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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
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.
;^)
:^)
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.
Er, am I revealing woeful ignorance or are there a lot of other people out there who haven't the faintest idea what the heck this is? Couldn't an informative link have been provided?
OK, 5 minutes with Google later, it appears to be something coming from the Scouting movement; kids build cars out of a basic kit of a block of wood and wheels, and then race them down an inclined track. So you get competition, woodworking skills, and parent/child bonding as Grandad shows you how to work a lathe etc. (OK, you don;t use a lathe to make a wooden car but obviously since I don;t know what it is I have no woodworking skills)
~~~~~ BigLig2? You mean there's another one of me?
Put all your weight in the middle, and as low as possible. If you put it up front, your rear wheels end up being a drag. Max out the weight as everybody suggested, it does help it run more smoothly. Just hollow out the shell as much as you dare, melt lead fishing weights into it, and goo the whole thing over with silicone to make it stay in place. I have a shoebox in a closet that I just happened to open a few months ago that has 25 year old cars in which the siliconed weights are still securely in place. The nice thing about lead is that you can actually re-melt it and experiment with distribution prior to siliconing it into the final location. For testing we'd just temporarily tape it in place using something strong like electrical tape.
I'll go counter to everything everybody else said: Aerodynamics didn't seem to matter at all. Many decades ago I consistently won with *everything* I built -- specifically, that included a big boxy "fire truck" design festooned with all sorts of plastic "equipment". I never took anything less than first place in the 20 or 25 cars I built and ran over a period of several years. I doubt the cars go fast enough or run far enough for aero to matter -- it's all weight distribution and free rotation of your wheels.
That said, most of my car bodies still followed a basic low-wedge shape, with a wide rounded-bottom groove down the middle. This was easy to build, sort of looked aerodynamic, and removed as much high-riding wood weight as possible, leaving as much low-riding lead weight available as possible. I've tried shaving weight off EVERYWHERE, in some cases even trimming wheels down to flat discs (leading to new rules in our group requiring the use of specific wheels from a kit).
"Real" car designs are boring, everybody does those. Go for the abstract. Confuse your opponents. :)
Slashdot quality declines as the number of hot grits posts decreases. - Provolt's Law, Apr-09-2005
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.
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.
Don't forget rolling friction!
Ideally, you want to stick the wheels on a lathe to minimize the amount of wheel surface touching the track. This should have more impact than any tweaking of aerodynamics!
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!!!
My father ran for several years a council-wide competition as well as the pack-level races. Just to try out different things, he made a test car that had different styles of wheels, solid axle vs. nails, etc.
WRT the wheels, he milled perfectly flat across (maximizing contact area), a very small sliver on the inside or outside of the car, and a v shaped wheels. It ended up that multiple runs ended up with near identical times. Wheel shape really didn't come into play. Didn't matter because altering the wheel shape was against the rules.
Weighting does come into play, but not the position of the weight. Just get it close to 5 oz (the limit at the time). If you have races over multiple days, remember that scales sometimes do change slightly over time, so it may be better to be a tad under.
Solid axles did nothing once you cleaned up the nails and put graphite on them.
Some other things that we found out with test cars and through observing the races that really don't affect times:
-Paint jobs. Smooth, polished, waxed. Didn't matter. Have fun with it, put stripes on it or flames, but don't go crazy trying to get the perfectly smooth finish to minimize microflaws in the paint. Regular ol' Krylon works fine.
-Percise "aerodynamic" shape. The winner was almost always a flat car about 3/4 of an inch think. Pointed, rounded, or angled noses didn't matter, but they usually had some type of "areodynamics" to them just for looks. A square nose ran just as fast.
-Drag, turbulance around the wheels, etc really don't come into play. They are light, start off slow, and reach the bottom quickly that it dosn't come into play significanly.
Things that did help with the car:
-Not buying the premium "pine car" accessories like the weights, "better" wheels, axles, etc. Any weight will do. We used old brass hardware. Lead shot also works. Feel free to have fun with model car parts like windshields, tailpipes, etc. Keep in mind overall dimension limitations.
