Bad Idea
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.
This year, I decided to follow his lead but with more power. I designed this motor to be printable on a 3d printer like a reprap or makerbot. All the files have been released on Thingiverse.
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.
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March 15th, 2011 - 18:36
Hmmm, what if the hammer came from above instead of below? then it is pushing downward…
Still, might be better to investigate an actual CO2 bike pump — those pierce the cartridge during insertion, and then the handle is used to dispense the air. Maybe you’d need to pierce the cartridge, and have a valve that opens when the car is launched.
March 15th, 2011 - 19:01
You might be able to have a scissors mechanism, where there is a second part (attached to the current lever arm) that moves opposite it and counteracting the force of the first arm on the lift of the car.
March 16th, 2011 - 12:44
Very cool ides…
Would hollow spike like a syringe needle help by not needing to have the “back and forth” movement of the hammer? Just slams closed over the end of the CO2 cart and the gas could escape via the “jet nozzle”. Might even help with the power…
Kev
March 16th, 2011 - 13:57
That was my original idea and I still think it would be the ideal solution but I wasn’t able to make it work with the tools I have. I started by using screws as the piercing pins and drilling a small hole through the center. I was never able to get it to work. Besides hitting accurately, the piercing pins have to be sharp and durable and they have to produce a pretty large bore to let a lot of gas out quickly. I seemed to be able to get 2 out of 3 of those goals but never all three.
March 20th, 2011 - 00:55
Based on the nature of the two “spectacular crashes” shown, it looks like uneven gas release causing a yawing moment (remember that flat spin in Top Gun? Same problem with an SR-71 Blackbird when one engine flames out unexpectedly).
I think the easy fix would be to stack the two canisters top/bottom, so that uneven gas release wouldn’t have any negative effect. This might be tough to do within the height restrictions for a car, but if one were mounted very low (between the rear wheels) it would probably work.
March 20th, 2011 - 13:58
I hadn’t though of this. I examined the cartridges after each run and adjusted the screws to try to get the resulting holes about the same size. There was usually some variability. In two (out of four) runs, the car stayed on the track and ran perfect. In all four though, the back end came off the track momentarily at the start. My suspicion is still that the force of the hammer is causing the rear wheels to lift. If all four wheels stayed down, I don’t think uneven gas release would be a factor but with the rear in the air, it’s very likely.
March 12th, 2012 - 16:26
I’ve had the same trouble with mine. With the force it takes to pierce the canisters, and the strength needed in the piercing pins, it does seem that there isn’t an answer to this – at least with this type of design. There have been designs that work but they leave the piercing apparatus on the track and the car goes down on it’s own. I think that’s tacky at best. I’ve come to the conclusion that the cylinders need to be pierced prior to setting down on the track and some kind if a valve system. I’m working on that one.
My son has a turbo-fan car (RC) that is hitting the finish line in less that a second so we are going to build the CO2 car and race them against each other. Keep a check on YouTube, when we get them done we’ll post a video “Pinewood Turbo Fan vs CO2 race”. Probably be summer or fall 2012, time permitting. Thanks for your great posts!
March 28th, 2012 - 11:33
I would suggest using the needles from a turkey injector. They are sharp, durable, and have large center holes. Cool designs though guys!
January 23rd, 2013 - 15:33
what times are you getting or how fast is the car
February 7th, 2013 - 03:10
I don’t have any objective measure of speed but it’s VERY fast. Times recorded on the finish line were just over two seconds. A typical car would run the track in about five.
February 16th, 2013 - 18:01
I want one