MadSci Network: Engineering
Query:

Re: Does it matter where you put the weights on a Pinewood Derby race car?

Date: Fri Jan 28 14:05:25 2000
Posted By: Mike Scannell, Powertrain Product Engineer, Ford Motor Company
Area of science: Engineering
ID: 948471035.Eg
Message:

Alan,

Wow, I remember the Pinewood Derby! I did that when I was a kid too. I don't know how good of advice I can give you though because I never won. But I think I took 3rd once. Hopefully I've learned a bit more about physics since then though.

Anyway, I believe that you will be best off with the heaviest car you can get. I remember at ours we had an "offical" scale that measured ounces to two or three decimal points, and everyone would try to get exactly 5.00 oz. Exactly where you put the weight in the car is not very important, but I would try to keep it in the middle. Where you put the weight will only have an effect on the car's "center of gravity," but in a simple race like this, center of gravity is not very important.

The physics involved in a Pinewood Derby race are fairly simple. Like you said, gravity does all the work. It's the only thing you have to move the car forward. Now, there are a couple things trying to slow you down, and those are wind resistance and friction. Some friction may increase when weight increases, but wind resistance is purely dependent on the shape of your car. So, the more your car weighs, the harder it is for these forces to slow your car down.

So how do you win? Well, first you should make your car as heavy as possible, or as close to 5.00 oz as you can. Then you should make sure you minimize any type of friction in your wheels and axles (the nails). I'd use a very light oil or graphite to lubricate the axles. The reason I say light oil is that with such a small, light car, a heavy oil may slow it down more than it helps. Make sure your car rolls straight, because otherwise it will be bumping into the track guides a lot, and that will slow you down. You may want to consider putting oil or graphite on the wheels to help reduce friction between the car and the track guides. Your car doesn't need traction, so actually lubricating the outside of the wheels won't hurt anything.

There are two things to consider when working with wind resistance. The shape of your car and it's frontal area. The frontal area is the size of the car when you look straight at it from the front. A large frontal area means the wind has something bigger to push on (like a sail). So you want it to be nice and small. You can't change the width of your car, but making it shorter will help. Getting the shape right is a little trickier. There's a lot more involved in aerodynamics than I can explain here. But just a couple of tips: keep the shape smooth and streamlined.

Good luck with the derby. Let me know if you have any more questions.

Your Mad Scientist,
Mike Scannell


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