How To Build a Pinewood Derby Car/Wheels
Pinewood derby wheels are made by an injection molding process that leaves undesirable blemishes and imperfections in the parts. Since the wheels are the only moving parts, their quality has a significant effect on performance. Stock wheels are 1.190 inch diameter and 0.415 inch wide with a 0.310 wide tread contact area. This chapter covers selecting wheels, sanding, shaving or lathing the tread, coning the hubs, polishing the bores, balancing and, as a final touch, highlighting the spokes and raised letters. As always, know the rules of your organization before starting your work.
BSA car wheels are pressed from 18 different molds, each with slightly different characteristics. The wheels are split up and packed into boxes randomly, but the best performance results are often obtained from wheels from the same select mold.
Out of round wheels can be corrected by lathing or shaving. Some wheels have a particularly bad wobble that can set up vibrations even when the wheels are lathed, balanced and run on polished axles. If you suspect that you have such a wheel, it may be advisable to discard it and use a wheel from a different kit. Note that a particular mold number does not assure a particular performance characteristic due to run-to-run variation within that mold. Always test your wheels and discard those that perform poorly no matter what the mold.
Sanding and Shaving
The simplest wheel treatment is sanding. Here, the wheel is held in a mandrel and inserted in the chuck of a drill or drill press. Sandpaper is used to dress the wheel tread surface to remove the sprue gate trim and leave the outer surface of the wheel flat.
Lathe work on a pinewood derby car wheel can range from simple truing of the outer diameter to the removal of substantial quantities of material from the wheel. A lathe can also be used to cut a V or H tread pattern in the wheel.
Light wheels are advantageous because a large fraction of the kinetic energy of the car is contained in the rotational energy of the wheel (see the Physics chapter). Light wheels mean less rotational energy in the wheels and therefore more linear speed. True wheels mean less wobble and straight tracking. You want the wheel to remain in the middle of the axle and have the minimum contact with the car body and axle head.
Approximate Wheel Weight Treatment Weight (g) Weight (oz) Set (oz) Stock BSA 3.6 0.13 0.51 Lathed 3.3 0.12 0.47 Light 1.0 0.035 0.14 Outlaw 1.1 - 1.4 0.039 - 0.049 0.16 - 0.20
Wheel coning is the process of modification of the inside wheel hub to reduce hub-to-body contact area and thereby the drag. Although the hub should not ride on the body, it will make intermittent contact as the car moves down the track.
The hub can be coned using a lathe, a rotary tool or drill press with a grinder bit, or a special hub tool for pinewood derby wheels. When the plastic has been removed, the hub surface should be sanded first with #220 and then #400 sandpaper and buffed with a plastic polish. The polish can also be used on the body where the hub makes contact and be sure to use lubricant here as well.
Bore reaming and polishing
Polishing the inner surface of the hub can reduce friction between the wheel and axle. Bore polishing kits consist of a cylindrical tool that mounts in the chuck of a drill and has two prongs that hold a pipe cleaner. A liquid abrasive with 1 μm (0.001 mm) aluminum oxide abrasive crystal is used to polish the bore. The polish can be used with the pipe cleaner holding tool or with a pipe cleaner or cloth alone. The abrasive is capable of removing scratches down to 5 μm (micromesh 4000) The aluminum oxide abrasive also makes an excellent polish for the car's paint.
Note: Do not use a high-speed rotary tool for wheel polishing! You can easily melt the plastic wheel due to the friction at high RPM. Use a variable speed drill at low speed or polish by hand.
With balanced wheels, an equivalent mass is distributed about the axle slot. To balance a wheel using a static wheel balancer, insert the rod through the wheel and suspend the wheel in the wheel balancing tool. Suspended in the magnetic bearings, the wheel will rotate so that the heaviest part is at the bottom. Mark the wheel, remove it from the device and remove a small amount of the material from the heavy side using sandpaper or a sharp knife (e.g. X-acto knife) . Remove material from the inside of the wheel by scraping or sanding so that the tread area remains unaffected. Replace the wheel in the tool and repeat the balancing procedure until the wheel remains stationary in any position.
Painting the spokes and letters
Painting the mock wire wheels silver and raised letters white won't make your car go any faster, but it will make it look better. Use a silver enamel model paint and a fine brush to paint the spokes. The brush should be nearly dry and gently touch the side of the brush to paint the raised spokes. To paint the raised letters, paint a small area of paper with white enamel and use the wheel like a stamp on a stamp pad to pick up a small amount of the paint on the letters. Let the paint dry thoroughly before doing any more work with the wheels.