By Randy Davis
Last season, I ran an experiment and published an article on the effect of the height of the ballast weight on performance (see Volume 13, Issue 9 – “Height of Ballast Weight Versus Performance”). You can read the article Here.
That experiment showed that for a reasonable (9/16 inch) variance in the height of the ballast weight, there was no measurable effect on performance.
After the article was published, I received quite a bit of feedback claiming that the experiment was under ideal conditions so it could not be applied to real racing.
Truly, the test was done under as ideal conditions as possible, as I wanted to ensure that any variance in performance was due to the height of the weight and not some other factor. For example, outlaw (disk) wheels were used to minimize friction, and the track was very smooth.
So, to address the feedback I determined to run the test again with the following changes:
1. BSA Wheels and Axles instead of Outlaw Wheels – these wheels have more track surface contact, and the wheel/axle fit is sloppy.
2. Rougher track
The same car was used as in the original experiment (see Figures 1 and 2). Of course the wheels/axles were swapped out; Pro-Stock BSA Speed Wheels and BSA Speed Axles were used and lubed with Krytox 100 lube.
Figure 1 – Top of Test Car
Figure 2 – Bottom of Car with Weight and Spacers Inset
As a recap, the car has a 1-3/8 inch hole drilled completely through the car, and a medicine bottle cap with a 1-3/8 inch internal diameter is glued over the hole. The resulting cavity can hold a 3.25 ounce tungsten round (9/32 inch thick) and two hollow plastic spacers (same OD and thickness), and a thin plastic shim to prevent rattling. On the bottom of the car, the hole is covered with a removable plate (shown in Figure 2). Additional ballast weight was added to bring the car up to five ounces.
To add “roughness” to the track, strips of Post-it material was applied to the track. At each track joint, a strip was placed on both sides of the center guide rail. At the half-way point of each track section, a strip was placed on one side of the rail (alternating sides down the track). When the car raced, it made a pleasing clickety-clack train track sound.
Figure 3 – Post-it Strips on Track
The experiment started with the tungsten round at the bottom and the two spacers on the top. Three heats were run with this configuration. Then the plate was removed, the round placed between the spacers, and the plate replaced. After three heats with this configuration, the round was placed above the two spacers, and six heats were run. Then the configuration was changed back to the round in the middle for three heats, followed by the round at the bottom for the final three heats. Thus, six heats were run for each configuration.
Other than the car being a little slower than in the original experiment, nothing else changed. The car was still very consistent, and performed the same regardless of the height of the weight.
2.5308 Sec – Low COG Average
2.5297 Sec – Middle COG Average
2.5305 Sec – High COG Average
.00156 Sec – Standard Deviation
The greatest difference in average times (between the low and middle COG was 1.1 milliseconds which was less than the standard deviation of the data, so the difference is statistically insignificant.
Thus, the same conclusion can be drawn as in the original experiment: within reason (9/16 inch for this experiment) don’t worry about the height of the COG. Certainly get the COG towards the back, keep your car aerodynamically sleek, and have fun designing your car. If you want to use a tungsten canopy, certainly don’t be afraid to do so.
From Pinewood Derby Times Volume 14, Issue 7
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Maximum Velocity Pinewood Derby Car Plans and Supplies
By Randy Davis