Tuesday, January 07, 2014

Effect of Wheel Weight on Performance

One of the major changes to pinewood derby racing during the past serveral year was the wide-spread availability of light-weight speed wheels. For BSA, wheels ranging from 2.5 to 1.0 grams were offered by various vendors including Maximum Velocity.

But what is the performance benefit of lighter weight wheels? To help answer this question, I ran an experiment comparing 1.0, 1.9, and 2.9 gram wheels, the results of which are documented in this article.

However, first I want to position these results. This experiment shows the general effect of wheel weight on performance, and can be used to estimate the benefit of weight reduction. On the other hand, the results cannot be used to absolutely state the benefit of using any given 1.0 gram wheel versus any given heavier wheel. Many wheel-related factors affect speed including the quality of the raw wheel, the machining method, the weight-reduction technique, and the skill/accuracy of the lathe and lathe operator. Therefore, wheel weight is only one factor that should be considered when selecting a set of wheels.

EXPERIMENT EQUIPMENT

The experiment used the following equipment:

- Pinewood test body - This body allows the weight to be readily changed without affecting the balance point.

- Pro-Ultralite Speed Wheels from DerbyWorx(1) - These official BSA wheels weigh 1.0 gram, and are machined with a high level of accuracy

- Speed Axles from Maximum Velocity(2)

- Eight, 1-1/8 x 7/8 x 1/8 O-rings - One or two of these rings were inserted into each wheels to increase the weight

Car Body
The Pinewood Wizard body was set up to weigh 5.0 ounces with the wheels and axles (no o-rings). Sufficient tungsten beads to account for eight o-rings were included as part of the weight. The car was centered weighted, and all four wheels were running on the track (see Figure 1).


Figure 1 - Wizard Body Weighted for 1.0 Gram Wheels

Wheels
The O-Rings were cut, so that they could be inserted into the wheel interior without removing the wheels from the car (see Figure 2). This technique eliminated experimental variance that would have been introduced if the wheels were removed and re-installed during the experiment. Wheel weight was as follows:

- No O-Rings - 1.0 gram

- 1 O-Ring - 1.9 grams

- 2 O-Rings - 2.9 grams


Figure 2 - Wheels with 0, 1, and 2 O-Rings Inserted

Axles
The axles were lightly polished and then lubed with Krytox 100. The wheels/axles were installed on the car, and the alignment was checked to verify that the car rolled virtually straight.

Track
A 32 foot aluminum Freedom Track was used with a Judge Timer. For each run the car was staged in the left lane.

EXPERIMENT PROCEDURE
The car was first run twice for lube break-in. Then, to minimize lube variance, the tests were made from low to high wheel weight, and then again from high to low wheel weight. The sequence was as follows:

1. Three heats were run with the 1.0 gram wheel weight. An o-ring was then added to each wheel, and tungsten beads were removed symmetrically to maintain the 5.0 ounce weight and center balance point.
2. Three heats were run with the 1.9 gram wheel weight. A second o-ring was then added to each wheel, and the weight adjusted to 5.0 ounces.
3. Six heats were run with the 2.9 gram wheel weight. One o-ring was removed from each wheel, and the weight was adjusted.
4. Three additional heats were run with the 1.9 gram wheel weight. The last o-ring was removed, and the weight was adjusted.
5. Three heats were run with the 1.0 gram wheel weight.

EXPERIMENT RESULTS
The following chart shows the results of the test. The red plot shows the results of the raw data, and the blue plot shows the results with the high and low run removed for each weight.


Figure 3 - Experimental Results

CONCLUSIONS
When racing on a 32 foot track, wheel weight has a significant negative impact on performance. Running with the lightest possible wheels (while still maintaining structural integrity) will offer the best performance possibilities.

It is possible that the results of this experiment would vary on a longer track, however, it is my belief that lighter weight wheels will provide better performance on all tracks, with the possible exception of extremely long 'monster' tracks.

(1) Pro-Ultralite Speed Wheels are available at:
http://www.maximum-velocity.com/wheels_axles.htm#proultra

(2) Speed Axles are available at:
http://www.maximum-velocity.com/wheels_axles.htm#outlawaxles
(scroll down to part #4095)

From Pinewood Derby Times Volume 7, Issue 3

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