Slanted66 - I started with my son two years ago and it's been a cool experience. His first car was the green one with the gold leaf. He took second in the race for his pack and first for appearance. Since that was his first year and he was only 7, he didn't operate the saw or drill press. But he did pick the design and color and he painted the primer on, sanded and painted the sublime green, sanded again but I added the flame and clear. All from a can though. The truck was my entry in the Dad's race, I ended up second (it was supposed to be a dodge ram). The next year we went for all out speed and he chose the wedge shape. He did a lot more, since it was a simple design. He (with my help) cut the block on the bandsaw, drilled the holes for the weight, set the wheels and painted it. He's a Bears fan, so he went with that theme. He won first in the pack and second in the whole district, where we got beat by a car that was "all dad built" but it was still lots of fun. The red car was my entry in the Dad's race. It was a last minute build and my son painted it but I ended up winning the Dad's race. Like others have said in this post, keep it simple. Tricks that I've learned that are legal - polish the axles. There is a ridge on the stamped nails that if you remove, will reduce the wheel friction. Graphite powder is legal and works great on the axles. Also before you put the wheels onto the car, rub some graphite right around the wood on the block where the wheel will touch the car. Cuts down on friction again.
Make the body as light as possible and position the weight with some of it behind the rear axle but most of it in front of the rear axle by a 1/2 inch. Use the axle furthest back from the front of the block as the front of the car. Lead is good but Tungsten is heavier and safer and you can concentrate the weight better. Before the race, run the car on a flat surface and see how it tracks. By playing with the axles, you can "align" the car so it runs straight. The straighter it runs, less friction running along the ridge on the track itself.
Check if you have a local Boy Scout Store. They have lots of good resources, books, decals, add ons like sidepipes, engines, even led lights now. This is the last year my son can compete before being too old but he is already drawing out designs on paper and talking about the color. He's learned how to operate a bandsaw, a drill press and to sand and paint. Some of the dad's can get out of control. One of the kids in his pack last year, his father bought his car off of ebay from one of those guys who claim to be a physic's teacher. They paid $100.00 bucks. But it got them second place, behind us. Anyway, it's a great thing you are doing have fun with it.
