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.

18 thoughts on “Bad Idea

  1. 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.

  2. 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.

  3. 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…


  4. 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.

  5. 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.

  6. 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.

  7. 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!

  8. I would suggest using the needles from a turkey injector. They are sharp, durable, and have large center holes. Cool designs though guys!

  9. what times are you getting or how fast is the car

  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.

  11. Quick question, how did you come up with the angle at which the co2 canisters are set? Would a differnet angle help keep the car on the track also? like a 45 degree angle?

  12. We have an adult derby every year the beckons creativity and one-up-man-ship. A few years back, I made a CO2 powered car that used a mousetrap as part of the launch mechanism. Worked about 80% of the time… the firing pin, aka sharpened bolt, would sometimes miss the sweet spot of the cartridge. Of course, I was able to take some liberties by creating a sort of launch pad that was separate from the car. At the time, that was the best I could come up with. I used a piece of floss to trigger the trap. The floss was held loosely to the gate pin by a magnet. Here’s a successful test:

  13. The angle was somewhat arbitrary. It’s set so the main axis of the cartridge is right behind the front wheels. My thought was that it would push the car down into the track and forward and avoid rolling over the front wheels. This seems to work fine. It’s the impact from the firing mechanism that causes the rear wheels to lift up, not the thrust. At least I think so.

  14. I want to buy one of this kits, where can I buy it?

  15. I’m sorry. I don’t sell the kits. The STL files are on thingiverse if you can find someone with a 3D printer to print them. If you get it going, send me a picture and I’ll add a blog post. I haven’t seen anyone else get the car to work. Good luck!

  16. Awesome design! I don’t have a makerbot (yet) so I build a version of this out of an oak block. Turns out oak still isn’t strong enough as it only lasted 4 runs before the hammer split. Should be an easy fix to reinforce or replace with a metal piece. We don’t have a timer on our track, but the first test run you see it appears to finish under 1 second. I may pull the video into some editing software to see exactly what the time is.

    I only had one issue where it barrel rolled off the track when one of the canisters didn’t get punctured. It is very fast when it punctures the canisters correctly. Here’s a video I put on Facebook.

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