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What a drag
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kevtherev
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PostPosted: Sun Aug 31, 2008 1:02 pm    Post subject: What a drag Reply with quote

I'd be interested if any folks has any information on the drag coefficient of a vanagon. (cue the jokes Very Happy )

Curious, because it seems that up to 60, things just seem to be fine on the fuel use and the noise is reasonably tolerable.
Yet beyond, at 70-75 that things get noisier and fuel usage is frightening.

Subaru fitted vanagon owners tell me that the bus noise seems to smooth out again at eighty miles an hour and beyond... and actually gets quieter.

is this some kind of aerodynamic barrier?

or is this there a simple explanation.
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Christopher Schimke
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PostPosted: Sun Aug 31, 2008 4:47 pm    Post subject: Reply with quote

Not that this is a direct answer to your question, but...

http://www.thesamba.com/vw/forum/viewtopic.php?t=258280
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fairweather
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PostPosted: Sun Aug 31, 2008 5:01 pm    Post subject: Reply with quote

If I recall correctly from my ancient physics class the resistance of an object moving through air is not linear, it is going to be exponential or logarithmic. When you approach and hit the speed you are talking about the curve on the scale gets much steeper. I would say the resistance would be on the vertical scale and speed on the horizontal. As far as the noise getter calmer at higher speeds I would venture a guess that the engine+drivetrain+tires etc "likes" that particular rpm better resulting in less vibrations etc.

Ok, now the physics majors can pick me apart Confused

TV
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PostPosted: Mon Sep 01, 2008 12:27 am    Post subject: Reply with quote

Quote:
As far as the noise getter calmer at higher speeds I would venture a guess that the engine+drivetrain+tires etc "likes" that particular rpm better resulting in less vibrations etc.

Harmonic vibration- the vehicle has reached its resonant frequency.
My Bus buzzes loudly at 3600 RPM. Then it calms down. Laughing
( I do not claim to have any special training in the field!).
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kevtherev
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PostPosted: Mon Sep 01, 2008 10:12 am    Post subject: Reply with quote

fairweather wrote:
If I recall correctly from my ancient physics class the resistance of an object moving through air is not linear, it is going to be exponential or logarithmic. When you approach and hit the speed you are talking about the curve on the scale gets much steeper. I would say the resistance would be on the vertical scale and speed on the horizontal. As far as the noise getter calmer at higher speeds I would venture a guess that the engine+drivetrain+tires etc "likes" that particular rpm better resulting in less vibrations etc.

Ok, now the physics majors can pick me apart Confused

TV


Quote:
resonant frequency.


Ahh like a sweet spot on a bat...


so as the pressure increases with speed,
the wind noise and buffeting is sort of left behind a little as speed increases past 80, could that be the pressure wave moving back over the van and the disrupted air pushed further behind

I shall ask my wife to give me a scientific evaluation on the noise level in the back... as we approach the sound barrier Very Happy

thanks loogy I didn't spot that in my search
it's interesting that the dehler looks the better displacer of air next to the tin top
My camper is shaped similar to that dehler.. but it's like a sail in crosswinds.

thanks
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PostPosted: Mon Sep 01, 2008 11:24 am    Post subject: Reply with quote

The drag coefficient (Cd) and frontal area (A) for the:

regular Vanagon are 0.45 and 3.06 m2;
later Vanagon with front spoiler lip,the values are: 0.44 and 3.08 m2;
Westfalia, the numbers are 0.52 and 3.17 m2;
later Westfalia with spoiler drops the drag coefficient to 0.51.

The European hightop camper (Dehler profile) takes the drag coefficient to 0.40 with a frontal area of 3.61 m2

The drag equation is:

Drag = 1/2 A * Cd * rho *v^2 (rho= density of air)

Hopefully that'll help answer the question...
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PostPosted: Mon Sep 01, 2008 7:25 pm    Post subject: Reply with quote

wbx wrote:
The drag coefficient (Cd) and frontal area (A) for the:

regular Vanagon are 0.45 and 3.06 m2;
later Vanagon with front spoiler lip,the values are: 0.44 and 3.08 m2;
Westfalia, the numbers are 0.52 and 3.17 m2;
later Westfalia with spoiler drops the drag coefficient to 0.51.

The European hightop camper (Dehler profile) takes the drag coefficient to 0.40 with a frontal area of 3.61 m2

The drag equation is:

Drag = 1/2 A * Cd * rho *v^2 (rho= density of air)

Hopefully that'll help answer the question...


It does, except what units does the equation return in? Nm, kW, ?
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PostPosted: Mon Sep 01, 2008 8:24 pm    Post subject: Reply with quote

tencentlife wrote:
It does, except what units does the equation return in? Nm, kW, ?


Drag is a measure of a kind of force. So in metric, N(ewtons), and in inch, typically noted "pounds force" (lbf)

Cd = unitless
A = m^2
rho = g/m^3
v = m/s

Which in metric, the units work out to g*m/s^2 = N.

-Damon
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PostPosted: Mon Sep 01, 2008 8:33 pm    Post subject: Reply with quote

Cool. Guess I could have figured that out. You made it easy. Thanks.
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jackbombay
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PostPosted: Mon Sep 01, 2008 8:41 pm    Post subject: Reply with quote

Image may have been reduced in size. Click image to view fullscreen.


