1. First and most important, this is the boys' race! Help them understand what works and why, but let them get their hands dirty and make mistakes. That's how we learn. Tigers can sand and paint. Wolves can start using coping saws. Bears can learn to use a drill press with supervision. Webelos can start learning laser guided CNC milling machines... There is an anything goes race for parents who just have to build a car, and kids that want to bend (or break) the rules.
2. The car's weight is the engine. 5 ounces is the max, so use it. Why would you run a 4 cylinder car at 2.5 oz. against V-8s at 5 oz? The higher the weight is on the track at start and the more weight you have, the greater the potential energy. Be careful, though - if the car is too tail heavy, you'll have stability issues causing braking. The center of gravity (balance point) of the car should be about 1" in front of the back axle. Remove as much wood up front as possible so you can add more weight in back where it counts. Dense materials make the best, most compact weights. Remember that lead isn't good for kids, so you handle that part of it. Some people opt for Tungsten, but it's really expensive and you don't gain that much. Also, tungsten putty and tungsten powder are actually less dense than lead.
3. Friction makes up the brakes. Think of everywhere the wheels rub. Against the axles, against the side of the car, against the side of the track rail. Polish the tires (but don't modify them - that's illegal). Take a needle file and get rid of any burrs on the axle. Polish the axle with increasing grades of wet/dry sandpaper. Use a good dry lube (liquid is illegal!) Hobby lube is good and widely available. Remember that all four wheels must touch the track! Three-wheeling, though a good speed trick, is illegal!
4. Stability makes a fast car. Weebles wobble, but they lose races. Get those axles absolutely square in the axle slots. If you have bad slots, flip the block over and cut them in square with a doubled up hacksaw blade. However, the slots must be the original distance apart (4 3/8"). Long wheel bases are illegal in Pinewood Derby (but really help for those of you who do AWANA Grand Prix). Before you glue the axles, make sure the car rolls straight on a level, smooth surface (e.g. kitchen table). Turning axles in the slots a little at a time can help align the wheels and get the car tracking straight.
5. Sleek and smooth (aerodynamics) has less to do with it than you think, but a well designed car following the tips above will end up pretty sleek. I tell my kids to focus on either making it cool (competing for design and creativity), or making it fast. The Oscar-Meyer Weiner Mobile, even though cool, doesn't make for a fast design, even though applying some of the other tips here can still make it faster.
A fast car is built from a dozen little speed enhancements, each giving just a little more edge.
Surf the Internet (there are links in another post from 2010), do research, do math, conduct advanced particle physics experiments, but remember, it's your son's car. If he does his best and has a sense of accomplishment when we're done, everyone wins.
Does anyone have a 3 1/2 oz. sphere of spent Uranium-235 laying around that I can have for my engine?
