How thin of a car is ok

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rpcarpe
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Re: How thin of a car is ok

Post by rpcarpe »

Sporty & FS are spot on.
Here's the mantra I've been using this year 'Less wood, More metal'.

Just don't let the bottom of the car meet the top.
Sporty - I definitely need a portable wind tunnel for my garage, but first I need another garage to hold a wind tunnel.
My wife started a new support group... Widows of the Pinewood Derby.
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Re: How thin of a car is ok

Post by idpwdnut »

Duane wrote:Making the front thinner and lighter means that the weighted back end then needs to be heavier and thicker, so air drag is increased by the thin front end. A thin or hollowed front end does help with moving the com rearward, which helps the total energy available. Does the changed com provide enough energy to overcome the added air drag?
I am afraid that I am not understanding this comment. If the weighted portion of the car is 5/16" and I then taper that forward portion of the car down to 1/8", is my air drag going to be more than a car that is a 7" long flat plank of 5/16"? I would think wind resistance would be a factor of largest cross section of the car, but that a smooth, sleek tapered front ends would be more of a help than a henderance. Most of your exotic supercars (fullsize) are sleek thin front ends, goinig thick enough to sit a person. I realize that PWD cars and fullsize cars are completely different, but there has to be some overlap in principles.
My seat of the pants results have shown my different. My sons and his friends cars which are 5/16" extended will base dogbone style cars were pack champion cars in their respected packs and were 1st and 2nd at their district races. In the fun race, the blue standard wheel base car pictured above beat them by have a car lenght. Same agressive com weigthing,
Duane wrote:Does the changed com provide enough energy to overcome the added air drag?
I would say absolutely, if done correctly as a part of an orderly stepped car design and building process. Why else would the wedge be so popular.

Open to be taught more about areodynamics though.
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sporty
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Re: How thin of a car is ok

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Sporty - I definitely need a portable wind tunnel for my garage, but first I need another garage to hold a wind tunnel.

Ahh, You don't really want to get crazy into this do you ? :idk:


I did some adult racing there in 2010. and I felt it was needed to try and do well. This was not my wind tunnel, It was my Former team sporty racing mate. Woodworx. he had his own wood shop and biz. He got it from a friend for us to use.

There are allot of car builders / racers in this sport, plenty faster than me. They either retired, like I am. or they adult race and when you are at that level, working so hard to go fast and stay on top. You can spend allot of time and money into this sport.

So it's not as easy for them to share there info and processes and little speed improvements change all the time. Some sell there stuff, they got into the sport and found a way to turn it into a biz.

I just happen to share all mine on here, or most of it any how. I been at this 12 year or was. lol.


You just got to really love it or want to go really fast. :pullhair: :pullhair:


Pinewood derby is just a small part of the overall scout goal. They do and learning many more things. But when dads get involved, building and speed. Look out, we just go crazy !.
:mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen:


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Stan Pope
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Re: How thin of a car is ok

Post by Stan Pope »

Duane wrote: ...
Making the front thinner and lighter means that the weighted back end then needs to be heavier and thicker, so air drag is increased by the thin front end.
...
This is true only if the purpose of the thinning is to move the CM. If the CM is to remain in the same place, the removed material can be replaced by a small dense nugget located at the CM of the removed material.

Now, since the weight of the front end plays a disproportionate role in the location of the CM, reducing the front end weight allows less extreme location of the CM of the added rear end mass, which, in turn, allows that rear end mass to be flattened out with its CM moved a bit forward.
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Re: How thin of a car is ok

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sporty wrote:Ahh, You don't really want to get crazy into this do you ? :idk:
Too late
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Re: How thin of a car is ok

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rpcarpe wrote: I definitely need a portable wind tunnel for my garage, but first I need another garage to hold a wind tunnel.
I'm jealous too!!

I tried to find some open-source wind tunnel sim software that was simple enough to understand, to run a 2-D car profile, or some spinning wheels. Didn't find anything I could readily use.

I have hand-held cars up through my car's sunroof, while driving 15mph. Useless for wood bodies, but did tell me whether plastic sails or dragon wings would flutter.
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Re: How thin of a car is ok

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Stan Pope wrote:Now, since the weight of the front end plays a disproportionate role in the location of the CM, reducing the front end weight allows less extreme location of the CM of the added rear end mass, which, in turn, allows that rear end mass to be flattened out with its CM moved a bit forward.
Thanks! So thinning the front end (everything far forward from the CM) will require/allow more metal in the back, but relaxes the need to pile all that metal aft of the rear axle. Relaxing that metal placement allows it to be a longer thinner pile, rather than the thicker pile I was assuming. Yes?
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Re: How thin of a car is ok

