Setting the Gap for Performance

One of the most common questions I am asked is, “How much room should there be between the car body and wheel hub?” My first answer is, “Well, our Pro-Axle Guide, Alignment tool, and the Gap Gauge in our speed kit will set the spacing for you.” Then the next question is, “If I don’t have those, what should I do?” My response is then, “Try using a credit card. It will set a gap that works.”
Of course, these responses beg the original question of, “How much room should there be between the car body and wheel hub?” Or better yet, “What is the optimum gap?” I have heard and read some distinctly different opinions on this topic. Several people have said that they set a fairly wide gap in an attempt to minimize the contact between the wheel hub and car body (and also to simplify making the needed observations when performing alignment using the Shim Method). On the other side, Michael Lastufka’s DOE (Design of Experiment) tests showed that best performance is attained with a small gap.
Which is right? I can understand the thought process behind the wider gap; less opportunity for contact. But, I also recognize that a wider gap allows the car more “wander room”, thus permitting the car to travel a greater distance during its trip down the track (remember that the shortest distance between two points is a straight line).
So, I decided to set up an experiment to measure performance versus the wheel to the hub gap. One car, one set of wheels, and one set of axles was used throughout the experiment. Outlaw wheels where used to minimize the effect of guide rail and track surface contact. Krytox
100 was used as the lube to minimize break-in and maximize the consistency of the runs.
The following gaps were tested. A set of feeler gauges were used to set the gaps.
– 0.015 (Approximate width of heavy business card)
– 0.025
– 0.035 (Similar to PineCar Alignment Tool, and Pro-Axle Guide)
– 0.045 (Approximate width of a dime (0.049)
– 0.060
– 0.090 (Approximate width of an axle (0.085)
– 0.120 (Approximately 1/8 inch (0.125)
To minimize experimental variance, the axles were inserted, the heads were marked at the 12 o’clock position, and the axles never removed from the car. As each gap setting was made the axle was adjusted and the axle heads were checked to make sure that the mark stayed at the
12 o’clock position.
In order to minimize the effect of lubricant breakdown, the test was limited to five heats per gap setting (total of 35 heats). On the first pass, three heats were run per gap, going from the largest gap to the smallest gap. Then two additional heats were run going from the smallest gap to the largest gap. The high and low runs per gap setting were discarded, and the three remaining runs averaged.
As can be seen in Figure 1, the smaller gaps outperformed the larger gaps. Note that there is no real difference in performance between the three smallest gap settings (the difference is statistical noise).
However, as the gap increases, performance decreases in an almost linear fashion.

Figure 1
Wheel to Car Body Gap Test Results

So what is the bottom line? Clearly the gap setting does affect performance. For best performance, use a small gap setting (such as provided by the available gap tools). If one of these tools is not available, then use a credit card (which is typically 0.030 inches).
From Pinewood Derby Times Volume 7, Issue 9
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