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#1
Old 01-13-2006, 01:25 PM
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Pinewood Derby: What’s the best lubricant?

Anyone who has or had a son in the Cub Scouts knows about the Pinewood Derby, where boys make a car out of a 7 inch long block of wood, 4 nails for axles, and 4 plastic wheels and race them down an inclined track.

Most kids use powdered graphite for lubricant on the axles and wheels. Is that the best thing to use, or is there anything better? One essential property is that the lubricant has to last for at least 8 races to make it to the finals because they can’t touch the cars after they’re checked in or between heats.

These races have gotten so competitive that the better cars are within milliseconds of each other, so any and every “go fast” technique must be used.
#2
Old 01-13-2006, 01:35 PM
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In my experience, graphite is the best lubricant. It's much better than the dry non-graphite lubricant sold in the Scout shops. The other critical thing is to ensure the axles are on straight and that the wheels are free to roll, but not so loose that they wobble.

My 9-year old son and I will be working on his car this weekend!
#3
Old 01-13-2006, 02:01 PM
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Make sure you check with the organizers with regards to graphite....


Way back in the olden days, some track owners got upset about graphite for some reason.

But it could be my 20years of time between now and my time in scouts.

-Butler
(Eagle class of '85)
#4
Old 01-13-2006, 02:09 PM
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I'm curious, what type of bearing do they use? Simple sleeve bearings? Are sealed ball bearings against the rules?
#5
Old 01-13-2006, 02:18 PM
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scr4, assuming you're not pulling our legs: It's a plastic wheel-shaped disc with a hole through the center, and a nail through the hole, and the nail is sticking into a channel cut through a block of wood.

Would buckminster-fullerene work better than graphite? Slippery at the molecular level...
#6
Old 01-13-2006, 02:23 PM
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When I was a scout my dad worked for a paper mill and got me some of the spray-on teflon lubricant they used for sticky spots on the conveyors. That stuff worked way better than graphite. I actually used it on the whole bottom of the car and sides of the wheels, so if it hit the track anywhere in its downward journey it slid better. The good thing about the stuff was that it is transparent so it didn't mess up the track. Parrafin wax works ok for this too if you can't find teflon lubricant.

However, there are other things to address first.

The wheels are plastic that ride on a nail driven into the side of the car.

The cheapo plastic wheels have a lot of molding flash on them that needs to be removed in order for them to track properly. They also can tend to wobble, which really affects rolling resistance. They also almost never are straight, so they tend to push the car against the track. This causes a lot of friction. You want to get the car tracking straight on a smooth surface like a hardwood floor before you ever go to the track. Another thing that you can do is chuck the wheel in a lathe or drill motor and turn a crown on them so that the contact patch is very small. Soapbox derby and solar car races use very skinny tires for this reason. Although the wheels are very hard on the pinewood derby car and do not deform much, having a narrower tread helps because it allows the wheels to skid easier if the track is not straight, helping the car lose less speed.

You are generally not allowed to use any other wheels or external parts but modifying the parts that are in the kit is considered fair game. If they are allowed, sometimes bronze thrust washers between the wheel and the car help reduce friction too.
#7
Old 01-13-2006, 02:24 PM
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We also used to sand the wheels. They always had a little bump on them from the molding process.
#8
Old 01-13-2006, 02:56 PM
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I have no experience in this, but here are some thoughts.

It might be possible to improve the nail. I'd try chucking it in a lathe (or drill) and making sure it's perfectly round and polished smooth. This could be problematic if nails are supplied in the kit and you have to use them, as making them narrower would increase wobble. If you can provide your own nails, you should be able to choose ones of optimum diameter.

I'd also look for spray moly (molybdenum disulfide) to coat the axle surface of the nail. Be sure to burnish it in. Spray moly is something a machinist would use, I last got some at an auto parts store. If it's not available, try (aerosol) spray graphite.
#9
Old 01-13-2006, 03:05 PM
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Aside from the part about Dad working at a paper plant and the teflon lube, I could swear orangetruck was my brother.The four of us collected quite a few trophies thanks to our efforts on the wheels. We would carefully chuck the wheels into a drill and smooth/shape them, but we used graphite. No washers were allowed.

You really can't just bang the nail in throgh the wheel's hub and hope for the best.

