Pinewood Derby Scale Measures CG

If you suffer from nostalgia, you might remember carving a block of wood into a car, adding some wheels, and racing it against other contestants in a pinewood derby. Today’s derby is decidedly high tech though, and we were impressed with this car scale that also figures out the car’s center of gravity.

Based on an Arduino, of course, along with a pair of HX711 load cells. Why a pair? That’s how the device measures the center of gravity is by weighing the front and rear of the car separately.

We really liked the wooden case and found the use of wood satisfying if not ironic. Our only input is that since you need the wheelbase of the car to do the CG calculations, we’d have glued a ruler down. On the other hand, probably any self-respecting pinewood derby creator knows their wheelbase by heart.

Why does CG matter? If you are too far forward, you lose some acceleration. If you are too far back, the front wheels might pop up. With this device, you can know exactly where your center is and make adjustments accordingly.

If you’d rather build something for the actual race, why not a photo finish system? Or, perhaps you need a jet-powered (illegal) entry.

23 thoughts on “Pinewood Derby Scale Measures CG

  1. i don’t want to give away this secret, but i can’t resist, you actually want one of the front wheels to be lifted off the track. 1 less wheel = less friction and less rotational momentum you have to spin up.

  2. Let’s have a moment of silence for poor cub scouts near Titusville, Manchester County in MO or any other place where all of the other kids have engineer parents.

    My last one had wheels and axles crc machined at mcdonnell douglas and was weighted by an alloy that melted at 110 F in a double boiler on the stove like chocolate since metallurgists just have that stuff laying around the house. I was the poor kid.

      1. I’m still younger than the last MD-80 you hopped a flight on, so no embarrassment there. The city was a theme park for kids interested in making things. I still have a t-square from ralston purina that some random engineer gave to me as I was going through the place.

  3. My pinewood derby “secret” was to put the CG as far to the back as practically possible. When the cars were in the starting blocks my CG was higher than everyone else and therefore it had more potential energy. As I predicted, I was still accelerating later than other cars as the track flattened out. I pulled ahead at that point for the win.

    Aerodynamics don’t matter much at these speeds but CG management, wheel alignment (to track true) and axle polishing (friction, duh) is the way!

  4. I worked at a large auto manufacturer. Every year we would have a pinewood derby contest. The winning group got a steak dinner. My cars won often. I am a EAGLE scout after all. A group leader came to me and asked me to make his groups car since no one wanted to build it. He said he just wanted it to win a few races to save face, not the whole thing. I said I would if I could make it from a plain block no sculpting. He came in 3rd. Might of won it all. But a large blast of wind blew his car off the outdoor track according to him. I was not there since it was held after work, no pay no stay. The next year almost every car was just a block of wood. They quit having it after that year… Guess it was my fault.

  5. Our weights were inlet in the bottom. Taught my sons how to use a chisel to do this, since they needed to build the cars. Not too many 9 year olds knew how to create a mortise.
    Shape design and painting was up to them. We had winners and losers. Always had fun.

  6. At a local makers’ fair there was a display of really beautifully made and painted pinewood derby cars. I said with a smirk to the guy associated with the display, “Beautiful. And I’m sure dad didn’t help.” He grinned.

  7. We also sanded the wheels to round them off and reduce the contact patch (not much material thickness to work with so it took a few tries, and polished the hole in the wheel, not just the axle.

    The biggest trick was constructing a regulation track at home to test different car modifications and tweak them to perfection.

    1. The only thing i never did was sand the wheels to round them off, It was “illegal” in our competition to modify the wheels. I got 2nd place every year, and the last year I was young enough to compete, I finally found out the guy who beat me every year was rounding his wheels. I look at those 2nd place trophies and see 1st place, because I know I played by the rules.

      One of my other tricks was to put a crown onto the axle so the wheel rides in the center and not on the edges. The other was to put a groove into the axles that would act as an oil reservoir (Yes, we were allowed to use oil). weight back as far as possible, 1 front wheel off the ground. machined axle slots so the car went straight. slotted front of the car so it sit in the center of the release pin and was slightly farther forward to trip the laser.

      interestingly, i discovered the wheel off the ground advantage after my best friend at the time built his car in his bedroom and didn’t have the tools/capability to make the axles straight and one of his front wheels came off the ground. his car was so fast that year even without any other tricks.

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