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Pinewood Derby Stories and Photos from Maximum Velocity
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Revisiting the Balance Point
By Randy Davis
In my first few pinewood derbies I had no concept of setting a balance point (also known as COG or CG). I knew that the car needed to weigh five ounces, so I added weight to the middle of the car and raced. We had mixed results (most everyone else didn't know what they were doing either). But there was one family who had very fast cars. From that Dad, I learned about rear-weighting (and about using a quality graphite). So after that we started rear-weighting the cars.
When I wrote the first few editions of our "Speed to the Finish" booklet (for free distribution at our local club - not for sale) I didn't even mention a balance point. But in the first edition that went on sale, I gave a range of 3/4 to 1-1/4 inches as the target balance point.
Although I don't believe my booklet really set the standard, somehow a 1-1/4 inch balance point became the de facto oral standard for the balance point. Even today, I regularly get calls from folks who state that they "set the balance point of their car to 1-1/4 inches, which is the recommended position".
Is/was a 1-1/4 inch balance point the best position? Has anything changed to cause us to rethink the best balance point? Let's take a deeper look.
MEASURING THE BALANCE POINT
The balance point of a pinewood derby car can be easily located as follows: (1) set a balance stand (or a ruler on its long edge) on a table and (2) lay the car (with wheels and axles in place) on the device as shown in Figure 1. Move the car forward or backward until it balances on the ruler. The balance point is the distance from the
center of the rear axle to the point at which the car balances.
Figure 1 - Locating the COG
BALANCE POINT AND STABILITY
The balance point directly affects the stability of the car. If a car has a balance point that is too aggressive for the track, the car will generally be very fast on the hill, but will become unstable on the flat portion of the track and will exhibit a rapid left-right motion known as the "death rattle". To correct the instability, either Rail- Riding(1) alignment must be implemented (or increased), or the balance point must be moved forward.
THE 1-1/4 INCH "STANDARD"
Likely, the 1-1/4 inch balance point standard came from repeated posting of this number on web sites and in speed tip booklets. At the time when this number became a standard (the year 2000 or earlier) most tracks were made of wood and were not as smooth and precise as modern tracks. So the 1-1/4 number was aggressive enough to provide good performance, but conservative enough to provide stability on the vast majority of tracks. Another factor to consider is that tungsten was not readily available for car weighting until about 2003, so making a car with a more aggressive balance point was a challenge for most car builders. So, all things considered, at the time the 1-1/4 inch balance point was a good target number for the vast majority of car builders.
NEWER TECHNOLOGY
But then, as always, technology changed and pinewood derby racing reaped some of the benefits. First, aluminum tracks came on the market, followed quickly by tungsten weight. Now, cars could be more easily rear-weighted, and smooth tracks were available to improve stability. Finally, the Rail-Riding alignment method was introduced to the pinewood derby community, which provided increased stability for cars running with aggressive balance points.
NEW BALANCE POINT STANDARD
So, we are now at a time when the 1-1/4 inch standard is a bit dated. When people ask, I now give the target balance point numbers listed below. Please recognize that these numbers are for people that generally do not have tracks to test on and have one shot at racing. Certainly, you can be more aggressive if you have access to a track, and time to fine tune the car.
Wood Track, no alignment consideration: 1-1/4 inches
Wood Track, Rail-Riding: 1 inch
Aluminum track, no alignment consideration: 7/8 inch
Aluminum track, Rail-Riding: 3/4 inch
There are other factors that affect these target numbers. For example, tracks of 50 feet or longer, and very rough tracks require a less aggressive balance point. But if the track is reasonably smooth and of a standard length, these target balance point numbers work well for the majority of car builders.
CONCLUSION
Certainly there are a lot of factors that affect the positioning of the balance point. However, I hope that this article provides some food for thought, and helps reset the standard for setting the balance point. But I am sure that I will keep getting calls from folks stating that they "set the balance point of their car to 1-1/4 inches, which is the recommended position".
(1) Rail-Riding is an alignment technique that generally improves performance. In November, there will be a complete article on Rail-Riding.
