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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.


The variance in wheel characteristics from mold to mold can be
determined by measurement and by observation. These characteristics

- Diameter variance
- Bore placement
- Bore angle
- Bore diameter
- Tread characteristics
- etc.

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.

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.

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.

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.

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!

From Pinewood Derby Times Volume 3, Issue 2

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