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kevtherev Samba Member
Joined: December 23, 2007 Posts: 897 Location: the 51st state
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Posted: Sun Aug 31, 2008 1:02 pm Post subject: What a drag |
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I'd be interested if any folks has any information on the drag coefficient of a vanagon. (cue the jokes )
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 Samba Member
Joined: August 03, 2005 Posts: 5390 Location: PNW
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fairweather Samba Member
Joined: August 26, 2007 Posts: 663 Location: Aspen, CO
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Posted: Sun Aug 31, 2008 5:01 pm Post subject: |
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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
TV |
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greenbus pilot Samba Member
Joined: March 14, 2008 Posts: 1285 Location: Wisconsin: Rustbelt, USA baby!!
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Posted: Mon Sep 01, 2008 12:27 am Post subject: |
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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.
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Harmonic vibration- the vehicle has reached its resonant frequency.
My Bus buzzes loudly at 3600 RPM. Then it calms down.
( I do not claim to have any special training in the field!). _________________ Sent from a white van down the street. |
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kevtherev Samba Member
Joined: December 23, 2007 Posts: 897 Location: the 51st state
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Posted: Mon Sep 01, 2008 10:12 am Post subject: |
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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
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
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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wbx Samba Member
Joined: April 11, 2005 Posts: 1254 Location: Monterey, CA
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Posted: Mon Sep 01, 2008 11:24 am Post subject: |
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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... _________________ '84 Westy (first owner).......but my daily driver has pedals
My "perspective" mantra:
A Volkswagen Vanagon is just a material thing,
As such, it is of the earth,
And if i need to, I can let my Van go. |
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tencentlife Samba Member
Joined: May 02, 2006 Posts: 10078 Location: Abiquiu, NM, USA
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Posted: Mon Sep 01, 2008 7:25 pm Post subject: |
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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, ? _________________ Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/
Please don't PM here, I will not reply.
Experience is kryptonite to doctrine. |
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wbx Samba Member
Joined: April 11, 2005 Posts: 1254 Location: Monterey, CA
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Posted: Mon Sep 01, 2008 8:24 pm Post subject: |
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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 _________________ '84 Westy (first owner).......but my daily driver has pedals
My "perspective" mantra:
A Volkswagen Vanagon is just a material thing,
As such, it is of the earth,
And if i need to, I can let my Van go. |
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tencentlife Samba Member
Joined: May 02, 2006 Posts: 10078 Location: Abiquiu, NM, USA
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Posted: Mon Sep 01, 2008 8:33 pm Post subject: |
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Cool. Guess I could have figured that out. You made it easy. Thanks. _________________ Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/
Please don't PM here, I will not reply.
Experience is kryptonite to doctrine. |
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jackbombay Samba Member
Joined: October 19, 2007 Posts: 2723 Location: Eastern Idaho
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90Doka_Guy Samba Member
Joined: April 08, 2007 Posts: 548 Location: South Jersey
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Posted: Mon Sep 01, 2008 9:09 pm Post subject: |
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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.
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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. _________________ -'87 Westy
-'90 Tintop
-'90 DOKA |
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MsTaboo Samba Member
Joined: June 02, 2006 Posts: 4087 Location: East Kootenay, British Columbia
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Posted: Mon Sep 01, 2008 9:09 pm Post subject: |
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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? _________________ Currently:
'90 Syncro Westy 3 knob w/Zetec
The information age has morphed into the age of disinformation and willful ignorance. Agnotology!
Help the fight against Truth Decay.
Defend democracy, support Ukraine. |
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jackbombay Samba Member
Joined: October 19, 2007 Posts: 2723 Location: Eastern Idaho
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Posted: Mon Sep 01, 2008 9:24 pm Post subject: |
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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. _________________
Gas struts to pop your top easily!
Pop Top strut kits now available for late Bay window Westies
Samba ad here.
DIY artificial rain gutters (ARGs) |
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wbx Samba Member
Joined: April 11, 2005 Posts: 1254 Location: Monterey, CA
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Posted: Mon Sep 01, 2008 10:40 pm Post subject: |
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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
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 _________________ '84 Westy (first owner).......but my daily driver has pedals
My "perspective" mantra:
A Volkswagen Vanagon is just a material thing,
As such, it is of the earth,
And if i need to, I can let my Van go. |
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kevtherev Samba Member
Joined: December 23, 2007 Posts: 897 Location: the 51st state
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Posted: Tue Sep 02, 2008 10:25 am Post subject: |
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fantastic... just fantastic... thank you all very much
I have just won a whole heap of beer.
you da man wbx. thanks for digging it all up. I have added this information to our club wiki
kev |
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levi Samba Member
Joined: February 11, 2005 Posts: 5522 Location: Las Vegas
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Posted: Tue Sep 02, 2008 3:29 pm Post subject: |
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jackbombay wrote: |
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?
_________________ One of these days I'm gonna settle down,
but till I do I won't be hangin round.
Going down that long lonesome highway,
gonna see life my way
https://youtu.be/cSrL0BXsO40 |
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jackbombay Samba Member
Joined: October 19, 2007 Posts: 2723 Location: Eastern Idaho
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Posted: Tue Sep 02, 2008 8:45 pm Post subject: |
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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?
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Not mine, I cut out and fiberglassed over the bin on a frame of 1/8" masonite an 2X2'. _________________
Gas struts to pop your top easily!
Pop Top strut kits now available for late Bay window Westies
Samba ad here.
DIY artificial rain gutters (ARGs) |
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Dogpilot Samba Member
Joined: October 03, 2005 Posts: 4205 Location: Flagstaff, AZ
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Posted: Tue Sep 02, 2008 9:36 pm Post subject: |
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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! _________________ Geology with a Syncro rocks!
86 Syncro Westy AKA "The Bughunter"
98 Disco I
08 Range Rover SC
08 VW Rabbit S
1951 O-1G |
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inkysocks Samba Member
Joined: April 13, 2010 Posts: 8 Location: New Orleans, LA
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ZimZam Samba Member
Joined: July 21, 2005 Posts: 562 Location: Talkeetna, AK
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Posted: Tue Apr 13, 2010 6:27 pm Post subject: |
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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. _________________ '85 Cabriolet (Margo)
'87 Westy (Heidi)
'75 Porsche 911S Targa
"Nobody trusts anyone, or why did they put tilt on a pinball machine.”
- Steve McQueen
"It's just metal, when rusted and dented , anything you will do will be better. Stop being afraid. What they (VW) did many year ago is not better then what you will do now." -Ben |
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