Do the "wafer" cars always win?

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resullivan
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Re: Do the "wafer" cars always win?

Post by resullivan »

I have to disagree somewhat to what has been posted above. I have seen two different videos from two different people in which they taped an index card onto the car and tested the speed. They gradually removed surface area from the card and it was very clear that the car got faster. One of the videos I believe was the physics PWD guy. I think he said as little as 1/16th of an inch less of surface area is worth removing to gain speed.

That being said, you might check your rules and see if you can have multiple entries. I know our pack as well as our district allows people to enter one car for looks and one for speed.
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Re: Do the "wafer" cars always win?

Post by FatSebastian »

resullivan wrote:I have seen two different videos from two different people in which they taped an index card onto the car and tested the speed.
One video is Physics and the Pinewood Derby. IIRC, the other may have been Pinewood Professor. Regardless, I think the context of the experiment might beg for clarification...
resullivan wrote:They gradually removed surface area from the card and it was very clear that the car got faster. [...] I think he said as little as 1/16th of an inch less of surface area is worth removing to gain speed.
Sully is correct that, all other things being equal, a thinner car has less aerodynamic drag. But, there are three factors that affect deceleration due to aerodynamic drag: mass, area, and shape (streamlining). Thus, a streamlined car that is 1/16" thicker could still have better aerodynamic performance than a thinner yet non-streamlined car. In the experiment, the index card was fastened perpendicular to the direction of travel, like a sail or parachute facing the wind and acting like an air brake. The experimenter trimmed away the card piecemeal, reducing the area significantly and the mass negligibly. While the experiment showed a significant relationship between area and elapsed time, it did so using the worst aerodynamic streamlining possible: a block with a sail fastened to it. This exaggerated the significance of the aerodynamic influences in the experiment.

I suppose my point would be that the activity should foremost consider the satisfaction of the child. Although a plank with wheels is about as streamlined as one can get, that shape may very well be a most uninteresting "car" to a child. Also, there are other physical forces and factors which can be more significant than aerodynamic drag.
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Re: Do the "wafer" cars always win?

Post by Curse You Red Baron! »

Our two cents. My son's 'wild wing' design won Webelos and was 3rd overall in Pack (pack is 50-65 cars depending on the year) with relatively poor alignment and super canted back axles at 5 degrees or slightly more (our first RR and i got the guide for my scout's drilling wrong :oops: ). It lost pack by about two inches, so yes it could have won, and I'm sure aerodynamics hurt us - but so did several other things we could have done better. My scout hollowed out the car using a scroll saw with the block on it's side, so that the remaining piece of pine looked like the stencil outline of the wildwing. Then we added balsa side panels (2 per side glued together, so that combined they were about as thick as the pine panels the BSA sells - it sanded and painted just fine, no issues with strength at that thickness.) We could have done more to remove wood, polish wheel hubs, and align - heck, there was a big hole in the top (cockpit) and that had to have caused air turbulence - we were kind of amazed at how well it did. Our two Pack/district winners (year before, year after) were close to planks, but at or over 3/8 of an inch tall in places, and had fins and curves, so we had some design fun with them too. The thinner cars both won at districts, but the Wild Wing came in 8th if I remember right, which was respectable. It was a really cool car, and fun to build. I've heard it said that your car's speed is limited by what you do the worst. If your aerodynamics are less than optimal, then you have to be that much better at everything else, which is a fun challenge! If the point is to have fun with your son and teach him something too, what could be better than a challenge you are both passionate about?
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Re: Do the "wafer" cars always win?

Post by Topspin.D »

The aerodynamics argument aside... we like thin cars because it's the easiest way to remove weight from places we don't want it and to add it in the good places.

Most of our car bodies are about 0.5-0.7 oz before we start painting and adding weight.
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Re: Do the "wafer" cars always win?

