I still have my Derby cars around here somewhere. Loved spending time in the garage with my dad.
- weight to the rear. Remember, the official scale is only so accurate, so go a tad light and have some very small lead shot to fine-tune.
- 3-wheels (one rear slightly lifted)
- put the axle nails in a drill and use a very fine emery cloth to polish
- get a little grit on another nail, chuck it in a drill, and polish the inside of the wheels, too.
- I've heard of guys chucking the wheels into a small lathe or drill press, and using a fine file to insure they are perfectly round and smooth.
- very fine graphite to lube
The shape didn't seem to matter. Not much "drag" in 20 feet. Let your son design to car and paint it up. That's the best part, anyway! The rest is just the "engineering genes" my dad passed down to me.
Unless the rules have changed since I was in it, both of those suggestions are against the rules. Are you going to get caught at the local level races? Probably not, but being dishonest goes against the very basic principles of scouting.and cut small grooves in the axel where the wheel will run for less friction.
I usually try to make the wheel beveled so that the car runs on the outer edge of the wheel only.