-As someone else pointed out, polish the supplied nails to remove the burrs or whatever they are from both the head and shaft. You want a very smooth nail. Easiest way is put them in a drill press, get some fine sandpaper and some water. Turn the press on and just polish away.
-Pop the wheels into a lathe or drill press and wet sand down the surface of the wheel to remove any moulding lines.
-Graphite, graphite, graphite. Don't use WD-40, oil, etc. graphite is the key. Put it on till everything is covered where a wheel may even rub (side of car, nail head, shaft) I think liquids or grease might even be against the rules (drips on tracks) as well as spacers.
-Getting a fast lane. This is just a chance. Some lanes run faster the others. They may be smoother at the joints or have a better finish or something.
Some other advice:
-Have fun.
-Take your time. Work with your son to figure out what you want to do before hand on paper. Have him draw out a couple of ideas and talk to him about the design qualities (aerodynamics, liitations of the wood, available tools to make it).
-Don't do it all. This is your son's project that you are helping. Give him some tips/help, but don't do it all for him. I remember when I was a scout seeing these wonderful looking cars magnificantly created and a flawless paintjob. It's very obvious when Dad does it all and the kid has didn't do anything. Son will get more value in the long run from spending time with you working then if you did it all and he wins. Looking back, I didn't win 1st, but I usually ranked high and I remember working with my Dad more then anything about the race.
-Both of you should remember it is only a race. It doesn't mean anything in the grand scheme of life. I remember kids crying because they lost. I'm sure some dad has gotten "track rage". It isn't worth it.
Common sense stuff here, but.....
1. Molten lead is hot, very hot. ADULT use only.
2. Molten lead puts off poisonous fumes (from the lead, and from impurities in the lead). Melt lead only in a well ventilated area (outside), and keep the kids away from it.
3. Above all, let the kids dictate the overall design of their cars. Suggest all you want, but if they want to build the coolest looking, slowest car around, let them. It's their race, not yours, and they need to be the ones who enjoy it for their own reasons.
We have a Pinewood Derby for adults here at work, with proceeds going to the local Boy Scout chapter. It's fun.
/. isn't very useful. :)
In our rules (which I believe are standard PWD rules), you must use the wheels and axle that come in the derby kit. And tests have been done proving that aerodynamics do not matter at all -- by far, the biggest contributors are center of mass and wheel friction. With these things in mind, you can see that most of the advice on
Use powdered graphite on your wheels. Raise one wheel slightly above the track, so that your car is racing on only three...but make certain that the racer rolls straight without wobbling. Make the center of gravity one inch in front of the rear wheels. Get as close as you can to the maximum weight. And build your car with a 'nose up' design -- have the front of your car touch the starting pole as high as possible, with nothing below. When the starting pole drops, a nose-up racer starts downhill a fraction of a second before a racer with a low nose.
The above is what you'll want to do if you want to win. My advice, especially if you're doing this with your child, is to build a neat design and go to have fun.
Genocide Man -- Life is funny. Death is funnier. Mass murder can be hilarious.
IAAP (I am a physicst)
From a conservation of energy standpoint, you should try to put as much mass as possible over the back wheels. If I remeber from my Boy Scouting days, the derby cars started out sort of pointing downwards. Having more mass "higher up" the starting ramp means that it will have slightly more gravitational potetntial energy than a car built with mass in the nose once the cars hit the flat part of the ramp near the finish line.
Of course, I think that it would only be a few % difference at most. Something like (length of car)/(height of ramp).
From an engineering standpoint, I think just having the maximum mass (and excellent wheel lubrication) is the most beneficial, so as to reduce the percentage of energy lost to drag.
Remember Galileo dropping the large stone and small stone from the tower of Pisa? (ok, an apocryphal story, but anyway...) The both hit the ground at the same time since they were dropped from the same height. If we negleted friction & drag from the Derby, which is actually a MAJOR concern, all cars would fall equally fast, EXCEPT those with more mass at the back of the car, and therefor furthere up the ramp, and therefore with more potential energy.
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