Post by bracketracer »

Vitamin K wrote:I ask, because when I help my son make his next car, I want him to be able to create the design that he wants (in terms of looks), but since I know that he wants to win as well, I want to help him choose a fast design as well.
I wanted to share my own thought on this. When my son's first Derby came along I was gently prodding him to build a plank car. It would be FAST! He said he already knew the car he wanted. The "Home Depot" stock car from the Saturday Kid's Workshop at our local HD. Holy Cow! How are we gonna win with that! But I talked myself into it, it's his car, I'm just helping. Glad I did! The only thing I didn't let him do was cut the body on the tablesaw. I wasn't ready to turn a 6 year old loose on the big saw just yet. He glued it up, drilled the wheelwells, sanded it, painted it, polished the wheels and axles, and helped with applying the decals. (He did let me help lighten it a little, 'cause when we were done widening the body we had a 9 ounce racer with no wheels!!) And he ended up with a permagrin after getting second in the Tiger cub race to a plank car by .028 cumulative for four runs. Third place was only .008 behind. Oh Well. Next year will be different! Maybe. Here's a couple pics:
Image
Image
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Re: Do the "wafer" cars always win?

Post by Vitamin K »

That's a spiffy looking car, Bracketracer! Props to you for going with what the boy wanted and making it speedy!
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Re: Do the "wafer" cars always win?

Post by bracketracer »

Thanks for the compliment! I think it could have been quicker, we left the COM at 1 1/4". I think we could have shortened it up more had we used tungsten instead of the plain cheapo steel weights. The weights you see in the pic go all the way to the rear of the car, we couldn't slide them back any farther!
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Re: Do the "wafer" cars always win?

Post by LightninBoy »

I built a "block" car for my 5 year old and let him decorate it. Pictures are here ...

http://www.derbytalk.com/viewtopic.php?f=19&t=7198" target="_blank

It was the fastest car in the Siblings category and would have taken 3rd overall in the Pack if it was a scout car. FS and others are bang on in this thread ... the big advantage of going thin is really about weight distribution.
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Re: Do the "wafer" cars always win?

Post by Vitamin K »

Does anybody have a recommended source for getting balsa wood? The pieces I see in places like Michael's seem too small or thin for my purposes, but maybe I'm not looking in the right places.
resullivan
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Re: Do the "wafer" cars always win?

Post by resullivan »

What size are you looking for? Hobby lobby has a good selection from veneers to blocks.
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Re: Do the "wafer" cars always win?

Post by bracketracer »

I get my balsa at a hobby store near my work. They are heavy into RC airplanes and stock a ton of gear. There's probably one like that near you? If not, I pm'd you a link to their online store.
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Re: Do the "wafer" cars always win?

Post by rpcarpe »

Two quick thoughts:

- In our popular workshops, when I explain to Scouts and Parents about the shape of their proposed car, I always emphasize 'All Else Being Equal' the thin car wins. Then I look straight at the Cub and tell him 'Your car, your call'.

- If the 4x6 sail on car experiement were re-done, perhaps shaving 1/8" off the car each time while compensating with a piece of weight to keep the COM the same would yield more accurate results.

Good ideas here on how to make a Hershey bar shaped car more interesting.
My wife started a new support group... Widows of the Pinewood Derby.
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Re: Do the "wafer" cars always win?

Post by J.Stein »

This thread has caused me great consternation.

After our first year (ran a hollowed out wedge), we cut one block into three separate "dog bone" or "dragster" style cars. We've been tweaking the design since and are getting down to pretty much a toothpick but have to build up the rear slightly (vertically) with wood putty to hide all the tungsten we put in the rear.

A few weeks back, I had the dremel out to sharpen the mower blade and told my boy to "go get the Pinewood box" and we wasted most of an afternoon finishing up the final "GEN III" body.

Now I wonder. We've only got two years of Webelos left....do we scrap our Dog Bone design that we've been trying to master and "go for it" and try to tackle a new "wafer" or "spoon" design? :?

Ah, heck with it. The boy already spray painted the thing. :mrgreen: :rofl:
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Re: Do the "wafer" cars always win?

Post by Noskills »

we cut one block into three separate "dog bone" or "dragster" style cars
This poses a good question. Is it the actual full width super thin car that does well or just the very light car that does well. What if you had a 0.4 oz "Sporty Spoon" wafer car vs a 0.4 oz dog bone car-both with same COM, wheels, prep etc etc. Would they not be pretty much equivalent? I would think the initial weight, amount and placement of tungston would be more important than shape (barring obviously poorly aerodynamic designs).

Thoughts?

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Re: Do the "wafer" cars always win?

Post by sporty »

my super thin cars, i found did well, because they were lower to the track aside from just 4oz of weight to it. the air flow, stays lower, does not go up as high, a small detail in the end.

Sporty
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