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Offset Weight Placement

The front to back location of the center of gravity of a pinewood
derby car (the balance point) has received much attention. Several
experiments have been implemented which clearly show that a rearward
placement of weight provides more speed on the typical track. (See
"Weight Experiments" in Volume 3, Issue 14, and "Weight: Place it
Carefully" in Volume 5, Issue 10)

However, the side to side location of the center of gravity has
received much less attention. From a potential energy viewpoint, the
side to side location would be immaterial. However, from an alignment
viewpoint, it would seem that the best side to side location would be
the location that equalizes the weight load on the rear wheels.

If the car is running on all four wheels, then the weight should be
centered (left to right) on the car. Figure 1 shows this pictorially.
The dashed line divides in half the rectangle connecting the four
wheels. So, assuming the car is symmetrical left to right, weight
would be centered around (or placed on) the dashed line.

Figure 1 - Center Weighting on a Four-Wheel Car

However, the situation changes considerably if the car is running on
three wheels. In Figure 2, a three-wheeled car is shown. If a
triangle is drawn between the three wheels, then the bisector of the
triangle (dashed line) would be the line around which the weight of
the car should be balanced. But note that quite a bit of the car (the
front left section) and one wheel lie outside the triangle. So in
order to balance the weight of the car around the dashed line, likely
most of the added weight would need to be placed right of the dashed
line. The actual placement would vary depending on the relative
weight of the car body and the required ballast weight.

Figure 2 - Center Weighting on a Three-Wheel Car

Of course this is just theory. In practice, the frictional forces
involved may be so large that left to right weight centering may make
little to no difference in alignment or performance.

To test this, I constructed a car on which the ballast weight could be
adjusted left or right. Two 3/8 inch holes were drilled from left to
right through the rear of the car, and two 1/2 ounce threaded
tungsten weights were inserted into each of the holes. The back of
the car was made relatively tall so that the weights could extend out
of the car on both sides without interfering with the wheels.

Figure 3 - Test Car With Weight Evenly Balanced on Rear Wheels

Figure 4 - Test Car with Weight Shifted Left

Figure 5 - Test Car with Weight Shifted Right

The weight was then adjusted such that the weight was evenly balanced
on the rear wheels. This was done by placing the two rear wheels on
two identical scales, and placing the front dominant wheel on a piece
of wood at the same height as the scale platforms. The weights were
then moved until the scales showed equal weight on both wheels. Note
that the weight is shifted to the right of the car and extends
partially out of the body.

Next, the front dominant wheel was slightly adjusted so that the car
would roll straight on an alignment board.

The weight was then adjusted both to the right and to the left to see
what affect the weight position would have on alignment.

Surprisingly, the position of the weight had little to no effect on
the alignment of the car. Regardless of the location of the weight,
the car tracked within a two inch band on every run. The variance can
be accounted for by the inability to place the car in the same exact
position on every test.

Placing the weight such that the weight is evenly distributed on the
rear wheels is the theoretical correct technique. However, a
different placement will not have any significant effect on the
performance of the car.

Read More at: Pinewood Derby Times Volume 10, Issue 8

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