Designing a pull-back racer

I recently offered a workshop at WPL for kids where we made wind-up, or pull-back, racers. Instead of the standard cardboard bodies, which in testing I felt bent far too easily, I opted to quickly design and laser-cut some basic frames made out of 6mm MDF the kids could attach their parts onto.

This led me to thinking: could I design a whole pull-back racer, save for the elastic, to be cut on one sheet of MDF?

After some internet sleuthing for ideas, I did see one, similar example to what I had in mind, and with that as a source of inspiration, I fired up Inkscape and went to work designing.

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After designing, I needed a quick trip to Kwartzlab for 20 minutes on the laser cutter to cut the design, and move closer to turning my idea into reality. I dry-fitted the components, and it started to look like a car.

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30 minutes of gluing later, and I had a fully-functioning pull-back car. My design is far from perfect: the rear wheels wander too much due to me over-worrying about tolerances, and the car is probably 2-3 times larger than it needs to be, but it feels solid and reliably works. See for yourself:

One thing I did learn, and will try to apply to the next time I offer this activity to kids, is the friction between the wheels and the floor is very important. In the video above you can see I wrapped one of the wheels in an elastic to increase friction. Otherwise, the car wasted too much energy spinning its wheels and didn’t go too far. I think cheap, adhesive-backed craft foam might be an effective and economical solution to that problem.

I am going to try a new design that balances between the cheapness of the crafty cardboard designs and my over-engineered single-sheet MDF one. A basic body, with side supports for skewer axles, and large wheels, all laser cut.

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