Friction - or minimizing it - is the single biggest factor in building a fast derby car. Look at the wheels and axles in your BSA kit: you’ll find wheels that often have rough molds inside the center hub or around the edges, or they may even be out of round entirely.
Your BSA kit axles will have a small burr just behind the nail head which must be removed. Place enough of the axle’s tip into a drill or drill press to secure it, and first give it a spin to ensure that it’s straight - some axles out of the kit aren’t quite straight. Straighten them if you can or discard them if they are not suitable.
With the axle spinning in your drill, use ~220 grit sandpaper to grind down the burr behind the nail head, and follow up with 600, then 1500 grit.
Advanced Tip: Finish each axle with metal polish on a clean rag. Be sure and completely clean the axle of polish and dry it, or your eventual graphite lube will turn into sludge!
Give your wheels a test-roll across a flat surface, and ensure that they are truly round - if any are not, then they are not suitable for use as a track surface contact wheel.
One area many racers don’t consider is the hub of the wheel, where the axle makes contact. Using a tightly-rolled strip of 400-600 grit sandpaper, ensure there’s no plastic mold flashing present at either end of the axle bore.
It is possible to improve the outer surface of the wheel, but it takes practice, and it is very easy to do more harm than good here. Some racers encourage working this area, but personally, as long as there are no obvious imperfections, I would leave them alone.
You already know that your car should weigh no more than 5.0 ounces, and you likely already know that for best performance, your center of mass should be toward the rear of the car, somewhere ahead of the rear axle. But, do you know how to determine where?
You want to place your CoM as far back as possible, while not placing it so far back that the front wheels lose the ability to lead the car properly. There must be some weight over the front axles, or your car will lose speed as the wheels slide around on the way down the track. Placement of the CoM is also dependent upon factors such as the shape and slope of the specific track. If the track were simply a straight slope all the way down to the finish line, you could safely place the weight entirely at the rear of the car. The typical derby track, however, is more of a ‘reverse S’ shape or otherwise has a long, flat segment before the finish line. These are the areas where more forward weight is advantageous, so the intent is to find the best balance between the two.
Common wisdom says to place the Center of Mass roughly 1 inch ahead of the rear axles. I like to be a little more aggressive than this, but the more aggressive you get in placing the CoM closer to the rear axles, the more likely things like alignment or even debris or imperfections on the track will negatively affect you, because of the lack of weight over the nose of the car. It’s a balance, and there are strongly diminishing returns in this area.
Alignment is another area that many racers do not consider, but I would contend it’s the most critical aspect of a winning car past the basics of weight and friction.
It is commonly accepted practice to raise one front wheel slightly so that it does not contact the track surface - this is a great use for any of those out-of-round wheels you may end up with. This turns the remaining front wheel into the “steer” wheel, and you will want to place a slight bend in its axle so that you can steer the car toward its center rail on the track. There are many resources on this - the alignment I’m talking about is the rear wheels.
Rear wheels are often canted inward at the top to reduce their contact patch with the track, and this works - but it also makes the alignment of these wheels all the more critical. The more negative camber (inward tilt) the wheels have, the more effective it is - and the less tolerance there is for imprecision in the alignment.
This is an area where you cannot trust that the axle slot in the BSA kit block is correct. Check it with a carpenter’s square, do everything you can to ensure that the rear axles and wheels are perfectly perpendicular to the body.
There’s no perfect way to apply dry graphite lubricant to your race car. Make sure everything is clean and dry, first of all - there cannot be any liquid moisture or polish from cleaning up your wheels or axles, or the graphite will slow you down rather than speed you up.
Apply graphite under the nail head of each axle, and down inside the bore on the car-side of the wheels as well. Apply a small amount to the inner side of the hub where it may contact the body of the car.
For dry graphite lube, less is generally more. This is another aspect of building a fast derby car that requires a bit of guesswork. If the humidity on race night is high, for example, you can benefit from going a little heavier on graphite. Conversely, in dry conditions, too much graphite will only create friction.