Most Scouts have an idea of their preferred design, whether it be one like the picture on the Official Kit, one like they saw someone else build, or one they thought of all by themselves. This section is designed to help the novice builder avoid a "Frankenstein Monster" that does not leave the starting gate, and the experienced builder enhance the design knowledge they already have.
Before you begin, you will need a safe place to store and transport your car while you are building it and after it is completed. A shoebox or cigar box works well for this. Place something soft in the bottom of the box to provide cushion from bumps and bangs, and to keep the car from rolling around. Some bunched up tissue, toilet paper, or and old piece of clean cloth will do nicely. You will also need a zippered sandwich bag or some other storage device to safely store your wheels and axles while you work on the body of the car.
Open the kit and put the sticker sheet, wheels and axles in your sandwich bag. Concern yourself with the body of the car first.
Examine the axle slots for imperfections, and to ensure they are square with the body. The front of the car is typically the end with the axle slot farther from the edge, but it really does not matter which end you decide to be the front of your car. Axle slots may be cleaned up or squared, but may not be extended. The wheelbase of 4 ½ inches must be maintained.
Axle slots that are severely damaged or out of "square" may be re-cut on the opposite face of the block. Special designs may require this action to be taken in order to have correct wheel placement. As long as the wheelbase of 4 ½ inches is maintained, you can design your car as you wish. If you have a design that requires 2 inches of wood in front of the front wheels, re-cutting the axle slots would be necessary; placing one slot ½ inch from the rear and the other slot 2 inches from the front, thus maintaining a wheelbase of 4 ½ inches.
Draw the design of your choice on the side, top, and/or bottom of the wood. Any design you choose will be fine as long as it is not too small or thin. Designs with thin or narrow spots may break if the car is dropped or jumps off the track. Also, you need to leave enough wood to attach the weights. Designs that are too short (5 inches total length) do not allow for the full advantage of weight placement. While it is not a requirement to maintain the full seven inches of length, we do recommend maintaining as much length as possible.
The physics of Pinewood Racing is detailed on many sites and engineering textbooks. I will not attempt to give you a physics lesson, but I must explain the key ideas. Gravity is the only starting force for your car. As your car sits at the starting gate, it has a fixed amount of potential energy (the weight of your car). Since energy can neither be created nor destroyed, this potential energy is changed into kinetic energy (speed of your car) as the car moves down the hill due to the force of gravity. The variables of friction and weight are what separate the cars from first place to finishing last.
The main thing that will slow your car down is friction of the rolling wheels, not wind resistance from the car body. To illustrate this point I'll relate a true story. A few years ago one of our Tiger Cub Scouts arrived the day of the derby with an orange painted block of wood for his entry. Nothing had been done to the body except paint (which was obviously applied with a 6" brush) and added weight. His car placed second in the Tiger division and fourth place overall. This goes to prove the fact that wind tunnel testing of your design is a useless waste of time and resources. Unless your car is going to reach speeds in excess of 35 mph, forget streamlining and go for fun, imaginative designs.
Designs may be anything you can imagine. I have seen antique formula 1 cars, semi-trucks, boats, a hot-dog, a fighter-jet, a skateboard, a rocket, an arrow, funny cars, drag racers, NASCAR replicas, space ships, sprint-cars, the Batmobile, a log, a coffin, a roller-skate, a pencil, a computer mouse, and a plain block of wood, just to name a few.
While the design of your car has nothing to do with speed, there are some things you can do to ensure a straight-running, balanced racer. Sleek, low-slung bodies do perform well; not because of their low profile, but because they are less likely to wobble, due to a low center-of-gravity.
Tall designs tend to be top-heavy. This causes the car to wobble on the track. A wobbling car can jump the track or will cause rubbing of the wheels against the center lane guide. This rubbing is friction, and will slow the car down.
One last thing to keep in mind is the left-right balance of your car. A car with too much weight on one side will cause the car to tip over, jump the track, or run sideways down the track. If the car does not run straight on its trip down the track, the wheels will bind against the center lane guide. This could actually slow your car enough to make it stop before crossing the finish line.
Tip: If your Pack uses an older style track with a starting gate that is "pushed" away by the initial momentum of the car, rather than newer designs that open automatically (or are manually opened by an experienced starter), This tip will be very useful. When designing your car, keep in mind to include an upward angle on the bottom, leading edge of your car's nose.
As the starting gate lowers, it actually rotates away from the nose of the car. When the starting gate is at a 45 degree angle, the car with an upward sloped nose will actually move down the track, edging ahead of a flat nosed design by a few millimeters due to the angular approach. By the time the gate is fully open, the car with the upward sloped nose will have a leading edge of ½ inch or more. This ½ inch could be all that separates the cars at the finish line.
Angles steeper than 45 degrees on the underside of the nose will not help. The steeper angle of approach does not force rotation of the starting dowel as effectively as a shallow angle. Angles of 20 - 35 degrees look natural and optimize this effect without being obvious with your intentions. It simply takes advantage of a design flaw in this type of starting gate. This design enhancement is legal in most races, but check with your Pack's rules before you build.
Tip: Avoid a front end that terminates in an extreme point, like a pencil. Most starting gates are a short, narrow dowel peg that may not hold a car of this design stationary before the release. An extremely pointed nose may slip to the left, right, or over the starting gate, resulting in disqualification, or causing the car to begin the race at an angle instead of pointed straight down the track. Beginning the race at an angle will enhance the effects of friction as your car rubs the center lane guide. You will not reach your maximum speed potential and could easily lose the race.
Tip: Avoid a front end that terminates in a V-shape, or slot. The entire racer must stage behind the starting gate, no exceptions. While it may seem beneficial to gain an inch of track by having the front-end wrap around the dowel peg, it does nothing more than create controversy. Theoretically, the part of the car that contacts the dowel peg will also be the part of the car that trips the electronic timer (gain an inch at the top of the track, but lose an inch at the finish because an inch of car must pass the finish line before the timer trips). This is not always the case due to inconsistency in the timer mechanisms. Why be disqualified because of your design? Why start heated arguments that may lead to a few bruised egos (or noses)? The Pinewood Derby is supposed to be fun for everyone. If you have a questionable design in mind, check with your Pinewood Derby Committee, Den Leader, or Cubmaster before you build.