C02 Powered Pinewood Derby Motor.

by sliptonic, published

C02 Powered Pinewood Derby Motor. by sliptonic Mar 14, 2011

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Note: My car was featured in the September 2011 issue of Popular Mechanics

This is a 3D printable motor for an 'outlaw' class Pinewood derby car. My son's cub scout pack has run an outlaw class for the last couple years. I've made other cars (http://sliptonic.com/?p=113) but this year I wanted to make as much as possible 'printable'. The trigger was inspired by a medieval crossbow mechanism. I welcome comments on improving it.


First a note: My name for this car was 'Bad Idea' and it probably is. It's completely illegal in regular pinewood derbies, possibly dangerous, and will certainly scare every mother in the room. You've been warned. If you decide to build one, please take extra safety precautions like the note mentioned below and maybe make everyone wear safety glasses.

The motor uses the following printed parts:

Tankblock: holds 2 12-gram C02 cartridges at a downward angle. A channel between and below the cartridges lets a stiff wire pass underneath from the front bumper to activate the trigger mechanism.

Trigger block: Holds the parts of the trigger mechanism is alignment.

Pawl: The pawl is pushed toward the back of the car when sitting at the starting gate. It rotates on a common axle with the hammer and keeps the sear from rotating.

Sear: The sear grabs the hammer and releases it when pressure is removed from the pawl.

Hammer: Pulled forward by springs, the hammer does the work of piercing the cylinders. The hammer has a large cylindrical opening to be loaded with weight. A brass rod rests in a groove in the hammer and connects the springs. As the pins pierce the cylinders, the rod pulls out of the groove. This lets the hammer fall back and clear the way for the gas to escape.


Print and clean up the plastic parts. I printed the hammer at 100% infill because it takes a beating and needs to be as heavy as possible.

Cut two pieces of brass rod about 60 mm long and thread the ends with a 3mm die. You want the wing nuts to thread on and not quite touch the sides of the block/hammer.

Cut two more pieces of brass rod. One 45mm runs through the hammer, trigger block, and pawl. One 25mm runs through the trigger block and sear.

I used one more piece about 160mm long as the rod between the bumper and the pawl. A 90 degree bend on one end fits into the bumper and a tight 180 degree bend on the other gives a rounded surface to push against the pawl.

Drill holes in the 'ears' of the wing nuts to connect the springs.

Twist in the drywall screws to the two holes in the hammer. They should protrude about 1.5mm on the tank side.

Fill the big hole in the back of the hammer with weight. You should be able to find some tungsten or lead fishing weight that fits snugly. If not, improvise and glue it in. Try to make the hammer as heavy as possible. The mass is needed to drive the pins into the cylinders and not just bounce off.

To Use it:

The pinewood derby block just needs to be cut to a thin slab about 5mm thick. If you use the standard axle positions, the block may be too thin and fragile. I drill my own holes for axles.

Mount the trigger block all the way to the rear of the car with the back edge lined up with the edge of the block. Mount the tankblock just in front and touching the trigger block.

Before using the springs, insert a couple cylinders and check the alignment of the piercing pins. They should meet squarely in the center of the cylinder seals. If not, make sure your cylinders are seated all the way down. Adjust the drywall screws so that both screws are hitting the seals at the same time. Remove the cylinders.

Load the motor by pulling the spring rod back and dropping it into the groove in the hammer. It has to engage the notch on the sear at the same time. You can then push the bumper rod back to push the pawl against the sear. It should now hold with minimal force on the bumper.

With the car in position on the track and the springs cocked, hold the hammer back with your thumb and carefully insert the cylinders. Gently remove your thumb.

Safety Note: Cartridges contain a LOT of pressure and the car can become unstable for any number of reasons. After racing this car, I wish I would have added a couple eye screws to one side of the wooden block. During the race, we could pass a fishing line through and run it from the starting gate to the finish line. This is a safety precaution to keep the car from flying into the crowd if something goes wrong.

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I had all my pieces printed and got the rest from Home Depot. After assembling and fine-tuning, it worked brilliantly! I will be running it Sunday in a challenge match against a ducted fan car.

Race Day and I loved it. Found a flaw but otherwise, great!

