It's Pinewood Derby season again and I kinda enjoy making derby cars. This year, my youngest son and I made an awesome car for his bear den division. It came out great and I'll do a blog post on that one as soon as I get some good pictures.
We also have an 'open' class where the rules are a little more relaxed. Last year's car was fast and I actually won but not because I was the fastest. I was really impressed by a c02 powered car that another dad had made. He had trouble keeping it on the track, but it was REALLY fast.
A few more details about the build.
The idea was to make as much as possible printable and make the pinewood part easy. In the end, it uses a 5mm slab of the original block, the wheels, and axles are prepared like any other car.
The hardest part was finding a way to both pierce the cartridges and get the piercing mechanism out of the way so the gas can escape. My solution was to have spring pull the hammer through a radius such that as the cartridges are pierced, the hammer reaches a point where the springs are released from the hammer. The hammer can then fall back leaving a clear path.
Here's the bench test video that might help explain.
One problem with the design: I think the force of the hammer striking the cartridges is lifting the rear of the car off the track and causing the instability. 2 of my 4 runs on race day were spectacular crashes. I'm not sure how to address the problem, but maybe adding weight to the rear of the car would help.