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Top Fuel Cars: My CO2 Car

(The seventh in a series of articles on cars that "stretch the
rules".)

In Volume 8, Issue 13 (March 18, 2009), a CO2 car developed by Rich
Cagle was showcased. Rick's car used a "trailer" to contain the
firing mechanism. At the end of that article, I mentioned that a self-
contained CO2 car of my invention had been completed. In today's
article I will describe that car, explaining some of the pitfalls,
design considerations, and other lessons learned.

BACKGROUND
As a quick review, 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.

My design goal was to build a car that was fully self-contained (no
trailer vehicle). In addition to Rich's car, I used two other designs
I found on the Internet as inspiration:

1. A self-contained car built by Lance Allen, described at:
Lance Allen Car

As seen in the photos below, when armed, the rear of the car is lifted
off the ground. I chose not to use this method of "cocking" the
trigger as I did not want to risk any track damage. But I did borrow
the spring and firing hammer design from this car.


Photo 1 - Lance Allen's Car Armed
Photo from web site listed above


Photo 2 - Lance Allen's Car Unarmed
Photo from web site listed above

2. Another self-contained car build by Jeffrey Rein, described at:
Jeffrey Rein Car

This car uses parts from a CO2 plane. My firing trigger is inspired
by this car.


Photo 3 - Jeffrey Rein's Car Armed
Photos from web site listed above


Photo 3 - Jeffrey Rein's Car Unarmed
Photos from web site listed above

FIRST PROTOTYPE
Photos 5 through 7 show my first prototype of a self-contained CO2
car.(1) All of the parts came from the local Ace Hardware, or from our
K&S metal inventory.(2)


Photo 5 - Prototype Side View


Photo 6 - Prototype Rear View - Firing Mechanism Armed


Photo 7 - Prototype Rear View - Firing Mechanism Un-armed

The basics of the firing mechanism on the prototype are as follows:

1. Hammer is attached to a spring via a short chain.

2. When armed, the brass plate holds the hammer in place, and is
itself held by a steel rod on the left side of the car. When the
steel rod is pulled forward, the brass plate is rotated by the hammer.
Note the small pin at the bottom of the car that stops the rotation of
the plate (keeps the plate from dragging on the track).

3. The steel rod is pulled forward via two springs in the front of the
car, behind the front "bumper".

4. The bumper is held back by the track starting pin. The two curved
wires at the front of the car keep the bumper in place. What is not
seen is a short steel rod extending back from the bumper, which slides
in a tube under the car. This allows the bumper to move smoothly back
and forth.

FIRST RESULTS
How did it work? After much tuning (adjusting spring tension, car
weight, firing pin diameter, etc.), the car did function. It easily
outran a standard pinewood derby car, but the performance was much
less than anticipated. This lack of performance was because the firing
mechanism required that the car be fairly heavy (9.5 ounces), so the
single cartridge did not generate enough force to overcome the weight
and give the breath-taking speed that you would expect from a CO2
car.(3) Nevertheless I ran it a few times, and then the hammer bent
and the firing pin broke. So, I went back to the drawing board.

LESSONS LEARNED
I had not anticipated the number of design considerations that had to
be taken into account when building a CO2 car. These considerations
included:

1. Hammer Spring

a. Strong enough to allow the hammer to puncture the cartridge.
However, a strong hammer spring requires a stronger front spring,
which requires a heavier car, which requires greater thrust (a "Catch
22" situation).

b. The Hammer Spring tension must relax just prior to puncturing the
cartridge, so that the CO2 gas can push the hammer back out of the way
(otherwise the firing pin plugs the newly formed hole).

2. Front Spring

a. Strong enough to pull the side rod to release the hammer.

b. Must be adjusted to be held back by the weight of the car, which
must be low as possible.

3. Hammer

a. Strong material - I used brass square tubing on the first design,
but it eventually bent.

b. High mass - The hammer must be heavy enough to puncture the
cylinders (too light and it bounces off). On the prototype, I filled
the brass hammer with tungsten disks to increase the mass.

4. Firing Pin

a. Proper Diameter - The hole in the CO2 must be of the optimum size
so that the gas is fully released just as the car crosses the finish
line. If it is too small, the cartridge still has gas remaining after
the finish line. If it is too large, all of the gas is released at
once, which blasts the car off of the track (quite comical, but pretty
rough on the car). I found that the sharp end of a large straight pin
works well.

b. Strong and sharp - The pin must not bend easily, and must be very
sharp.

5. CO2 Cartridges

a. I started with 8 gram cartridges (used on CO2 dragsters), but
quickly progressed to 12 gram cartridges (for airsoft guns) - heavier
but longer thrust.

SECOND TIME'S THE CHARM
For the second pass (as seen in Photos 8 through 10), I went with a
two cartridge design and a steel, T-shaped hammer.


Photo 8 - Side View


Photo 9 - Rear View - Firing Mechanism Armed


Photo 10 - Rear View - Firing Mechanism Un-armed

The firing mechanism is the same as the on the prototype. The weight
of the car increased to 10 ounces, but the thrust was doubled, so the
performance was satisfactory.

Here is a Quick Time (.mov extension) video of the CO2 car racing
against a typical pinewood derby car:
CO2 Video

CONCLUSION
Although this project consumed a lot more time than I had originally
anticipated, it was a very satisfying experience. I enjoy tinkering,
and this project required a lot of it! If you enjoy building projects
and solving problems, then a CO2 car may be just the project for you.
If you do build one, please send me photos and a description.

(1) Not mentioned in the article is the modular design of the car.
All of the firing mechanism parts are held in place by removable pins.
This allows the springs, chain, etc. to be quickly changed out without
removing screws. Screws were only used to attach the CO2 cartridge
bracket.

(2) K&S metal products are available at:
Maximum Hobby

(3) This is the big advantage of Jeff Cagle's trailer design. All of
the excess mass can be put into the trailer, which allows the actual
car to be relatively light weight. This allows a single cartridge to
provide excellent performance.

Read More at: Pinewood Derby Times Volume 9, Issue 3

A feature article is a regular part of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit:
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