How to Build a Fast Pinewood Derby Car
Going fast in pinewood derby racing is all about maximizing weight and reducing drag. There's 2 types of drag that you'll need to minimize. Drag from friction: The wheels on track and the axles on wheels. And drag from aerodynamics which is the air flowing over the car. We'll cover all of this here and keep the terms as simple as possible. Make sure your group allows these modifications before purchasing.
Most groups will have a maximum weight allowed for a car. The closer you can get to this without going over the faster your car will "fall" down the track. Location of the weights is always hotly contested. Some people believe it is best to evenly distribute the weight among all 4 wheels. The most popular belief is to make the center of gravity 1/2-1" in front of the rear axle. You can check this by balancing your car on a pen. What is definitely important is to keep the weight distribution even from the left to right side of your car or it will pull to one side going down the track. Getting the weight lower down on the car will also help reduce the car from swaying side-to-side as it travels. But make sure your car won't rub the bottom of the track using a tool to check proper ground clearance. Finally, don't add your weight until the car is painted as sealer and paint do add weight.
Reducing the friction from the spinning wheel to the axle is extremely easy and important to being competitive. Most groups won't allow you to oil the axles because they drip on the track so the number one way to lubricate is by putting graphite powder on the axles. Using Super Speed Axles will capture more graphite powder in the channels and reduce contact surface between the axles and wheels. You can also polish either style of axles for a smoother finish to reduce drag.
Reducing friction between the wheels and the track is another big speed difference between cars. Getting wheels with a smoother surface is an easy way to achieve this. If you rules don't allow wheel changes then consider sanding a smoother surface onto the wheels by spinning them in a drill against the right grit sandpaper. You can also use this method to put some angle (camber) on the wheel tread which will reduce the amount of wheel that contacts the track.
The Camber and Toe on your wheels is a huge factor. If the wheels aren't pointing perfectly straight (toe) then they will rub on the rails of the track as it travels down the course. As mentioned before, the camber will reduce friction on the track. But if all 4 wheels aren't cambered the same then the car will pull to one side and rub the track rails. To get the camber on all 4 wheels even and all 4 wheels pointing perfectly straight needs a proper gauge tool.
Body Friction (aerodynamics):
Your car has to push air as it travels down the track. The easier it is for the body to push through the air the faster you'll go. The simplest way to do this is to have less body touching the air which is why cars that are carved to be minimal and skinny are so popular for speed.
Shaping your car like an airplane wing to create lift is a popular idea but you'd have to balance lifting the car which essentially reduces car weight with needing the car heavy enough to fall down the track as fast as possible. Without wind tunnel testing you're more likely to get this wrong so focus on making the car lift-neutral by avoiding wing shapes and spoilers.
Making the car as smooth as possible is also a great way to reduce drag. You want the air to slip over the car. Sanding the surfaces very smooth is key. For the smoothest surfaces follow the following steps:
1. Sand the carved body as smooth as possible.
2. Apply sanding sealer
3. Sand again
Repeat steps 2 and 3.
4. Paint the car your favorite color.
5. Apply stickers for cool factor.
6. Apply a clearcoat to harden the paint.
7. Wet sand the finished product.
8. Just before racing the car apply a coat of wax.
Please remember that the primary goal of Pinewood Derby Racing is to learn building skills that will last a lifetime. Sure winning would be nice too. But getting the chance to do something you've never done before and celebrate the accomplishment is the real win. Remember to thank anyone who helped you build your car and cheer your friends on when they're racing you down the track!