Another point to watch is the weight. You want the car right at the maximum allowed weight. We handled this by boring holes in the underside and filling them with solder, which was easily trimmed (melted out or carved with a knife) to achieve the desired weight. I don't have my car handy, but think we put the weight at about 1/3 into the car's length.
#10
Old 01-13-2006, 03:14 PM
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They must have changed the rules since I was a Cub Scout in the early '80's. We were required to use the stock wheels as they came in the official kit. Modifications weren't allowed, and this included lubricants. The block of wood for the body, OTOH, could be (and usually were) chucked, so long as the final car was still made of wood and fit into the required overall dimensions.
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#11
Old 01-13-2006, 03:22 PM
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The best lube IME is sewing machine oil. Far, far better than graphite. Comes in a grey squeeze bottle with a red cap. Get it at fabric stores or the sewing dept. of Wal-Mart type places. Very good general purpose oil. It does not dry out like the cheapo oils (3-in-1, WD-40) that way too many people buy.

But: Check your rules. Some places only allow graphite.

Remember:

Test, test, test. Make up a ramp and make sure it runs straight and smooth.

Add weight up to the max. Weight does matter here.
#12
Old 01-13-2006, 03:55 PM
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I have heard that one should set the car up so that only three wheels touch the ground. Cuts rolling resistance by 25%.
#13
Old 01-13-2006, 03:58 PM
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My 9-year-old cub scout is racing next weekend, so I'll be working on our car this weekend. In past cub scout races and in indian guides, we've always done very well. Last year our pinewood car finished first (out of six cars) in every race it competed in.

We used powdered graphite for the lube. Also, I used 600-grit sandpaper to take the burrs off the wheels, and I used a Dremel tool with a sanding attachment to take the flashing from the underneath the nail head, and the bumps from the nail, then I buffed the nail to a high sheen with the Dremel and a buffer attachment. When I put the wheel on and would spin it with my finger, I timed it spinning for 25 seconds - that's a long time!

Another tip for adding weight is to put the weight in the back of the car, as far back as possible. When the car is sitting at the starting gate, it's pointed down, with the rear higher than the front. You want the weight as high as possible, to convert gravitational potential energy into kinetic energy.

And be sure your cub scout understands how to handle the car and how delicate it is, and how it's important not to bang it around.
#14
Old 01-13-2006, 04:00 PM
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Oh, by the way, the reason that skinny wheels would be better is that there are many little gritty bits on the track, and a wheel that's half the width will hit only half as many as a full-width wheel. But modifying the wheels is not allowed in pinewood.
#15
Old 01-13-2006, 05:31 PM
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I'd like to test a couple of different lubricants before we mount the wheels on the car, say one wheel and axle with graphite and another with moly or teflon. But I'd need to have a way of giving each wheel exactly the same spin to see which one spun longer.

Any ideas on how to do this?
#16
Old 01-13-2006, 05:39 PM
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Quote:
Originally Posted by jebert
I'd like to test a couple of different lubricants before we mount the wheels on the car, say one wheel and axle with graphite and another with moly or teflon. But I'd need to have a way of giving each wheel exactly the same spin to see which one spun longer.

Any ideas on how to do this?
Mount a rubber wheel (maybe one of those mandrels that sandpaper rings fit onto) on an electric drill, spin each wheel with it.
#17
Old 01-13-2006, 05:43 PM
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I second the "all the weight in the back" suggestion. When I went through the pinewood derby, there was a weigh-in scale at the registering table. We had a few ounces to spare, so we superglued coins to the back.

Long story short, I won district.
#18
Old 01-13-2006, 05:58 PM
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Throw away all dreams of cool-looking bodies. My brother won with a section of stair rail carved to a point on the front. Lead was glued into a pocket underneath to bring it up to within a gram of the limit. Axle nails were chucked into a drill and sanded round
#19
Old 01-13-2006, 06:52 PM
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I need to clarify on using moly. It's not meant to be the sole lubricant. When burnished to metal, it helps fill microscopic gaps to make the metal smoother. You still need powdered graphite, or teflon, or oil. Whichever you choose, also having moly on the nail will enhance its slipperiness.
#20
Old 01-13-2006, 07:29 PM
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Quote:
Originally Posted by Rocketeer
I have heard that one should set the car up so that only three wheels touch the ground. Cuts rolling resistance by 25%.
I didn't think of it back in my scouting days, but I'd like to try making a car with only three wheels. On one end of the car, cut an oblong hole right through the body, hold the wheel in place and drive the nail in from the side. (Raise it up slightly, since it will be riding on the raised center of the track.)

I don't know if the rule makers thought to prohibit that, or if would even work very well, but it seems like an interesting thing to try.
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