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Effect of Track Configuration on Pinewood Derby Car Performance
I have often been asked about the specifications of the 'official' pinewood derby track. A standard length, a standard height, a standard radius for the transition: if it were only so.
As you likely know, there is not one standard for pinewood derby tracks, so manufacturers are free to offer their own design. And of course, 'do-it-yourselfers' build tracks based on existing plans, or their own conception. All of this leads to tracks of varying lengths, heights, transitions (some with no transition), and surface materials.
With this wide variety of racing conditions, the question for car builders is, "For a given track configuration, what car design elements will produce the best results?" This is actually quite a broad question, and thus cannot be fully answered in this brief article. However, I will provide some basic design considerations when confronting various track configurations. To simplify matters I will assume a center-guide track with an initial slope, a curved transition, followed by a flat section.(1)
CENTER OF GRAVITY
Before discussing track configurations, a brief review of the Center of Gravity (COG) is appropriate. The COG is the point at which the completed car balances front-to-back. A COG further back on the car will produce more speed, but will sacrifice some stability. A COG of 1 to 1-1/4 inches in front of the rear axle is appropriate for most tracks. A more aggressive COG (closer to the rear axle) can achieve better results under optimum track conditions, while a more conservative COG (further from the rear axle) is more appropriate for adverse conditions. With this in mind, let's discuss tracks.
HEIGHT OF STARTING GATE
The height of the starting gate from the floor dictates the amount of potential energy that is available to the car. Thus, a higher starting gate will produce higher speeds, which in turn require more stable cars. The starting gate of most tracks is approximately 4 feet. At this height, no special consideration is required. However, if you will be running on a track with a higher starting gate, then a less aggressive COG may be required if warranted by other track factors.
TRANSITION RADIUS
A large transition radius (see Figure 1) is slightly more 'gentle' on cars, thus allowing a more aggressive configuration. On the other hand the front end of cars equipped with a very aggressive COG will tend to lift slightly when negotiating a small transition radius. Most of the manufacturers offer a large transition radius, while one manufacturer offers a slightly smaller transition radius.(2)
Figure 1 - Transition Radius Comparison
TRACK LENGTH
Longer tracks have a longer horizontal run-out, thus providing more opportunity for cars to slow down before reaching the finish line. To maintain momentum on longer tracks, cars must be at maximum weight, have good aerodynamics, and must be as frictionless as possible. In addition, a less aggressive COG will generally provide better results on extremely long tracks (over 50 feet), as the frictional forces will be more evenly divided among the wheels.
TRACK SURFACE
Smooth track surfaces produce faster results and allow for more aggressive car configurations. Thus, on smooth tracks, the COG can be moved further to the rear of the car. Rougher tracks require a more stable configuration with a COG closer to the center of the car.
TRACK TILT
Tracks should be virtually level side-to-side. However, it is of course impossible to achieve a perfectly level state, so all tracks will exhibit some amount of side-to-side tilt. This results in cars moving towards the lower side until the guide rail is encountered.
The fastest times will be achieved when the amount of guide rail contact is minimized. However, recognizing that some guide rail contact will occur, and that the side-to-side tilt of the track cannot be predicted, is there any advantage to purposely 'steering' the car left or right? As it turns out, if the car is equipped with a raised front wheel (the car runs on three wheels), then it is best to minimize contact with the raised wheel.(3) To accomplish this, the car can be aligned to purposely 'steer' towards the side of the car on which the raised wheel is mounted. This eliminates any contact between the guide rail and the raised wheel, possibly increasing performance.
PRACTICAL CONSIDERATIONS
1. Each track has its own performance characteristics. This article discusses general design considerations based on track configuration, however, to accurately ascertain the performance characteristics of a given track, tests must be run on that specific track.
2. Sometimes the track configuration at a local event and the subsequent district event are quite different. In this case, consider designing the car with adjustable weight so that the COG can be changed between events.
3. Regardless of the track, smooth and round wheels, polished axles, good lubrication, and good alignment will overcome many obstacles.
NOTES
(1) All of the commercial track manufacturers produce a track with these design features, except for one (SuperTimer) which has side guides.
(2) The original BestTrack transition was quite small, however, that was replaced several years ago with a larger radius.
(3) One advantage to a raised wheel is that it does not have to spin while the car is accelerating. However, if the raised wheel contacts the guide rail, it will be spun-up, thus robbing some energy from the car.
A feature article is a regular part of the Pinewood Derby Times Newsletter. To subscribe to this free e-newsletter, please visit: www.maximum-velocity.com/subscribe.htm