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idpwdnut wrote:
Duane wrote:Making the front thinner and lighter means that the weighted back end then needs to be heavier and thicker, so air drag is increased by the thin front end. A thin or hollowed front end does help with moving the com rearward, which helps the total energy available. Does the changed com provide enough energy to overcome the added air drag?
I am afraid that I am not understanding this comment. If the weighted portion of the car is 5/16" and I then taper that forward portion of the car down to 1/8", is my air drag going to be more than a car that is a 7" long flat plank of 5/16"? I would think wind resistance would be a factor of largest cross section of the car, but that a smooth, sleek tapered front ends would be more of a help than a hindrance. Most of your exotic supercars (fullsize) are sleek thin front ends, goinig thick enough to sit a person. I realize that PWD cars and fullsize cars are completely different, but there has to be some overlap in principles.
If the plank car has a rounded or beveled nose, it will have essentially identical air drag to a car with a thin wedge profile and same rear thickness. (Ignoring CM and weight stuffing issues.) At these speeds, simple rounding of the nose is adequate to get perfect airflow and streaming all the way until the unfortunate back end. Narrow wedge/spear noses are necessary at supersonic speeds, and useful at subsonic speeds, and useless at 200mph. Think of the front edge of a 747 wing; it is rounded. It's the back edge of the wing that must be knife-edged for low drag, at any speed. A teardrop streamline shape has the pointy part in the rear; the front merely needs to be smooth.

In all these cars, the air drag occurs primarily aft of the blunt rear end, and aft of each wheel, as a big suction. The amount of suction depends on the cross section before the trailing edges. There's little pressure drag in front of anything, and little friction drag along the sides.

If you ran the wedge cars backwards, they would have much lower air drag. But lousy CM energy.
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Re: How thin of a car is ok

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Duane wrote:
Stan Pope wrote:Now, since the weight of the front end plays a disproportionate role in the location of the CM, reducing the front end weight allows less extreme location of the CM of the added rear end mass, which, in turn, allows that rear end mass to be flattened out with its CM moved a bit forward.
Thanks! So thinning the front end (everything far forward from the CM) will require/allow more metal in the back, but relaxes the need to pile all that metal aft of the rear axle. Relaxing that metal placement allows it to be a longer thinner pile, rather than the thicker pile I was assuming. Yes?
Yes! You can play with a variety of configurations rather quickly with Computing Center of Mass and Weight Distribution. Note that the page includes a dimensions - weight work sheet for a variety of ballasts along with a worksheet for compounding a series of additions to a car. I really should include some "macros" to quickly describe "standard BSA Axles and Wheels", etc. But If you're serious, you're gonna trim those down, too! :)

When I said "worksheet" above, I'm talking about a calculation capability that takes what you supply and fills in the blanks with what can be deduced from it (with just a few exceptions.) From what I've seen around the web, that page is rather unique in concept.

The "thick pile", I think, was a result of the assumption that trimming the front was done in order to move the CM back as far as possible from its prior location.
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How thin of a car is ok

Post by geauxturbo »

At low speed, wouldn't creating turbulance (like a golf ball surface) be better? Not that I or my 8 year old have the paitience to dimple a pwd car. But I bet one of you would be up up the task :)
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Re: How thin of a car is ok

Post by sporty »

geauxturbo wrote:At low speed, wouldn't creating turbulance (like a golf ball surface) be better? Not that I or my 8 year old have the paitience to dimple a pwd car. But I bet one of you would be up up the task :)


No, the golf ball effect is really for 1 directional air flow, not effected by the ground but open air, no close surfaces by it and thats more for really fast speeds.

Many others have tried it, it does not work.


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Re: How thin of a car is ok

Post by sporty »

Duane wrote:
idpwdnut wrote: I am afraid that I am not understanding this comment. If the weighted portion of the car is 5/16" and I then taper that forward portion of the car down to 1/8", is my air drag going to be more than a car that is a 7" long flat plank of 5/16"? I would think wind resistance would be a factor of largest cross section of the car, but that a smooth, sleek tapered front ends would be more of a help than a hindrance. Most of your exotic supercars (fullsize) are sleek thin front ends, goinig thick enough to sit a person. I realize that PWD cars and fullsize cars are completely different, but there has to be some overlap in principles.
If the plank car has a rounded or beveled nose, it will have essentially identical air drag to a car with a thin wedge profile and same rear thickness. (Ignoring CM and weight stuffing issues.) At these speeds, simple rounding of the nose is adequate to get perfect airflow and streaming all the way until the unfortunate back end. Narrow wedge/spear noses are necessary at supersonic speeds, and useful at subsonic speeds, and useless at 200mph. Think of the front edge of a 747 wing; it is rounded. It's the back edge of the wing that must be knife-edged for low drag, at any speed. A teardrop streamline shape has the pointy part in the rear; the front merely needs to be smooth.

In all these cars, the air drag occurs primarily aft of the blunt rear end, and aft of each wheel, as a big suction. The amount of suction depends on the cross section before the trailing edges. There's little pressure drag in front of anything, and little friction drag along the sides.

If you ran the wedge cars backwards, they would have much lower air drag. But lousy CM energy.

