Weight and Distribution

One of the most fundamental rules in the Pinewood Derby is the maximum weight limit of 5.000 ounces, or 141.643 grams. Simple physical principles dictate that a heavier car will run faster. How is that? We learned in freshman physics that acceleration due to gravity is independent of mass. That is true, but 2/3 of the track the cars run on is flat. What does that mean?

On the sloped part of the track, both a light car and a heavy car will accelerate at the same rate. Assume, for sake of argument that both cars have the same shape (aerodynamics) and the same drag due to friction. The combination of aerodynamic drag and friction will therefore exert a similar force on both cars. We also learned that the same force will accelerate (or, in this case, decelerate) a lighter object more quickly than a heavier object.

Thus, once the cars are rolling along the "flat", the lighter car will decelerate, or slow down, more quickly than the heavier car and will therefore reach the finish line later than the heavier car.

Fundamental Rule Number One: The car must weigh exactly 5.000 ounces.

Rule number one applies regardless of any distinctions that your organization may make in other rules. Whatever the weight limit is, you want to be exactly at the limit. What is "exactly"? After all, there may be a difference between the scale you used to weigh the car compared to what the "official" scale says. It doesn't matter if you used a precision balance calibrated to 0.0001 gram if the "official" scale says you're 0.1 grams over.

How do we get around that idiosyncracy? Well, you could leave the car just a shade light, say 4.97 ounces or about 142.5 grams. Or, if your rules permit last-minute weight tuning, you build in a little extra weight in a form that you can easily remove it in half-gram or less increments until the car is just barely under the maximum weight.

My preferred trick for that is to drill about 5 countersunk holes into the bottom of the car, sized for a #4 flat-head screw. You will find that 1/2" #4 flathead brass wood screws have a mass of just under 0.5 grams. Then build up the rest of the weight in the car so with all five screws in place, it is 1 to 1.5 grams over the 141.643-gram limit. At tech inspection, you pull out the screws one at a time until the car passes, and you are assured of being within 0.5 grams of the limit. (I consider 0.5 grams "good enough", as the other variables, including how the car is placed at the starting line, matter at least as much as the last 1/3 of 1% of the weight limit.)

Fundamental Rule Number Two: The weight must be as far to the rear of the car as possible, but...

Let's go back to that freshman physics course again. Our gravity-powered cars depend on their potential energy at the starting line to give them their velocity as the car travels down the slope and transitions from the slope to the flat. All of the cars are spotted at the same height on the track, but a car whose center of mass is higher up on the track will have greater potential energy with which to work. Raising the center of mass is simply a matter of moving the weights toward the rear of the car.

You can overdo the "move the weight back" trick, though. If the front end of the car is too "light", it will tend to bounce on any irregularity in the track and may even fly off the track or interfere with another car, which can lead to disqualification.

It has been our experience that locating the center of mass of the car about 3/4" to 1-1/4" in front of the rear axle represents the best compromise between stability and potential energy. Any higher, and you will have trouble with the car leaving the track.

There are numerous ways of adding weight to the cars. The most important considerations are that the weight not project below the bottom of the car body, or it may drag on the guide strip, and that it be very well secured so it does not fall out or fall off. I had access to some brass rod stock which was about 0.40" in diameter. We would cross-drill three half-inch diameter holes almost all the way through the body, two just in front of the rear axle and one just behind it. Once the body was painted (this adds appreciable weight!), we would cut three pieces of brass rod and trim them to bring the weight to the "just under 5-ounce" point. The rods are then hot-glued into the holes.