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Open Competition Races

Many organizations give parents and siblings an opportunity to race by
sponsoring an open competition race. Typically the rules for these
races are somewhat relaxed, allowing more innovation and creativity in
car design.

In today's article I would like to first share some typical ways to
'open-up' the rules for this type of race. Then I would like to explore
a more radical way to add excitement to your derby event.


Here are some common ways to allow more flexibility in open

1. Wheelbase - If the organization event mandates the use of a standard
wheelbase (distance between the front and back axles) then allow the
wheelbase to be modified as desired, as long as the overall length
specification is maintained.

2. Alternate axles - Instead of requiring the use of the organization
standard axles, allow the use of after-market 'speed axles'.(1)

3. Wheel shape - Typically, standard rules allow minimal wheel
modification (e.g., only a light sanding). Try removing this
restriction, thus allowing wheels to be lathed flat, narrowed, equipped
with a grooved tread surface, and/or overall weight reduction.

4. Length - Try changing the maximum length from the typical seven
inches to twelve inches (or one inch less than the distance from the
starting pin to the back edge of the track).

5. Weight - Consider changing the maximum weight to one or more pounds
(with standard wheels and axles, I don't recommend exceeding 1 pound).


Wouldn't it be fun to add a whole new dimension to your open
competition? I'm thinking of power!

Okay, I know what you're thinking. Someone will strap a rocket engine
on the car, and damage the track, a person, or the building! But of
course, restrictions would be necessary to minimize any risk. So, I
suggest the following rules.


-- Dimensions --
(Adjust to fit your track, and leave an open lane between cars)

- 11" max length
- 5" max width
- No limit on height (remove the electronic finish line)
- 3" max inside wheel base
- 3/8" minimum track clearance
- 32 ozs. max weight

-- Power Sources --
In addition to gravity power, other power sources may be used as long as
they do not pose a risk to the spectators or to the track.

1. Allowed power sources include, but are not limited to electrical,
mechanical, and pneumatic.

2. Power sources which use combustion of any type are explicitly
disallowed. This includes combustion engines, rocket engines, and

-- Wheels, Axles and Body --
A pinewood derby kit may be used, but it is not required. Any material
can be used for the car as long as the finished product meets the
specifications. However, the wheels must be made of a material that will
not damage the track. Metal, ceramic, glass, or other hard substances
may not be used for wheels. Allowable materials include plastic, rubber,
and wood.

-- Race Procedure --
The owner of each car will stage the car on the starting line, and pick
the car up at the end of the run. Cars may be adjusted between races as
long as the adjustment does not delay the race. However, cars may not
be lubricated during the race.


I have heard of all sorts of ideas for adding power to pinewood derby
cars Listed below are a few that have merit.

-- Powered Rear Axle --
- Spring wind-up mechanism
- Rubber-band powered
- Mouse-trap powered
- Electric motor powered (R/C type)
- Weight powered (suspended weight drops and turns rear axle)

All of these ideas can be made to work. But two problems must be worked

1. How to apply power when the starting pin drops, or
2. If power is applied before the pin drops, how to apply the maximum
torque without the wheels spinning-out.

-- Non-powered Rear Axle --
The design ideas below eliminate problem number 2 above, but problem
number 1 must still be resolved.

- Compressed CO2 or Air (acts like a 'jet' engine)
- Propeller powered (push or pull)


I have had some experience with propeller power, so I would like to
share the design of a car that I built a few years ago. I have also
included photos of a few cars submitted by readers.

-- My Propeller Car --
Please don't laugh too hard. If I had the opportunity to do this again,
I would shorten the car and reduce some of the infrastructure.
Nevertheless, this car smoked the competition, and easily beats any
standard pinewood derby car.

Front View

Rear View - v3_i5_propcar2.jpg

Here is a parts list and some notes on implementation.

- Motor: 12 VDC motor, RF-370CA by Mabuchi Motors:

This is a VCR motor, but other motors would work as well. I drove the
motor at 18V for more power. This would eventually burn out the motor,
but the on-time is so short that the motor doesn't get a chance to

- Motor Mounting: The two screw holes on the font of the motor are for
mounting. The required ISO screws can be purchased at a hardware store.
I used a piece of thin stainless steel (hardware store item), drilled
holes for the mounting screws, shaft, and assembly bolts (see front view

- Propeller: I used a plastic prop intended for rubber band powered
airplanes. The hobby store I visited sold them in packs of three. The
prop is about 4" long and I had to trim the tips a bit for clearance.
The shaft hole was too small, so I drilled it out to fit snuggly on the
motor shaft. I then epoxied it in place. I believe this type of
propeller works better than an R/C airplane propeller. They are too
heavy and the blade angle is smaller, providing less thrust.

- Infrastructure: I had access to an old erector set, but you can use
any light metal strapping. It does need to be securely mounted as some
torque is generated. Make sure there is clearance for the propeller. I
had to remove some metal from the erector material. If you look
carefully at the front view, you can see where it is removed.

- Motor Shroud: Quaker Oats oatmeal container. I put this on for two
reasons: (1) To keep hands from touching the blades (it hurts, but
doesn't cut if you touch the spinning blades), and (2) To keep the blade
from breaking if the car rolled over. The propeller is epoxied to the
shaft, so if the blade comes off, the race if over.

- Motor Alignment: The motor does need to be pointing as straight
forward as possible. Add/remove small washers where the stainless steel
strap fastens to the infrastructure to adjust alignment.

- Starting Pin Switch: I used a contact switch (similar to part #
275-016 at www.radioshack.com). It is normally on. When the car rests
against the starting pin, the weight of the car closes the switch
turning the motor off. Thus, when the pin drops, away it goes.

- Kill Switch: If you look carefully at the front view you will see a
small toggle switch. That is used to turn the motor off when not in use.
Just make sure to turn it on at the starting gate!

- Batteries and Holders: Two standard 9V batteries. The battery holders
and clips are Radio Shack items.

- Wiring: Light gage electronics wire. Positive side of the battery
goes to the Kill Switch, then to the Starting Pin Switch, then to the
motor. The negative wire goes to the motor. Batteries are wired in

- Wood: Pine, 1-3/4" wide. The car is 10" long, but it could be
shortened to 8".

- Wheels/axles: Standard BSA issue. Axle holes are drilled with a drill
press. All four wheels touch the ground.

-- Reader Propeller Cars --
Here are a few propeller cars sent to me by readers.

Keith Gosselin

Like my car, this one has a starting pin contact switch and a kill
switch. Hanging the propeller (tri-blade in this case) off the back of
the car, permits the use of a longer propeller.

Mark Pugsley

A similar design using a rubber band airplane propeller (and liberal use
of rubber bands in the infrastructure).


A dual prop model that uses an R/C type battery and control. (This photo
was sent to me by another participant in the race. If this is your car,
please let me know so that I can give you credit).


Maybe this all seems a bit ridiculous to you. But I guarantee that if
you have a group with a lot of handy parents, an open competition race
with power assist is a blast and a certain crowd pleaser!

(1) See Speed Axles for aftermarket axles.

From Pinewood Derby Times Volume 3, Issue 5

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