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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?"
Let's see. my body was no thicker then my pinky in the front of the car which came to a rounded point in the front. The back was no thicker then my thumb.
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I put wood puddy (putty? hrm, it's late) in the middle where most of the fishing weights and lead were hidden. It looked like a cockpit/bubble like thing on the top middle. But I also had a door for access to a small amount of loose wieghts on the bottom for find tuning.
I even put a spoiler over the back wheels for kicks. With 2 wood dollies holding up a trianglar piece of wood on top. Hey, it looked cool
But after all is said and done, it is all about the wheels. I have seen people take 3rd with an un carved square block with weights.
Sand the wheels with some of the finest sand paper you can get (220grit at least, but the last of the sand papers was almost like paper it was so smooth...dont remember the grit on that one, though). I remember sanding the wheels for more hours then builing the thing. Also, I used graphite powder on the wheel nails for lubrication.
I won two 1st places in cub scouts with this design and one 2rd place later racing against the adults as a cubmaster.
Have fun
Graphite on the axles. I did very little to the thing other than that. No fancy slick low-drag paintjob. Put some weights in the bottom of the car to meet the maximum weight allowed, and graphite on the axles. That's all you'll likely need. But a low-drag paintjob wouldn't hurt. That's really about all you're allowed to do, anyway. And what else is necessary? Low-friction for the axles, low friction paintjob, and enough weight to maximize the 'thrust'.
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You don't need to worry about center of gravity - all you're doing is running a car down a straight track. No turning or anything like that - center of gravity has no effect on the type of performance you're looking for whatsoever.
Oh, don't forget the racing stripes - that's critical. Plus it should be red.
For a sweet low-friction paintjob, here's my advice:
Carve the thing into a blob shape - remove as little as you can get away with, as you'll just have to replace with weight later on (melt some lead into some holes till you get what you need). Once you have your shape, wet the thing down a bit and sand it smooth. Let it dry. Wet it down again and sand again with a smoother grit sandpaper. Repeat the process until you get a sweet, sweet shine. Using pine, you'll eventually be able to get the wood so smooth you can see reflections in it (no exaggeration). Once you've got it to that point, you're ready for painting. Enamel paint, for sure.
Put the weight in after painting, and get it up to the maximum weight allowed.
Graphite the axles before each run, of course, and whack your opponents on the knee before competition.
Weight tips: pretty simple. Make everything symmetrical. Putting more weight on the front, or more weight on the back, will simply screw it up - you want the wheels to have equal weight on them.
Good luck, and don't forget to be a geek and put pix of the whole build process online somewhere for us to Slashdot.
When I was a cub-scout, the best "drivers" quickly figured out to use graphite lubricant instead of oil such as WD-40. Spread it all around the axels and insides of the wheels. Also, be sure to sand off any of the plastic burs from when the wheels were manufactured.
When it's all lubricated and assembled, try spinning the wheels with your hand. I remember we'd all have competitions: spin all four wheels at once as fast as you could, and time whose wheels would spin the longest.
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..
OK, I ran a den for a few years. Our pack alternated between Pinewood Derby, Rocket Derby, and Raingutter Regatta.
Pinewood Derby - Race little wood cars down a track.
Rocket Derby - Race little rubber-band propeller-driven "rockets" down a piece of fishline; you go for distance, not speed.
Raingutter Regatta - Race little sailboats down a length of raingutter, powered by Cub Scout breath.
The biggest challenge to all three of these is to make sure the boys, not the fathers, do the bulk of the work. For most hand tools this isn't too hard, though the boys usually want the slicker looks that Dads can produce. You also want the boys to be able to have win-capable racers, without stepping in too much. We built the Pinewood Derby racers in den meetings, and I insisted that fathers (where available, a sad story in itself) attend with their boys. We settled on a basic design, and each boy got two cuts on the radial arm saw with me, while the fathers supervised other work like polishing axles. My hands were on top of theirs, the cars were clamped, but they got to run the saw back and forth, and learned a little about power tools, safety glasses, and hearing protection.
Oh, a tip, speaking of polishing the axles. Those little nails they give you for axles often have a burr on the inside of the head. You want to chuck the nail into a drill, and polish that burr off. It's also a good idea to tilt the angle of the inside of the head a little obtuse from the shaft, so that the inside of the wheel rides on a slight cone - less area for friction.
Of course my son didn't win, so maybe you should ignore this. But I will second any vote for graphite.
The living have better things to do than to continue hating the dead.
Having done this with my son for 3 years (started our fourth car last night), I have a little experience. There are a few things that have changed since 1980. No oil, it isn't allowed now in most races, use dry lubricants like graphite or Molysulphide instead. Weight is almost everything. If you spend 10 hours on the car, 9 of them should be on the wheels, even if you just attach the right weight to the block of wood and have your wheels working properly, you will do well.
BTW, we learned this by finishing last place the first year, third place the second year, and second place last year. Just two more races to go before Boy Scouts. I raced back in the good old days of the early 70s and got two first place trophys myself.
I have no sig, does anyone have one to spare?
First place the axle nails in a drill and using very fine wet/dry sandpaper buff the axles while spinning the drill. Then, using a rag apply some graphite to the axle in the same way.
Third mount the wheels themselves in the drill and lightly sand the outsides of them, as there is ussually a ridge of plastic left from molding. Now mount the wheels on the axle and the axle in the drill. Spin it up again and apply some more graphite to the spinning wheel. The idea is that a small amount of lapping takes place and smoothes the wheel's spin on the axle.
When mounting make sure to mount the wheels so that they spin freely but yet don't wobble on the axle.
Finally, yes bulbus shapes (think falling water droplet cut in half) are the most streamlined but I felt that keeping the weight in the back was more important as it gave gravity a split second longer to act on the car. in which case a wedge shape is best.
Just my thoughts and three first place trophies talking so take it with a grain of graphite!!!
I agree with the above with one exception. Melt the lead into a rectangular hole. Make it so that the weight is distributed evenly in the horizontal direction across the body. Don't burn yourself if you use a torch to melt the lead instead of an iron. I found the best way to do it without burning the wood was to use lead rod (you can get it at a crafts store in the stained glass section) and drip it in from a few inches above the car. Also, make sure you have an accurate scale and make your car as close to the maximum allowed weight as possible. The weight wins the race.
Also, mount the wheels in your drill and run them over some 220 grit sandpaper for a fraction of a second and then graphite the nails before you attach the wheels to the car. (I've heard of using oil like the parent mentions, but I had great luck with graphite).
As for cutting the wood, the fastest design I ever made was just to pass the block through the table saw with a tapering jig so it was almost full height at the rear and a sharp point at the front. The weight was 3/4 of the way back. I built the whole thing in an hour.
We found at a hobby shop some weights that were made for pine woods. Had different shapes and weights. They had a hole in the middle and you could just screw it down or I think we used tape or funtack then covered the weights with Black Electrical tape. Puttying the weight in is a stupid idea unless your for sure your car is not going to need a tweak. Remember, just cuz it's 5 oz on your scale does not mean that it is on THEIR scale.
Gorkman