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gnarly 928
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Joined: April 17, 2006
Posts: 46
Location: Lyle, Washington
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 Posted: Tue Feb 07, 2017 10:37 am Post subject: Re: What a drag |
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Just went through most of the thread here, being interested in adding a high top to my ABA tintop.. I've done a lot of aerodynamics, trial and error, mainly on Porsche. We used our 928 Porsche GT in the Nevada and Texas Open road "rallies" in the supersport category with speeds up to 190 on closed highways..Later, I gutted the car for tracks. I learned a whole lot, doing all the work myself, other than the motor.
First, even small changes can help a lot. The drag can be reduced. Think...70 mph along the road then you encounter a gust.of 50, or a headwind...Now you have 120 mph apparent wind...
Things at the front that cause turbulence are the biggest drags... I'd estimate at least one mile per gallon better mileage if one changed mirrors, another 1mpg if those gutters weren't there. I added some gargen border from Lowe's as a spoiler and got 1mpg on my 84.. little things add up.. and if I bust over a rock, that gutter crap is flexible and cheap..
The deahler top looks pretty slick... I'm doing a copy of another Euro high top without the link, matches the frontal angle then radiuses to full height ..
One interesting way I saw some guys checking aerodynamics at the track was putting drops of oil on wings and such, then running at speed to see where the oil blew. "Streamliners"? |
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MarkWard
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Joined: February 09, 2005
Posts: 17155
Location: Retired South Florida
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| Posted: Mon Nov 24, 2014 2:25 pm Post subject: |
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| Merian wrote: |
| you are assuming that the text is correct without any methodology to go on - but that is par for the course here |
Great 444th post.
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?Waldo?
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Joined: February 22, 2006
Posts: 9752
Location: Where?
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| Posted: Mon Nov 24, 2014 2:22 pm Post subject: |
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Published results of wind tunnel testing don't contain methodology? What does?
Merian, you are posting without any pertinent information, punctuation, capitalization or forethought. That is very much sub-par almost anywhere that adults are present. |
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raoul mitgong
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Joined: July 05, 2009
Posts: 1338
Location: Denver, CO
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| Posted: Mon Nov 24, 2014 2:13 pm Post subject: |
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And don't forget to calculate that the closer you approach the speed of light, the slower time actually moves, and the more massive the object becomes.
You are welcome.
-d
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84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
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91 Westy (Only the top 12 inches of this van (a burn victim)) |
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Merian
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Joined: January 04, 2014
Posts: 5212
Location: Orygun
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| Posted: Mon Nov 24, 2014 1:58 pm Post subject: |
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| you are assuming that the text is correct without any methodology to go on - but that is par for the course here |
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dobryan
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Joined: March 24, 2006
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Location: Brookeville, MD
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?Waldo?
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Joined: February 22, 2006
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| Posted: Mon Nov 24, 2014 12:44 pm Post subject: |
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| randywebb wrote: |
| I can pretty much guarantee that thing will have a higher drag. |
| clift_d wrote: |
| Coefficient of drag on the high-top is less than the pop-top although the cross sectional area is greater - see: |
| Jake de Villiers wrote: |
| Yes but - Cd x Frontal Area = drag |
My German is not all that, but my understanding is that even multiplied out, **it is still less drag with the high top than with the pop top.** They even show it multiplied out for clarity.
Pop top = 0.51 (CoD) X 3.17 (Frontal Area) = 1.6167
High top (Club Joker 3) = 0.42 (CoD) X 3.80 (Frontal Area) = 1.596
As you can see, despite randywebb's 'guarantee' to the contrary, the total drag (CoD X Frontal Area) of a stock Westfalia Pop-top is **MORE** than the total drag of a stock Westfalia High Top. |
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Jake de Villiers
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Joined: October 24, 2007
Posts: 5911
Location: Tsawwassen, BC
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| Posted: Mon Nov 24, 2014 10:58 am Post subject: |
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| clift_d wrote: |
Coefficient of drag on the high-top is less than the pop-top although the cross sectional area is greater - see:
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Yes but - Cd x Frontal Area = drag
The interesting numbers are in German on the right hand side. The fuel consumption data is informative but the noise levels (in dB) are the real nuggets on this page! 
