LCHIENThe rules are attached. which summarize the rules included with the kits. The emphasis is on using the 1-3/4" W x 1.25" high x 7" long Pine block they give you that has two grooves cut in the bottom for guiding the nail axles. They also provide in the kit four plastic wheels and four nails for use as the axles and there is great emphasis on using those parts.
Pretty much as I read it they allow cutting away the block and attaching stuff of other materials to the block as you wish as long as you stay in the envelope. Overall it has to be less than 2.75" wide to not hit the other cars which means you can add fenders and fairings.
I read the rules that I could discard the block (100% cut away) and "attach" my own block which was 2.5" wide) to have recessed wheels (see the green LEGO car). I suppose you can make the body of plastic or tin. Using a 3-D printer or a bandsaw is probably no different. They key thing is the weight limit the whole car must come in under 5.00 ounces so material makes little difference. I usually weight mine to 4.99+ or so ounces tuned with a couple of small screws to be added or removed at weigh-in time since you have to use a uniform scale. .05 ounces extra (a bit more than a 1/2" #6 screw) is 1%. Given that the energy imparted to the car is M x G x H and we all have the same Gravity and starting Height a 1% mass advantage translates to a 1% speed advantage all other things being the same. A thin tin car body you will probably have to add a lot of weight.
Aerodynamics makes some difference. A large blocky car is definitely slower than a streamlined car as I found out. The fast cars seems to be thin wedges. You make it too thin and you can't make the 5 Ounce weight so you lose out. So maybe a denser material helps. The cars cover 45 feet dropping 4 feet and do it in 3.1 to 3.5 seconds... the scale speed is claimed to be about 190-195 mph according to the display. I haven't worked out if the scaling applies to aero drag. My car was tapered and only 1/2" thick at the center except for the disk weights half-recessed in its back. That's about 14.5 ft/sec or 9.8 miles per hour actual speed.
What I have found to help a lot besides mass and aerodynamics is
- wheel alignment, if the car veers to one side or the other it will lose energy rubbing the wheels against the guide rail - for most people its a matter of how straight the nails are and its hard to change but there are some tricks for adjusting wheel alignment with toe-in. I used
- Camber of the wheels slightly positive so the wheels don't rub against the body and they run on the edge of the wheel instead of the flat.
- nail treatment - I polish them through at least four or five grit levels to a high shine and inspected with a microscope
- Lubrication - dry lube like graphite is the only stuff supposedly allowed and helps a lot
- wheel mass - it takes energy to spin up the wheels and that rotational energy (1/2 MRW^2) steals energy from the dynamic energy 1/2 MV^2 that comes from the initial MGH energy. Less wheel spinning mass is better.
- car center of mass location - farther to the back increases the initial potential MGH energy product since the car is inclined 45° at the start. Extra half inch of elevation is 1% extra energy out of 48 inches.
Oh yeah I don't have screw slot files either.
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