Pinewood Derby Stories and Photos from Maximum Velocity
Top Fuel Cars - CO2 Car
(The sixth in a series of articles on cars that "stretch the rules")
In previous articles, we have discussed propeller cars (fast, and relatively easy to make - Volume 6, Issue 15), direct drive cars (faster, but more challenging to build - Volume 8, Issue 3), and rocket-powered cars (fastest, but tricky to build - Volume 6, Issue 8). But, today's topic will be the most challenging of them all, a CO2 cartridge powered car.
One of our readers, Rich Cagle, succeeded in building and racing a CO2 car in 2007. This article will detail his design.
Figure 1 - Rich Cagle's Car(1)
BACKGROUND CO2 cartridges contain pressurized carbon dioxide. When the end of the cartridge is punctured, the gas is released at high velocity, resulting in considerable thrust. Typically, these cartridges are used on cars that are run along a string in a parking lot. A starting mechanism is used to puncture the cartridge to initiate the run. To adapt this technology for use on a pinewood derby track, a mechanism must be developed which will puncture the cartridge when the starting pin drops.
RICH'S CO2 CAR Rich solved the starting problem with a "trailer" which contains the puncturing mechanism.
Figure 2 - Trailer Attached to Car, and Ready to fire
The trailer contains a spring and a firing pin. The pin is cocked by pulling backward, and then engaging the pin in a large washer (See Figure 2). The washer is moved downwards by a sharp force, which frees the pin to move forward. This force is provided by a mousetrap mechanism. When the mousetrap is released, the trap strikes the plate at the top-rear of the trailer. The plate pushes against the washer, moving it downward and releasing the firing pin. When the cartridge is punctured, the car shoots forward, leaving the trailer to follow at its own pace. The trailer is weighted so that it is not blown backward by the exhaust force.
In Figure 2, the mousetrap is armed and ready to fire. Figure 3 shows the trailer in the un-armed position. Notice the sharp pin at the front of the trailer (for puncturing the cartridge), and the position of the mousetrap lever.
Figure 3 - Trailer in Un-armed position
STARTING PIN MECHANISM The mechanism to release the mousetrap is built into the car. A flexible rod (white in the photos) is connected to a spring mechanism on the front of the car (Figure 3). When the spring is relaxed, the rod moves forward on the car. But when the car rests against the starting pin, the rod is pushed backwards. When pushed backward, the rod can latch the mousetrap, and then be lodged under the mousetrap spring (see Figure 2). When the starting pin drops, the rod moves forward, which frees the mousetrap to start the firing sequence.
Figure 4 - Starting pin mechanism
ADDITIONAL DETAILS A 12 gram CO2 cartridge is used, which discharges at roughly 900 PSI. The CO2 cartridge is angled in the car to create some down force, keeping the car on the track. When the starting gate drops, the car is tough to see (it finishes the track in about 0.5 seconds) - you hear it more than you see it. To stop the car, a folded blanket was placed at the end of the track.
RICH'S COMMENTS - The previous track record was held by a compressed air pop bottle car. That car did not even get out of the gate before this car finished.
- In its first race the exhaust blew a competitor's car off the track onto the floor! Oops... (Sorry, no video).
- Safety was a huge concern. I tested the car to see if it would take flight by firing it in the air off of a ramp; it always spun and flew to the ground. To take no chances I had the entire crowd stand behind the starting gate. I also warned kids and parents how the car was powered and that it was not a toy. Please caution anyone building one of these to think safety first. Don't take chances, especially with kids around.
- Building this car with my six year old was a lot of fun, and many lessons were learned. I hope this is something your readers will enjoy.
EDITOR'S NOTE After seeing this car, I decided to build my own version. But I thought it would be great to not have the trailer. So I built a CO2 car with a self- contained mechanism - just this week I got the car working. Next season I'll include an article about that car, explaining some of the pitfalls, design considerations, and other lessons learned.
(1) All of the photos in this article were shot by Rich Cagle.