Graphite on the Tread
Do you put graphite on the tread surface of your wheels? Come on, you can admit it. I started rubbing graphite onto the tread surface about eight years ago. It seemed like a good thing to do, and it made the tires look nice and shiny. But honestly, during this time I didn't know whether it did anything to improve the speed of the car.
So I decided to test whether applying graphite to the tread surface has any effect on speed. While doing this, I also tested the benefit of applying graphite to the inside edge of the wheel (the part of the wheel
that touches the center guide rail).
The experiment used the following equipment:
- Extended wheelbase Wedge Body(1), weighing 5 ounces, with the balance point at one inch in front of the rear axle, and a raised front-left wheel.
- Pro-Stock Speed Wheels from DerbyWorx(2) - These official BSA wheels are accurately trued, but are not weight reduced.
- Speed Axles from Maximum Velocity(3), polished with Brasso.
- A 32 foot anodized aluminum Freedom Track with a Judge Timer. For each run the car was staged in the left lane.
EXPERIMENT SET UP
Before mounting on the car, the wheels were thoroughly lubricated with Max-V-Lube(4). To prevent the graphite from getting onto the tread surface or inside edge, the wheels were wrapped in paper (Figure 1).
Figure 1 - Wheel Wrapped in Paper
After lubrication, the paper was removed and the wheels were mounted on the car. The car was then aligned, and given a few break-in runs.
1. The car was run six times, the high and low runs were removed, and the heats were averaged.
2. Graphite was applied to the tread surface of each wheel. This was done without removing the wheels. To prevent graphite from getting on the inside edge of the wheel, a piece of cardboard was pressed against the inside edge of the wheel.
3. Again, six heats were run, the high and low runs were removed, and the heats were averaged.
4. Without removing the wheels, graphite was applied to the inside edge of each wheel.
5. A final set of six heats were run, the high and low runs were removed, and the heats were averaged.
The following chart shows the results of the test. As you can see there was a slight improvement in performance when graphite was added to the tread surface, and then again when it was applied to the inner edge.
Note that the amount of overall improvement is only four thousandths of a second. However, the heat times were very consistent; the standard deviation (amount of deviation of the heat times from the average) is quite small (ranges from 0.0005 to 0.0017). Thus, the improvement, albeit small, cannot be completely attributed to statistical noise.
Figure 2 - Experimental Results
Although the small improvement could be important in a tight race, lubricating the tread surface and inside edge is an extremely small factor in overall performance. If allowed in your race, I would certainly do it, but you would not suffer a significant performance penalty if you choose to not lubricate the tread.
Another factor to consider is that the anodized aluminum track on which the experiment was run is extremely smooth. It is possible that if the experiment was run on a rougher track, there could be a larger performance improvement. However, rougher tracks do not generally provide consistent heat times. So the standard deviation of the heats would likely be much higher, and any improvement could be buried in the statistical noise.
(1) Part 5622
(2) Part 4080
(3) Part 4095
(4) Part 5104
From Pinewood Derby Times Volume 8, Issue 2
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