My kids cars won their derby's, as well. The biggest mistake most people make ls that they don't make sure the car runs straight. The biggest way to lose momentum is to hit the side rails.
Mine were all legal, but fast. If you don't sand the axles smooth and use a good graphite, you are slowing the cars by friction. Weight distribution makes a fairly big difference, too. You don't have to be a physics PhD to make a fast car, but you have to at least use some common sense.