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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.
THEORY 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
EXPERIMENT SETUP 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.
RESULTS 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.
CONCLUSION 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.
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