Your forgetting a few factors, these asumptions and info, compared to a pwd car on a track with a center guide rail.

The results are not the same and I'm sorry to say, that the information is not true for the pwd cars.

Those testing and known data and results are not the same for pwd cars.

1- the pwd car, creates its own wind, there is no wind in the room, where the race is held, this is a factor, versus more common testing of car and trains.

They will have factors or wind coming from the outside, along with there own speeds they are creating along with the wind.


2- theslope of the track is a factor for a certain percentage of the race, which is also where the car accumulates most of its speed at. At which also is where the biggest and most wind resistance / air flow will occour, by the pwd car creating its own wind resistance by the forward movement of it's self.


3- So the designs and shapes you mentioned, will have a different outcome on a pwd car, then one might think.

4- I initially though the same thing, until we begin testing with a piece of track under the car. up until that point, allot of what you say and what more common testing has shown, with the basic thinking of shapes.

However, this was not the case with the factors of angles (track) and also that darn center guide rail.

Not to mention the wind that the forward moving pinewood derby creates, is enough to do something about, but rarely ever reached the back of the car or back wheels, during the testing we did.

The cars would have to go faster !
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Re: How thin of a car is ok

Post by rpcarpe »

So, Rpenning... does that answer your question??
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Re: How thin of a car is ok

Post by Duane »

sporty wrote:
geauxturbo wrote:At low speed, wouldn't creating turbulance (like a golf ball surface) be better? Not that I or my 8 year old have the patience to dimple a pwd car. But I bet one of you would be up up the task :)
No, the golf ball effect is really for 1 directional air flow, not effected by the ground but open air, no close surfaces by it and thats more for really fast speeds.
Many others have tried it, it does not work.
Tennis balls are made fuzzy for the same reason as golf balls are dimpled. Making the airstream boundary layer slightly turbulent rather than laminar does help reduce the huge air drag aft of a spherical ball, by about a third, so the ball travels further before slowing. A turbulent boundary layer is "stickier" and stays attached longer to receding walls than does a laminar boundary layer. The equations of aerodynamics and professional wind tunnel work shows that this principal works in air at all non-microscopic sizes and all non-sonic speeds. I suspect honeybees have hairy bodies rather than hard shells for this reason.

But the place where it is useful and effective is only at the widest part of the 3-d shape, just before the object begins narrowing and the air is brought together again by sucking pressures. The golf ball and tennis ball are coated with dimples or fuzz all over, only because the ball normally tumbles and has no fixed front and back ends. When Mythbusters coated an entire car with clay and dimpled it all over (and got better mileage despite the added weight), they didn't realize that all they needed to do was put a 1-inch wide strip of fuzz width-wise across the top of the car to get all the drag-lowering benefit.

Designers of glider sailplanes put a strip of "turbulators" along the top of the wings, at its thickest points. This allows the aft portion of the teardrop-shaped wing to have a thicker, shorter wedge shape and still do a perfect job of converging the top & bottom airflows without causing large-scale vortices. Otherwise the aft portion would have to be a much longer, heavier thin wedge. Turbulators are also used on some commercial jets. There, they take the form of stubby 1-inch fixed rotor-shape blades, sticking out here and there from the wing surface.

Turbulators only help if the aft does converge in a wedge or cone with moderate angle. In golf balls and tennis balls, they only help the airflow wrap slightly around the waist of the ball a ways, before the spherical shaped aft becomes too steep and the boundary layer detaches anyhow. The fuzz allows the cross section of the aft vortices to be about 80% of the sphere's diameter, instead of 90% without fuzz. And this reduces the cross section area to (80% squared) = 64% of the sphere. The induced drag is proportional to that cross section area.

In glider sailplanes, the turbulator or tripwire trick allows the aft vortex area to be reduced to near zero, without having an impractically wide wing.

Keeping the airflow totally laminar up to the point of the turbulator is important, as it reduces surface/air friction. Making the boundary layer turbulent aft of the turbulators does increase friction, but much more is gained by reducing the form drag from the large aft vortices.

In pinewood cars, the challenges are the stubby back end of the car body, and the volumes immediately aft of the wide BSA wheels. The problem of front wheels can be maybe helped with light, hollow fenders aft of the wheel. If the wheel is exposed, it is likely acting as its own turbulator already. But for the rear wheels and body back end, there is not enough room to add much tapering.

The points where wheels touch the track and center guide will always have a lot of large-scale vortices. Airstreaming there is both impractical and fruitless.
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Re: How thin of a car is ok

Post by PhantomVirus »

If you were to go ULTRA thin could you not embed a carbon fiber rod or 2 the length of the car? Thinking of using a small dado bit on a table say to cut a single notch the length of the car then making it ultra thin (next year's project) and epoxying the carbon fiber rod into the block.....

We went pretty thin this year (0.08Cm = 1/3 inche) without issue but I made sure I had the right block with the proper grain pattern to minimize warping (heating the block in the oven probably didn't hurt either).
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