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Pinewood Derby Stories and Photos from Maximum Velocity
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The Big Pinewood Derby Debates - Part 1
What really does affect the speed of a Pinewood Derby Car? As I read e-mails, discussion group debates, and printed information on Pinewood Derby racing, I notice a large amount of agreement. No one seems to doubt that:
1. Friction is bad, 2. Weight is good, 3. Wheel alignment is good.
But on the other hand, I continue to see disagreement and confusion on several topics. I call these topics "The Big Debates." They are:
1. Best Weight Position 2. Number of wheels on the ground 3. Canted wheels 4. Best wheelbase 5. Importance of Aerodynamics 6. The Best Lubricant
In this article we will discuss the first two items from the list above. The others will be reserved for future articles.
WEIGHT POSITION Why is there so much debate about the best weight position? I believe mostly because of two factors:
1. The advantage/disadvantage of weight placement is dependent on the track type. There are at least three track types in use, and all of them vary in length and slope. So the weight placement that produces best results on one track, may not be best on another.
2. The center of gravity (COG), the balance point of the car is the key measurement, not the location of the weight. Of course the weight location helps define the COG.
So, let's look at these two areas in detail.
Track Types
There are three track types in use today (see Figure 1), with the first two types being the most predominant:
1. Continuous-slope - One continuous slope from the starting line to the finish line
2. Ramp-flat - Starts on a slope, then transitions to a long flat section
3. S-shaped - Starts on a slope, transitions to a steeper slope, and then transitions to a flat section)
Figure 1 - Track Types
For the continuous-slope track, the COG location of the car has little to no effect on car performance. However, if your organization has district races, the track at that race could be of the ramp-flat type. So, I recommend locating the COG closer to the rear of the car in anticipation of racing at another event. There is no disadvantage to rear-weighting on a continuous-slope track, and who knows, maybe you will get the opportunity to race in a district event where a rear-positioned COG is an advantage.
For the ramp-flap track, best performance is generally obtained by locating the COG about 1-1/4 inch in front of the rear axle (possibly closer on very smooth tracks, further away on rough tracks). This location provides a longer 'fall distance' (see Figure 2) than center-weighting, and still provides stability (note that rear-weighting a car does put more weight on the rear wheels than on the front wheels. Therefore, make sure to thoroughly prepare and lubricate the wheels or the advantage could be lost).
Figure 2 - Fall Distance Based on COG Position
For the S-shaped track, the best location of the COG totally depends on the length of each section. So, in the absence of track testing (see below) it is probably best to locate the COG similar to the Ramp-slope track.
Locating the COG on a Car
The lengthwise location of the center of gravity of a Pinewood Derby car can be easily located as follows: (1) set a ruler on its long edge on a table and (2) lay the car on the ruler as shown in Figure 3. Move the car forward or backward until it balances on the ruler. This balance point is the lengthwise COG.
Figure 3 - Locating the COG
Testing the Track
Testing for the best COG location for a given track is fairly easy if you have a track timer. Build a lightweight car without added weight, but with 3 dowel rod pieces sticking up out of the car (one in front, one in back, and one in the middle). Using steel washers, weight the car in the front, back, and middle and compare the results. Mix and match to find the best COG for the track.
I ran time trials with this type of test car on a ramp-flat track and found that rear-weighted cars outperformed front-weighted cars by up to one car length. Other testing results show similar results.
In addition, Michael Lastufka used a computer model to generate data and reached similar results.
So before debating the best weight position, make sure to find out the track type, and clarify whether the person is referring to the COG or the weight position.
NUMBER OF WHEELS ON THE GROUND Should you run with four wheels on the ground or with three? How about two? Before we answer these questions let's first discuss why you would consider anything other than four wheels on the ground.
Running with less than four wheels can be beneficial in several ways. Lifting one front wheel:
1. Reduces the amount of energy required to start the wheels spinning by one-fourth, 2. Simplifies wheel alignment (one less wheel to align), 3. Provides the opportunity to eliminate the worst performing wheels (get it out of the way by raising it), 4. Improves car stability.
The first three items above provide a small amount of speed increase. But - as proven by Michael Lastufka in some recent tests - the increased stability of a three-wheeled car generally provides a large advantage. Let me explain.
Unless you use precision drilled axle holes, it is very difficult to adjust the wheels/axles such that all four wheels touch the ground equally. Usually, most of the weight of the car sits on three wheels with the other wheel only slightly touching the ground. As the car travels down the track, the car will rock back and forth imperceptibly between the wheels, causing a loss of speed as the fourth wheel continually speeds up and slows down. Purposely raising one front wheel eliminates this issue if the COG is placed behind the center point of the car.
How about raising two wheels? A 'two-wheeled' car would be created by raising two diagonally opposite wheels. The car would then rest on the remaining two wheels if the car was perfectly balanced. The trick would be to locate the center of gravity at the balance point.
If the four wheels were each located 1-inch from the end of the car, and if the car was symmetrical in all directions, then the balance point would be exactly in the center of the car. However, since the standard wheel spacing does not place the wheels in this manner, and since most cars are not symmetrical in all directions, weights would have to be carefully placed to establish the center-of-gravity at the correct spot for a given design. This would be a difficult task.
Even if the car was perfectly balanced, the car would tend to rock back and forth between the two raised wheels as it hit the joints or other rough spots on the track.
All in all, I think that time is better spent in making a fast three-wheeled car, than in attempting to create a two-wheeled car.
CONCLUSION I hope that this has cleared up any confusion on thse two topics, but I am sure that the debate will not cease. Many factors affect the speed of pinewood derby cars, and isolating the affect of one factor is quite difficult. So debates like these will likely continue.