On a 35’ track, the car covered the distance in 1.515 seconds for a conversion of 504.064 MPH

Do you have a Thing file for those CO2 cartridge?

I made a rough version of your design on my car with slight mods based on available supplies/tools. Our first time was 1.022 and our third got down to .921.

What length track was it?

Comments deleted.

For springs, Menards SKU 2027066...$1.89 each....5/16" x 2-13/16" x .036 WG Extension Springs

My husband and I are having trouble getting ours to work. It constantly wants to go off. My first thought is the spring is too tight but my husband believes otherwise. Any thoughts?

Have you considered selling these as a kit or completed car?

I have tinkered with the idea of a 3 cartridge car. I am just not a cad designer. My idea is to open the bottom two with larger holes for a quicker gas release and the third cartridge on top with a smaller hole for a slower gas release to sustain and possible continue to accelerate the rest of the way down the track. I have built two cars, but haven't had the chance to run them yet. I have also done some of the gravity powered car tricks to reduce friction on the hubs and wheels. I am trying to break a 1 second mark. I also made the bottom a little wavy to attempt to equalize air pressures between the top of the car and the bottom. Can't wait until I get to try it and see what happens.

I used 2 13/16inch springs from Menards Home improvement. Track time was 1.714. Advice, make the screws that puncture the co2 long and file them to a point.

Has anyone figured out what size springs to use for this?

"Bad Idea," LOL. This is awesome. I'd love to see an "Outlaw" class at our competition this year. If I recall correctly, Estes (of model rocket fame) used to sell rocket-car kits that were powered by an aluminum rocket engine that you filled with some sort of cryogenic propellant delivered from an aerosol can.

Edit to add: The cryogenic propellant was FREON! Amazing. No wonder they don't sell these anymore. Here's a link to a catalog page: http://ninfinger.org/rockets/catalogs/estes75/75est42.html

Can you provide the specs for the springs? Or a source?

Did you ever find our what springs to use?

Thank you for sharing plans. I'll post pictures after I try it.

Thanks for the design. I was able to get the car down the track in 1.089 sec.

What size springs did you use?

Thanks for the design. I was able to get the car down the track in 1.089 sec.

Hi Sliptonic, Great design. What OD brass rod did you use? Is the rod that goes through the hammer, trigger block, and pawl the same diameter as the two 60mm pieces?

The piece that goes through all the trigger parts is 1/8" (as is the push rod). The other two pieces are 5/32 and threaded on both ends. If I remember correctly, I chose that size because it's just slightly smaller than a #8 screw (.156" vs .164") but still threadable.

Thank you! I plan to try it this weekend.

Hello Sliptonic,

After watching your trial run of this CO2 car, you mentioned making modifications to make it release faster. What did you end up having to do to make it release the CO2 faster? Thanks! Dan

I use two drywall screws through the hammer as piercing pins. They're just screwed in through small holes printed in the plastic. This way, each screw can be turned in and out independently to tune how it pierces the cylinder.

The goal is to get both screws to pierce at the same time and to the same depth before the spring releases.

The effect was that it was releasing gas all the way down the track, and accelerating all the way. I think to release more gas faster, especially without a nozzle, would really decrease
stability and that's already a problem.

Sliptonic - do you have a recommendation where to get the 3D printing done?

What a great design! Thank you for sharing!

I did my own printing so I really haven't tried any of the services and can't recommend one. If you get it printed, please add a 'thing' with pictures. I'd love to see how it looks professionally printed.

Cars powered by a single cartridge were a favorite hobby back in the late 1940s, but the puncher was held by the builder of the car, not a part of the car. It was a straight tube that fit over the small end of the cartridge and contained a spring that, when released, drove a small steel point into it. As I recall, the guide we used was a piece of the steel wire also used for flying control-line model planes, available from the same hobby shop that sold the carbon dioxide cartridges, and we put the screw-eyes under the center of the car. The community-center building that provided a meeting place for the model-builders club had a long hall that allowed an impressive long run for the cars.

Fast and potentially dangerous.. I like it!

Sweet - if it is worth doing - it is worth overdoing.. pretty sure that is a quote from mythbusters.!

Very neat design. What program did you use to do the CAD work?

I used HeeksCAD for the 3d work and LibreCAD for 2D. LibreCAD is a fork of qcad. Both apps are open source.