Used to love this when I was a young lad. Dad let us do whatever we wanted to cars from design to paint. My last year competing and having learned from years prior I built a very streamlined car with weight in the rear. Requested Dads help with weight location and balance. Mounted washers in the rear under the car to make weight. Dad still made us do all the work. Graphite for lubrication on wheels (Dad's idea). Cant remember if it was Co2 cars from shop class or soap box, but I also used small straws pressed through the wood for easier turning axles. Thinking about it now, I think maybe that was a Co2 car where the axles turned. Either way, good luck and remember to have fun. I still remember making it through our local competition and moving on to a regional race. The evidence of Dads and Grandpas building 100% the car was ridiculous. The old guys were more competitive than the kids. I still finished high, but was disappointed not to win. Being beat by kids half my age with near perfect cars sucked, but as my Dad told me after the race, I wasn't beat by a single kid that day. He still has my car from that year in the shop.