i've built a few, i like to have the wheels cambered in a little, so they ride on the inside edges, simply put the nail/axle in at a slight angle to achieve this. i also like them to "3 wheel" meaning to have one rear wheel not touching at all.
i bring the weight up with lead fishing bobbers melted into place (i use sheet metal with vice grips for a handle, bent into a triangle so it can be poured easily) heat it from the bottom til it melts and pour. watch the fumes they aren't good for you.
lasty put the wheels on last, this way if your son plays with it it won't hurt the axles and his chance of winning
the last 4 cars i've built have always finished in the top 2.........for my boss' kid
