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Pinewood Derby Stories and Photos from Maximum Velocity
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The Wheels Go Round and Round - or Do They?
John Lennon wrote that he loved to spend time "watching the wheels go round and round", and during this pinewood derby season kids and parents will have a similar desire. In households all over this country (and in many other countries as well), kids and parents will be opening a pinewood derby car kit - dumping out the block of wood, the axles, and the wheels. As they inspect the contents, they hope that those raw materials will become a fast and good-looking car.
How many kids will build a car this season? I have no idea, but I am sure that the number of kits manufactured per year for consumption in the United States would boggle the mind. Clearly, kit manufacturing is a high-volume business.
In high-volume manufacturing, price is the name of the game. Organizations look for manufacturers who can produce the required quantity of kits on time, and at the lowest price. Quality is certainly considered, but since the product is considered a 'toy' or craft item, precision is not a factor.
This certainly applies to the pinewood derby wheels. To achieve the required volumes, wheels are injection molded in multi-cavity molds (BSA wheels have at least sixteen different mold numbers). The intent is that the wheels produced by the various molds will look and perform similarly; and to the casual eye, this is true. But a closer look reveals that: (1) Each mold produces a wheel with unique characteristics, and (2) No mold produces a 'perfect' wheel. Injection molding is great for producing parts cheaply and quickly, but not necessarily with precision.
WHEEL CHARACTERISTIC VARIANCE The variance in wheel characteristics from mold to mold can be determined by measurement and by observation. These characteristics include:
Although all of these characteristics can affect performance, this article will specifically focus on the first two characteristics mentioned above.
Diameter Variance - To achieve best performance, each wheel should of course be perfectly round, and all four wheels should have the same diameter. In reality pinewood derby wheels are not perfectly round. Some are close, but others are considerably out of round with an oval or egg-like shape.
Bore Placement - To roll properly, wheels should have the bore placed perfectly in the center of the wheel. Some wheels have the bore virtually perfect, but others have the bore offset several thousandths of an inch.
These two characteristics, Diameter Variance and Bore Placement, together determine the 'trueness' of the wheel. A wheel with zero diameter variance, and a perfectly placed wheel bore spins 'true'. That is, when viewed from the side, the wheel describes a perfect circle as it spins.
MEASURING TRUENESS The trueness of a wheel can be measured with calipers by making measurements from the wheel bore to the tread. To view the results of caliper measurements on a typical BSA wheel see:
The measurements on this typical wheel demonstrate that the wheel is not true. If used on a car, the body of the car would rise up and down by as much as 0.009 (0.562 minus 0.553) inches on every revolution. Thus, the car would vibrate down the track.
Worse yet, if the balance point of the car was close to the rear axle (best performance), the front wheels would be lightly loaded. An untrue front wheel could cause the front end of the car to slightly lift off the track repeatedly. This could cause several undesirable results including weaving from side-to-side and derailing.
CORRECTING UNTRUE WHEELS Unfortunately, using a wheel mandrel and some sandpaper will not true a wheel. The sandpaper will remove surface defects, but it cannot remove material in the proper locations to create a perfectly circular wheel.
In fact, until recently, a machine lathe was the only tool that could true a wheel. However, now a new option exists. Introduced this pinewood derby season, the Pro-Wheel Shaver XT II is essentially an affordable, hand-powered lathe for truing pinewood derby wheels. The shaver mounts on a Pro-Hub tool, the pin of which serves as the center axis for the tool. For photos, and more information about the Pro-Wheel Shaver XT II please Click Here.
ALTERNATIVES If you use the official BSA kit, then there are two alternative ways to get better wheels.
1. If your local race allows lathed wheels, but you don't want to do it yourself with the Pro-Wheel Shaver, then consider purchasing wheels that have already been lathed. Lathed wheels are available from several sources on the Internet, but for an added benefit, Maximum Velocity offers lathed wheels that are mold number-matched for even better results. For more information, please Click Here.
2. If your local race does not allow lathed wheels, then note that some of the molds produce superior wheels. By using matched wheels from one of the better molds, better results can be obtained. For more information on matched wheels, please Click Here.
CONCLUSION If you want to watch your "wheels go round and round" as fast as possible, then consider the characteristics of your wheels. The more true the wheels, the faster they will go round and round!
