An annual event in many Cub Scout Troops is the pinewood derby. In the derby, a father son team builds a very simple car and races it against the other cars on an inclined track, with an electronic timer.
Cars must be made from the BSA kit, using the enclosed axles (nails) and wheels. My goal in the race was to help my son have a fast enough car that it would not be eliminated on the first heat. It would also be a project in which he could learn that perfecting some simple parts would require some thinking, and result in a better operating machine.
The car is so simple that you would not think that one car could be made to go much faster than another car. Surprisingly, a person can invest a lot of time and creativity into making this simple car go faster. All the tips to make the car go fast are on the net, but here is my experience in making a fairly fast pinewood derby car.
Weight: it is essential to have the car weigh the most that is allowable. The scale (in our Pack) was a digital scale that weighs to the nearest tenth of a gram. The best way to get the car as heavy as legally possible is to remove weight to get it under the 5.0 g limit. If a car starts out weighing 5.1 and you remove weight, the scale will show it at 5.0 grams when maybe it is really 5.04. You would rather be 5.04 grams than 4.95, but both weights will read 5.0 grams on the scale, and be legal weigh ins. I got the car close but slightly heavier than 5.1, and then to get to just barely 5.0, I removed one at a time the small lead fishing weights that I had pounded into holes the underside of the car body. When I got down to 5.0, that was the best weight I could have.
Wheelbase: You want to have the longest wheelbase allowable, which means not using the grooves in the base as the axle slot. You need to drill holes in the side of the car, being careful to end up wtih enough clearance to clear the guide strip. The longer wheel base results in fewer bumps of the wheels against the guide strip, and thus fewer bumps which reduce speed.
Axle Preparation: Smoothing the axles and alligning them is where all the work is. The "axles" are actually crude nails, with ridges under the head and on the nail shaft. Those have to be removed and those surfaces made as smooth as possible. The surface under the nail head is smoothed using first a small triangular file, then files of various grits down to 400 grit. I have small metal files with diamond grit that I use to sharpen ski edges, and these are perfect for the underside of the nail head. The files are in different grits down to 400.
To smooth the nail head and shaft I put the axle in a drill or dremel, and put the dremel or drill in a vise. With the axle spinning, I use the triangular file for a rough pass, then hold a narrow strip of sandpaper on the shaft of the nail, and go through grits of 80, 120, 220, and 400. One sheet of paper of each grit is plenty for the four axles. As a last step, I put diamond paste on the backside of a strip of the sandpaper, and run it back and forth on the spinning axle. I have used the diamond paste before in metallography work, and it was the polish I knew, but there are probably others that work well.
The Wheels: All the wheels have to be regulation BSA wheels, but there are wheels, and then there are wheels. Four wheels come with the kit, but sets of four wheels are also available from BSA. I would buy 20 extra wheels, and my son and I would give then spin tests. He would run the stopwatch, and I would spin each wheel on a polished axle, with my finger. We would time the coasting time of each wheel, and found that some would spin for 1 minute and 30 seconds or more, and some would stop at 40 seconds or less. You use the four fastest wheels, of course. After getting the best spinners, you smooth the wheel surfaces to take off any mold ridges and make them perfectly round.
Alignment: The wheels are held on the car body by the nails, which are also the axles. Depending on the allignment of the wheels, the car can angle left, angle right, go straight, have the wheels press against the car body, or press against the nail heads, or run on the middle of the axle. You want the wheels to run in the middle of the axle, and for the car to run straight. What I did to help this was to tilt a long dining table slightly, by putting books under one end of the table. I ran a strip of masking tape down the table, then ran the car down the strip. By running the car down the tape, and observing one wheel at a time, you can bend the axle slightly up or down, forward or backward, to attempt to get a neutral (and perfect) axle orientation. If all four axles are thus alligned, the car will run straight. If it runs straight, it will have fewer bumps against the guide strip. If you reef too hard on the axle, you can break the axle out of the wood, so keep some super glue handy to glue the wood chip back in place.
Lubrication: I got a tube of graphite powder for lubrication. The trick is to get enough of the graphite onto the axle, then to spin it alot, to get it worked into the porosities of the metal and plastic. I put the axles in a baggie of graphite and jostled it for days. Then I assembled and alligned the axles before the race. Before the race I would add graphite, spin the wheels, add more graphite, spin more, etc. Immediately before the weigh in, I gave it a last shot of graphite. If you add too much at this point, the first few heats will be slower, but the last heats will be fast. If you dont' add enough, the first heats will be fast, but the last ones will be slower. The trick it so add just the right amount, and that is difficult.
Our first race resulted in our car taking about 5th place. Our car the next year did better, and I think I got lucky with the last minute lubrication. We won every heat and placed first in our pack. Jim was thrilled, and I felt like I was the envy of the Dad's. We decided to compete the car in the next level of race, and took it to the district race. Every car in this race had placed first in their pack, and I told Jim not to be surprised if we finished dead last against these cars. At the weigh in were some beautiful cars, and kids were evaluating which were the cars to beat. They didn't see our car as a contender, because it didn't look very good. As the race started, we won the first heat by inches, and ours and several other cars won heat after heat. Finally the fastest cars were run against the other fastest cars. Heat after heat, our car won, sometimes by a fraction of an inch. It came down to the last heat, and we edged out the other finalist, by millemeters!! Some kids were crying at this point, because they were beaten by a younger scout, and because they thought their car would win, and our car looked so plain.
Photo: Bob and Jim with the big district trophy. Jim with road rash from a recent bike crash.
I hope you have the chance to do this project with your son and have fun with it!
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