All Steps Are to Minimize Friction and Vibration, Which Will Maximize Gravity's Ability
All steps are to minimize friction and vibration, which will maximize gravity's ability to accelerate the car downhill, and for inertia to keep it rolling beyond the track incline without a loss of speed.
Axle preparation:
The objective is to get an extremely smooth finish on any part of the nail that has contact with the wheel.
Chuck each axle into a dremel tool or drill press, spin at 500-1000 rpm. Use a file to slightly bevel cut the underside of nail head, this will help reduce contact with wheel hub, which tends to act as a small brake.
Polish any part of axle that will have contact with wheel with very fine sandpaper, follow this with polishing compound.
The more of a shiny mirror finish you can achieve, the better.
Wheels:
The objective is to eliminate imperfections left over from the plastic molding process so as to make the wheel’s diameter as even possible around the entire crcumference, and as smooth as possible where it meets both the track and the axle. The majority of a car's speed is in proper wheel and axle prep. If your wheels spin on their axles as smooth as a finely balanced top, with no wobble or vibration and seem to take forever to stop spinning, you're there!
Get a nail that fits snugly inside the wheel hub, or use a wheel mandrel tool. Use this to chuck the wheel into a Dremel tool or drill press. At 500-1000 rpm, lightly sand circumference of the wheel, just enough to remove any high spot created by the injection-molding process. Use something flat and solid behind the sandpaper to ensure flatness of sanding.
Be careful not to go too far, the tread pattern seen on outer edge of the tire's sidewall must remain. It is against the rules to intentionally reduce the wheel diameter, or reshape the surface of wheel where it meets the track to anything besides flat.
Follow this step with polishing compound....get the outer diameter of wheel as shiny as possible.
Next chuck a pipe cleaner into a Dremel tool or a power drill, apply polishing compound to the pipe cleaner, and use that to polish the inner surface of wheel hub, where it will ride on the axle.
Clean wheels thoroughly when through.
Car Body:
Since Pack 79 rules allow it, the optimum car design will have the front wheels as far away from the rear wheels as possible. This makes for a more directionally stable car..it wants to roll in a straight line with less vibration moreso than a car that has the front wheels closer to the rear wheels.
Aerodynamics have little effect on these cars, since they are quite small and are moving at relatively low speeds.
More important is the car's center of gravity. Try to set the center of gravity approx one half inch ahead of rear wheels…this easy to determine simply by where the car will balance on a pen held horizontally under it, or on your finger.
Shiftng the center of gravity toward the ear of car puts the majority of the car's weight "further up the hill" when it is on the starting line, yet keeps just enough weight on the front wheels to prevent wheelies from track imperfections that might otherwise cause the car to leave the track. If the center of gravity is closer to rear of car as described, it gives gravity a little more time to accelerate the car downhill than a car whose center of gravity is centered, or nearer to the front of car.
Weight: cars are limited to 5.0 oz. We bought a small digital scale from Harbor Freight. Once we had cut, carved and sanded the car's shape we wanted, we weighed it together with the 4 axles and wheels and any other item that would be part of the finished car (such as a plastic engine, driver, etc.). Then we added more weight to the car to get it right at 5 oz. The best way to add a lot of weight is to cut a hollow in the underside of car body just ahead of the rear wheels, insert a few small screws as anchors, and pour molten lead into that cavity, or otherwise attach lead weights. Then drill out the lead to get car right at 5 oz. It is tricky to get the weight set to 5 oz while maintaining the optimum center of gravity just ahead of the rear wheels, but it can be done. Small screws can also be used to fine tune the weight and center of gravity, by may require countersinking if they are on underside of car body, to clear the track guide rail. On race day, screws can be added or removed to help get the car to pass the track scale.
Axle attachment: The axles must fit very snug in the car body, not loose at all. A drill bit that is about 3/4 of the diameter of the axle should be used. The holes must be drilled as straight, level and perpindicular to the car body as possible....really this should be the first step prior to any cutting\shaping of the car's body.
Rules require the car to have four wheels, however another speed secret is to slightly raise one front wheel...about 1/16 of an inch higher than the other three. Three wheels on the track have less rolling resistance than four, and make it far easier to get the car to roll in a straight line, and not repeatedly bounce off the center guide rail, which only slows the car down.
On race day, a fine dry graphite lubricant is essiential. It must be carefully worked into the wheel hubs, then roll the car to "bed it in." Then rub the graphite all around the outer diameter of each wheel.