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?"

Comments

Weight is everything

[Picass0]

The following assumes that adding weight to your car is legal within your rules.

1. weigh everything
2. make a note of your max. allowed weight. That's your target.
3. drill several hole in your pine body and fill them with enough lead shot to make your weight without going over.
4. Fill the holes in you pine body with wood putty.
5. sand and paint body of car
6. spray wheels with 10w-40 for less friction
7. go kick ass
8. 
   
   1st place, cub scout pinewood derby, 1980

Re:Weight is everything

[swillden]

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.

Assuming you can manage without unbalancing the car, it's useful to raise one wheel up a millimeter or two, so that your car runs with only three wheels touching the track. Ideally, this reduces the wheel friction with both body and track by almost 25%. Make sure the car still rolls straight, though, and make sure it doesn't wobble. Wasting energy bouncing up and down or -- even worse -- bouncing off the sides of the track will kill you.

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.

Which, unfortunately, means you should absolutely never allow a Cub Scout to touch it. The biggest problem with Pinewood Derbies, IMO, is that you have to choose: will it be Dad's car and have a shot at winning, or will it be the boy's car and lose badly? Next time around, we're going to make two cars, one mine and one his, so that he can have some fun building and racing his own but we can also have a real shot at winning.

A few points

[zero_offset]

The guy who said use graphite is absolutely right, it's the best way to get your axle to spin. That said, don't use "wheel nails" as he indicates -- use an actual axle running through a hole drilled into the body. Find the most rigid, lightweight "rod" you can for an axle. Get the graphite that comes in a squeeze tube, which makes it easy to squirt into the axle hole. Make the hole relatively tight (bouncing around is wasted motion) but not tight enough to restrict rotation or make it unreasonably difficult to add graphite. Be aware that graphite is powdery, messy stuff, so carry the car in a big ziploc or something along those lines.

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. :)

Friction

[aaarrrgggh]

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!

Be careful meltin glead into these...

[billmaly]

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.

Nose up, and/or have fun.

[Remus Shepherd]

We have a Pinewood Derby for adults here at work, with proceeds going to the local Boy Scout chapter. It's fun.

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 /. isn't very useful. :)

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.

Center of gravity

[The Iconoclast]

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.