Wheels
There are none so important as the wheels. They must be mounted so they run free, track straight, and be able to be lubricated. Don't paint your car with the wheels on. If possible avoid putting the wheels on at all until you have finished the painting.
You must ensure that the wheels roll smoothly, in a straight line and roll very easily. There are things to check and fix on each of the wheels. First, the wheels must be perfectly round. Some moulds may produce slightly out-of-round wheels which are slower than others. To check for this put the wheel on an axle and spin it. It should turn with the outside surface at a single reference point never varying. The run-out or the wheel movement along the axle axis should also be minimal. There isn't much you can do to correct a bad wheel. The wheels are or much better quality in recent years and bad wheels have been greatly reduced.
Wheel adjustment - After the wheels are attached place the car on a flat surface such as a kitchen countertop. Use a ruler to measure the distance from the countertop to the bottom edge of each corner of the car. Adjust each axle up or down in the axle slot until the four corners of the car are the same height above the counter top and all four wheels touch the surface at the same time.
Wheel alignment - Attach a piece of masking tape (1 metre long) to a smooth,
flat surface. Use a straightedge to make sure the tape is straight. Roll the car
on the surface using the tape as a guide. The car should be able to roll 1 metre
without veering off course. If the car turns within that distance, a slight
correction can be made by swapping wheels from side to side or front to
back.
Attaching wheels/axles - Attach the wheels/axles to the car after all body work is complete. This includes drilling, cutting, shaping, sanding, painting, weighting, and decorating.
Gluing axles - After aligning the wheels, glue the axles in place using a small amount of white glue or epoxy. Apply the glue to the axles at least 24 hours before the weigh-in. Do not use a thin glue such as super glue.
Check the wheel for burrs on the running surface of the tire and hub areas. These need to be freed of any extra plastic residue or moulding marks.
The nail type axles that come in the Pinewood derby kit must be used in the construction of your of your car. These provide no bearing surface so there is friction between the plastic wheel surface and the metal axle. Since this friction reduces speed we need to minimise the contact surface area, make the surfaces smooth and lubricate the mating surfaces. It is against the rules to machine the plastic wheel and these procedures usually require a lathe or other tools. That still leaves the axle open to "play with". The following steps suggest things you can do with simple hand tolls to improve the performance of the axles.
First, the heads of the nails used as axles in the kit will often have a old or casting mark in two places just where the head attaches to the shaft the nail. Remove this web of metal with a file being careful not to gouge or scratch the running surface of the shaft. This will prevent the axle from grinding the plastic hub area and slowing down your car. This is usually best done with the axle chucked up in a drill press or drill motor that is secured into a stable position.
Optional
step. This step can be performed before actual polishing but is designed for
those creating "the ultimate" racing machines. It's not necessary for
the average racer. Use a fine flat file to reduce the overall diameter of the
axle. To do this, chuck the pointed end of the axle into a drill press or drill
motor that has been secured with a vice or clamp. Place the file against the
rotating axle and apply even pressure while moving the file slowly. Do this
until the area within 12mm of the head is smaller than the rest of the axle
body. The more metal that is removed the less contact surface available to
create friction. The drawback to removing too much metal is that the axle
becomes weaker and will not tolerate being dropped or withstand rough handling
without bending. This is often a trial and error procedure with much testing
required to result in a fast turning wheel.
The axle can be finished to a high lustre by following the steps detailed here. First, mount the axle in drill motor chuck exposing the head and the first 20mm of the axle. Secure the drill so that it doesn't move. Now cut a piece of 400-600 grit wet or dry sandpaper to a strip approximately 12mm wide and 10 to 15 mm in length. Wet the surface of the sand paper with water or light machine oil, start the drill and loop the sandpaper over the axle and pull the paper back and forth like a shoe polish cloth. Work the paper until the metal is smooth in the wheel running area (next to the head of the axle). This usually takes about a minute for each axle. Now, using either pumice paste or metal polish in a soft cloth (like a tee-shirt), start the drill again and press the cloth and polish compound into the axle with a slight movement back and forth. This will also take about a minute. The finished axle will be very smooth and bright in appearance