wood Derby is a classic event for Cub Scout
s (though most often a challenge between fathers of the boys). There are three different versions of this: the car, the boat, and the airplane. Of these, the pinewood derby car
is by far the most well known (the boat is a challenge between the lung capacity
of boys and the airplane is twisting ability of some real long rubber bands).
The first Pinewood Derby was held in 1953 (as I said, it was classic) by the Cub Scout pack 280C located in Manhatten Beach, California.The following year the Pinewood Derby was referenced in Boy's Life (a scouting publication) in October.
For the car, the essence of the rules is:
- Weight: 5 oz. max
- Wheels: all four wheels must be touching the track surface. While sanding is allowed, there may be no extreme peaks or ridges.
- Axles: Removing of flashing or burrs and polishing allowed
- Lubricant: Graphite
- Propulsion: Gravity only
- Width: 2 3/4 inches - the car must be able to fit on the lane guide and thus very little narrowing may be done.
- Length: 7 inches
- Height: 3 inches (it must be able to fit under whatever is used to detect which car got to the finish line first)
- Clearance: 3/8 inches is the standard height of the lane guide and thus the car must be able to ride on this properly. No low-rider cars.
The track itself which the cars race on varies in size (the longer tracks are often used at county level or higher races). Typically the track meets the following standards:
- Length and Drop: 28 feet long with a 4 feet drop
- Slope: The slope of the drop is about 30 degrees. The slop of the straight away is about 0 degrees.
- Lanes: more than 3 lanes, normally 4 lanes though sometimes as many as 8. Each lane has a wooden strip in the middle that is
1 1/2 inches wide and raises up 3/8 of an inch.
- Starting mechanism: vertical pins
There are a vast number of suggestions as to how to make the car a winner. Here is a sampling of suggestions and design considerations.
The shape of the body is the most focused upon part of design of a
pinewood derby car. Often people make these to be either fancy
roadsters or other similar - hey, it looks neat. The essence of a
good design is that of the wedge. Quite simply, the car should be shaped
like a very thin wedge. Indeed, this is how most cars are shaped.
Furthermore, a reduction in the amount of 'clutter' on the car is a good
thing - various weights and other decoration often slow the car by
disturbing the airflow.
Unknown to many, the wheels are a large source of friction in the car.
Everyone who has raced a pinewood derby car knows of the wonders of
powdered graphite and its use - squirt it between the wheel and the
axel (nail). This reduces the friction of the wheel spinning on the axel.
However, there are a number of other things that can be done
(with varying degrees of legality depending upon your local pinewood
Please note that of all the modifications that can be done to a car, the wheels are the ones that are most likely to be restricted. Please verify with the rules before undertaking such modifications to avoid disqualification.
- Round the wheels. This reduces the amount of surface that the car touches the track - and the friction from it.
- Some stores will sell wheels that either have an 'H' shape or a 'V' shape. Both of these serve the same purpose as rounding the wheels (above).
- On wheels with solid walls, drill holes in the walls. These holes must be drilled symmetrically within the wheel and the opposite wheel must have the exact same characteristics.
- Polish the axles - and don't use on the car until just before the race to avoid damaging them. This requires several levels of sand paper to do.
For this, it is recommended to use a 30 micron (600 grit) to 9 micron (1500 grit) sand paper, however a 5 micron and 3 micron sand paper are suggested. The later two are not typically commercially available, but are often found in conjunction with pinewood derby cars for polishing the axles.
- While avoiding the use of the axles is a good thing (to avoid getting them messed up again after polishing them), it is also a good thing to 'break in' the wheels by spinning them with lots of graphite in them for several days prior to racing.
- Furthermore, essentially 'soak' the wheels and axles in graphite. To do this, pour a large amount of graphite in a zip-lock bag and put the wheels and axles in there with light shaking.
Adding weight to a car is necessary - the original block of wood does not weigh anything close to the 5 oz maximum necessary for a high performance car. Density here is critical to proper performance. The most often used weight is that of lead - a rather dense metal. Consider also that of tungsten which is even denser (1.7x the weight of lead for the same volume) and not toxic.
The location of weight, in theory, should be high up in the back, though this should not make the car stand on its back two wheels when sitting normally. The reason for this is to give the car as much potential energy as possible - the weight in the back is further up and remains on the incline for longer time than that on the bottom or in the front. If the center of gravity is too far back the car will loose stability when it comes to the flat portion.
To accomplish this, the car should be as long as possible and the back wheels as far back as possible while the front wheels are as far forward as possible giving the longest allowed wheelbase.
It is critical for the car to go straight when rolled naturally. This shows that none of the wheels are at an odd angle. If it is the case that the car turns when rolled naturally or wobbles as it moves that means that as the car goes down the slope of the race precious kinetic energy would be used pushing the car up and down or to the left or right.