Note how the "luggage bin" absolutely ruins the aerodynamics, its even significantly worse than the gigantic camper at the bottom, the westy is far and away the least aerodynamic vanagon there is.
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PostPosted: Mon Sep 01, 2008 9:09 pm    Post subject: Reply with quote

wbx wrote:
The drag coefficient (Cd) and frontal area (A) for the:

regular Vanagon are 0.45 and 3.06 m2;
later Vanagon with front spoiler lip,the values are: 0.44 and 3.08 m2;
Westfalia, the numbers are 0.52 and 3.17 m2;
later Westfalia with spoiler drops the drag coefficient to 0.51.


Where in the world did you dig that up?? I have always wondered this too.

One thing to add, if you want to figure out how much hp is required to push these puppies at a certain speed (not including rolling friction or losses in the drive train), multiply the drag force from the above equation (in newtons) by the speed (in m/s) and divide by 750. (1 m/s = 2.24 mph, the density of air at sea level is about 1.225 kg/m^3).

wbx, not to nitpick but rho has to be in kg/m^3 for the force to be dimensionally correct.
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PostPosted: Mon Sep 01, 2008 9:09 pm    Post subject: Reply with quote

I have wondered about that "luggage bin" being an air drag and thought about enclosing with either a "lid" or one of the G-westy lock boxes.

Thoughts anyone?
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PostPosted: Mon Sep 01, 2008 9:24 pm    Post subject: Reply with quote

71MYSTABOO wrote:
I have wondered about that "luggage bin" being an air drag and thought about enclosing with either a "lid" or one of the G-westy lock boxes.

Thoughts anyone?


Depends on how well your lid fits. The GW lock box would help some, but not as much as a well fitting lid, which is not particularly easy to make as the surface curves 2 directions.
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PostPosted: Mon Sep 01, 2008 10:40 pm    Post subject: Reply with quote

Keane wrote:

wbx, not to nitpick but rho has to be in kg/m^3 for the force to be dimensionally correct.


Yeah... you are absolutely right. When i just clicked back on the thread and saw that i used grams, i wondered who would correct that... You win Wink

And fwiw, that info was dug up from the vanagon.com archives. I remembered that thread from a long time ago (about 4 years). There is a freaky smart/tech geeky guy on there who comes up with that stuff. I just knew where it was, and can only assume it is correct. Some of it is corroborated by old road and track articles, though, so it should be good.

Using Keane's equation at 65mph:

regular Vanagon uses 27.6HP
later Vanagon with front spoiler lip uses 27.2HP;
earlier Westfalia uses 33HP;
later Westfalia with spoiler uses 32.6HP.
Dehler Profi uses only 28.9HP.

Hmmm... seems like there could be a market for an aerodynamic "luggage tray". I never use mine, anyway.

-Damon
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PostPosted: Tue Sep 02, 2008 10:25 am    Post subject: Reply with quote

fantastic... just fantastic... thank you all very much

I have just won a whole heap of beer. Very Happy Very Happy Very Happy Very Happy

you da man wbx. thanks for digging it all up. I have added this information to our club wiki

kev
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PostPosted: Tue Sep 02, 2008 3:29 pm    Post subject: Reply with quote

jackbombay wrote:
Image may have been reduced in size. Click image to view fullscreen.


Note how the "luggage bin" absolutely ruins the aerodynamics, its even significantly worse than the gigantic camper at the bottom, the westy is far and away the least aerodynamic vanagon there is.


...and, the only pop-top westy they show has a front lower lip spoiler. Interesting.....
So then, most westy's, not having that front spoiler have an even higher drag?
Crying or Very sad
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PostPosted: Tue Sep 02, 2008 8:45 pm    Post subject: Reply with quote

levi wrote:

...and, the only pop-top westy they show has a front lower lip spoiler. Interesting.....
So then, most westy's, not having that front spoiler have an even higher drag?
Crying or Very sad


Not mine, I cut out and fiberglassed over the bin on a frame of 1/8" masonite an 2X2'.
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PostPosted: Tue Sep 02, 2008 9:36 pm    Post subject: Reply with quote

Oh I see a possibility here. If you make a luggage rack cover that can lift up at its rear, you can approach the Dehler Profi's profile. You could make it like the speed activated spoilers on many high priced sports cars, lifting as the Van speeds up. What fun!
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PostPosted: Tue Apr 13, 2010 6:08 pm    Post subject: spreadsheet Reply with quote

i made a google spreadsheet that calculates these numbers for any given velocity for several types of vanagon body styles.

http://spreadsheets.google.com/ccc?key=0AiYu0jDliiExdEkyX0V2VE9LcGxtUXFidXgtOG0xeEE&hl=en

I wonder if anyone's developing the rear spoiler theorized by Kim, Geng and Chen to reduce drag by 5% and lift by 100% on bluff backed vehicles such as the Vanagon:

http://www.greencarcongress.com/2008/11/novel-spoiler-d.html

-i


Last edited by inkysocks on Tue Apr 13, 2010 6:34 pm; edited 3 times in total
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PostPosted: Tue Apr 13, 2010 6:27 pm    Post subject: Reply with quote

Mathematics and physics were never my forte. That being said my hat's off to those who paid attention in class, and can still remember the formulas. However, it doesn't take a rocket scientist to know that a Westy going to windward is a slug. On a reach is slightly better. Wished I would have listened in class though.
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