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Shifting the Wheelbase
By Randy Davis

The standard pinewood derby kit from BSA (as well as PineCar and Maximum Velocity), has an offset wheelbase, with one slot approximately 1-3/4 inches from the end of the block, and the other slot approximately 1 inch from the end of the block. The 1 inch slot is generally used as the rear axle, as this places the rear axle as far up the hill as possible, thus allowing the center of gravity (COG) of the car to be as far up the hill as possible.(1)

Some races do not place restrictions on the wheel base, thus allowing the builder to extend the wheelbase by moving the rear axle back and the front axle forward. Other races place restrictions on both the location and position of the wheelbase, thus preventing any adjustment to the wheelbase.

But some races restrict the distance between the front and rear axles, but do not place a restriction on the location of the front and rear axles. In this case, the wheel base can be shifted to potentially gain an advantage (see Figure 1). But does this really provide an advantage, and if so, how much advantage is attained? Let's find out.

Figure 1 - Shifted wheelbase


There are several ways to shift the wheelbase:

- If axle holes are allowed in your race (as opposed to requiring the use of axle slots), then new axle holes can be drilled with a Pro-Body Tool, Pro-Body Jig, or with a drill press.

- If axle slots are required, then new slots can be cut with a Pro- Body Slotter.

- If the original slots must be used, then a piece of the block can be cut off the back and glued to the front. (See Figure 2).

Figure 2 - Shifting the Wheelbase While Retaining the Original Slots

In our experiment, we want to avoid changing any factor that would affect the performance of the car, other than the shifted wheelbase. So we will leave the original wheelbase intact, and effect the shift by moving a part from the back of the car to the front of the car (See Figure 3).

Figure 3 - Test Car
(Note the styrene piece at the back of the car in the first photo, and the front of the car in the second photo)

The body of the test car was shortened by 1/4 inch, and then a thin slot was cut crosswise on both the front and back of the car. A piece of styrene was cut such that when installed into one of the slots, the car measures seven inches long. By moving the styrene between the front and back of the car, the wheelbase is shifted by 1/4 inch without changing any other factor on the car.(2)

The car is equipped with polished speed axles and Pro-Stock Speed Wheels, and is lubed with Krytox 100. The car has a raised front left wheel, and is aligned to rail-ride. A 3.5 ounce tungsten canopy provides the majority of the weight, with a small amount of tungsten putty (in holes behind the rear axle) to bring the total weight up to five ounces. The COG of the car is 7/8 inch in front of the rear axles.

Five heats were run with the styrene piece at the back of the car (standard wheelbase), then ten heats were run with the styrene piece at the front of the car (shifted wheelbase). Finally, five heats were run with the styrene piece at the back to even out the heats to ten in each position.

Standard Wheelbase Average - 2.542 sec; Std. Deviation - .0016
Shifted Wheelbase Average - 2.539 sec; Std. Deviation - .0008

The data shows an improvement in time with the shifted wheelbase of .003 seconds. This is more than the standard deviation, so the difference is statistically significant.

Going into the experiment I expected an improvement from the shifted wheelbase, but I expected the improvement to be more than 3 ms. It is possible that the improvement due to the wheelbase shift would be larger if other factors could be minimized. For example, if disk (outlaw-type) wheels were used, the improvement due to the shifted wheelbase may be amplified.(3)

(1) More information on modifying the wheelbase can be found in Volume 10, Issue 13.

(2) The main issue with an experiment of this kind is that moving material from the back to the front changes the COG of the car. The advantage to the styrene piece is that the change is negligible since the styrene weighs less than 1/10th of a gram.

(3) I've added this to my list of future experiments.

From Pinewood Derby Times Volume 14, Issue 11

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