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Pinewood Derby Tips?
PizzaFace asks: "My son and I have to start building his first Pinewood Derby racer, and I'm looking for tips. I've found conflicting advice online about even the basic science (e.g. high vs. low centers of gravity) so I'm hoping to tap into some of Slashdot's expertise." Might someone have some pictures of cool designs that might useful as a starting point?"
1st place, cub scout pinewood derby, 1980
Slashdot readers will look at a pine derby car and wonder if they can cut a window in the side and add neon lighting.
- billn
It's not too hard. As others have pointed out, center of gravity means nothing, except that a lower center would tend to have less cross-sectional area (which is good). All you have to worry about is friction and weight.
.001g over, use a sharp knife and remove a sliver of tape to finish the tuning. You'd be amazed how much a bit of vinyl electrical tape can weigh...
Friction is the hard part, and you've got two kinds to deal with: Friction with the air, and friction between the wheels and their axles.
For wind resistance, just start slicing the car down as much as you feel you can without sacrificing strength, or capacity for added weight. Round off both ends of the block into something that looks reasonably aerodynamic - I used a bench-mounted belt sander. You want it low, and flat, so as to displace as little air as possible as it moves down the track. Using similar curves to a high-speed train will yield better results than mimicking an F14 or Lamborghini. That the resultant form is rather boring is not an accident.
Friction against paint is probably not a big deal. My finish was a hand-brushed yellow-and-black flame paint job, and was nowhere near smooth. But it certainly wouldn't hurt to make it as mirror-like as possible with careful painting and sanding, along with a coat of well-buffed wax.
One trick I used which I've never seen repeated: A small circle of electrical tape, placed over the outside of each wheel, to further reduce drag. They were cut with a ball peen hammer and a coin of appropriate diameter, and fit precisely. It should be flat or slightly convex, but not concave, when applied to the wheel. This will increase rotational inertia (==bad) somewhat, but it seems like a beneficial thing to do.
To reduce friction between wheel and axle, I first removed the burr on the supplied axles using progressively fine sandpaper - there were two axial burrs, along with one across the back of the head. I also removed burrs on the inside of the wheels, and slightly rounded the flat part near the middle, inside of the wheels where it can touch the body, to reduce contact area.
Use graphite on the wheels and bearings, and use it as much as a polish as you do a lubricant. Work it into the microscopic texture of the axles, the inside of the wheels, and the bearing surfaces between the axle and the body by whatever means you can find, and then add some more and roll the car around a bit.
I used feeler stock (thinner-than-hair bits of flat metal) to get each wheel to within a thousandth-or-so of an inch of being on the same plane, such that they'd all be in contact with the track surface. I don't know if this helped, but it seemed like a good idea.
At least, try to keep your axles as straight and square as possible with eachother and the body. You do not want any pressure on the points where the wheel contacts the body, or the head of the axle.
And adjust the axles so that there is as little play in them as possible. The wheel should not move appreciably side-to-side, nor should it forcably rub on any point. Slop here translates to lost energy. It may take several iterations of gentle tapping on the axle and even-more-gentle pulling on the wheel to get this right.
Weighting is a science. You want the car to be as heavy as it possibly can be. Remember that you won't be immediately disqualified for having a very slightly heavy car, and that you'll get at least a couple of chances to bring the weight down.
Drill holes in the bottom of the car, and pour lead into them. I used an antique balance I had at home and added weight until I measured it being just -over- specification, and so the lead was protruding slightly below the bottom of the car. This allowed me to use the official scales to tune the car at the event, removing a bit of lead with a file and re-weighing several times before nailing it precisely, while in the process filing the lead flat and reducing drag.
Place a strip or two of electrical tape over the lead, to further reduce drag and add the nth degree of tuning: If you file a bit of lead off and the car still measures
If your track levels out toward the bottom, put the weight as far toward the back of the car as you can. You'll get a few more inches of "thrust" by doing this, vs. your front-weighted opponent.
The devil's in the details. When it's all done, handle the car as if it were full of nitroglycerine - don't even look at it funny, or all of the precarious work you've put into it will begin to undo itself. Never let others handle it, and absolutely never leave it unattended at the event. It's a finely-tuned, very delicate instrument, and is deserving of respect.
Kid-proof tablet..
Here's my advice.
Ask your son to draw some pictures of what he wants it to look like. Go over them with him. Pay particular attention to any pictures that go on the sides -- that will tell you his interests in a way conversation can't.
When it comes time to begin actually carving the block of wood, take a dummy piece of wood and make sure he knows how to cut. Then let him cut the dummy piece. Then let him cut the car.
Use a scale and some lead shot (or whatever), break out a piece of paper for a real application of subtraction. How much lead shot do you add to get to the maximum allowed 5oz? (Wow, subtraction is useful in real life, dad?) Ask him how best to attach your weight. Perhaps he can think of other materials just as good? (Paint will add some, so it may be helpful to keep it possible to remove a little.)
Now, start painting. Maybe you sand first, maybe you don't. Chances are, he's getting tired of this project and just wants to see the final shiny piece. High gloss enamel spray paint in the color of his choice. Anybody older than the Tiger Scouts (if they're still around) is old enough to handle spray paint with supervision. Try to coach, not lecture. Help, not instruct. Ask the boy how best to apply the artwork from the first paragraph.
Overall, the goal is to spend quality time with the boy. Sure, the boy says he wants to win. You or I may have won when we were kids, but do we remember? I don't. All I remember is how little help I got. My first car started out as a team project but it ended up as the metaphorical "Team of One". For my second car, I was told that I knew where the table saw was. These are the times he'll remember. And the times you will wish you had back. Try to imagine what'll happen to your son when he's 18. What will you wish you had done? What will you wish him to remember? Does a trophy really matter?
In the end, which would have more value to a boy like yours: winning first prize in the Pinewood Derby Competition, or his dad taking him and his den out for pizza? My money's on pizza. Judging from the winners in my pack, the pizza would have been cheaper.
Have fun. I wish I was in your place.