing for the short attention span of digitally addicted youngsters, progress has to be continuously apparent.

With these considerations in mind, the strategy is to come up with a concept that is easily realized but flexible enough to allow continuous development. That means doing a workable solution now and maybe upgrading it to the optimum solution at a later date. Once complete and running, it should in theory be possible to gradually work through the entire car replacing different parts to refine it. Eventually you might end up with a new vehicle that is better in every respect and have enough parts left over to recreate the original vehicle. But meanwhile, back in the real world, the primary objective is to get it sitting on its wheels and running as soon as possible.

Tuesday, 16 June 2009

How we got here

This all started with a phone call from my brother. He said that he and some friends were organizing a Bogie race in their local village. I should point out here that in the North of England, Bogie is familiar term for a soapbox racer. Elsewhere the term has less savory connotations.

‘You must enter,’ he enthused, ‘build something and bring it up. It was hardly what you might call a no-brainer. The four-hundred-mile journey to the venue was the least of the obstacles. Finding the time and resources to build a suitable vehicle in a suburban home were more demanding challenges.

Instead I called on a favour and borrowed a professionally built machine that was constructed to promote the company that created it. We took it north, had a great day and were very competitive. More importantly, for the first time in all my activity with vehicles, my sons felt engaged in the project. It had no engine and was relatively free of highway regulations so my offspring could sit in it, help work on it and even drive it. It occurred to me that a gravity racer project would be something we could all be involved in and enjoy.

With that in mind and the promise that my brother’s event would be reprised for next year the project was born.