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clift_d
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| Posted: Sun Nov 23, 2014 5:58 pm Post subject: |
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Coefficient of drag on the high-top is less than the pop-top although the cross sectional area is greater - see:
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1988 LHD T25 1.6TD Westfalia Club Joker syncro |
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randywebb
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Location: Greater Metropolitan Nimrod, Orygun
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| Posted: Tue Jun 28, 2011 3:30 pm Post subject: |
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total drag will generally increase with the cross-sectional size of the monstrosity you are trying to push thru a fluid
only exception is if it is so well designed to be "slippery" (low Cd) as to negate the effect of the X-sectional area.
I can pretty much guarantee that thing will have a higher drag.
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offroad.
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Location: llanelli, s wales
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| Posted: Tue Jun 28, 2011 2:01 pm Post subject: |
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| i was going to ask the same question. |
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SyncroGhia
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silverbulletuk
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Location: NW Surrey, UK
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| Posted: Thu Jun 23, 2011 2:25 am Post subject: |
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FWIW, in Europe we have a Renault Trafic van of similar size to a SWB T5 (and often re-badged as a Nissan or Vauxhall)
It's basically just another wedge-fronted, slab-sided, flat roofed box van with corners you could cut yourself on, but for one detail:
The roof immediately behind the top of the front windscreen and over the cab area has a pronounced "hump" before transition into the very flat roof.
It's ultra-fugly without a doubt, but they must have done it for a very good reason. I'm guessing aerodynamics as all vans round here are driven at 85 mph all day long 
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Orbitald
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Location: Oakland, CA
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| Posted: Wed Jun 22, 2011 1:06 pm Post subject: |
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Its not just the coefficient of drag that matters, it is also the frontal area of the van. So although you may make modifications to create a more aerodynamic Vanagon most also increase the frontal area as well. The high top increases the frontal area substantially which most likely makes the overall drag of the vehicle higher even though it may indeed be quieter.
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J Charlton 
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| Posted: Wed Jun 22, 2011 12:55 pm Post subject: drag |
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For what its worth - and I know that anecdotal evidence is highly subjective - 2 weeks ago I went from a westy pop top to a high top
The van is powered by a 2.5l subaru engine. I took it fully loaded with my 2 little guys to Busfusion - about 250 miles each way
Some observations:
1. The hightop is lighter than the westy top by about 40lbs
2. The drive is much quieter with the hightop
3. Fuel consumption seemed to be about the same
4. I tended to drive faster with the top - the noise of the turbulence around the westy poptop seemed to mitigate the desire for speed
5. No real discernable differences in head or side winds - quartering winds from the rear could be felt.
IMHO the 2 factors that really impact air resistance - drag - that I have on the van are the truck mirrors that stick out on both sides and the 10' Fiama awning that really "dirties up" the passenger side, although, again, the effect was less noticeable with the hightop.
As I said, its anectotal - but those are my observations.
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randywebb
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Joined: February 15, 2005
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Location: Greater Metropolitan Nimrod, Orygun
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| Posted: Wed Jun 22, 2011 12:30 pm Post subject: |
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| IdahoDoug wrote: |
Someone mentioned adding a Dehler top reduces Vanagon drag by 10% "if that's how you read CD" (which is an auto industry term Coefficient of Drag). Coefficient of Drag is a ratio of the vehicle's frontal area, to its total drag if I recall correctly. So you can put a taller top on and reduce the CD. But you have much higher total fuel-sucking drag despite a CD "improvement". So don't use that as a measure of aerodynamic improvement that will reduce drag.
* * *
It is total drag, plus or minus any mass you've changed that will determine which way you move the fuel consumption needle when making body changes.
DougM |
Exactly right.
(tho the terms are not just for the auto industry, but are in general use in science & engineering)
Cd is a "figure of merit" that lets you compare "how good" different shapes are independent of size
| IdahoDoug wrote: |
I cannot accept that putting a heavy object atop a vehicle that makes the vehicle taller can cause the vehicle to move through the air more easily. No way.
DougM |
Way! it CAN happen - and it HAS happened. not real likely, but quite possible.
Aerodynamics is a very complicated field of study, and the basic equations that govern fluid flow have NEVER been completely solved (i.e. analytically)
People working in the field use semi-empirical methods - algebraic, differential equations, along with computer simulations (somewhat like you might have seen in modern chassis design - using a grid of points to form a mesh over the whole vehicle (or bird or fish)), along with measurements of physical models and even real full scale objects, if they have enuff $$
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silverbulletuk
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Joined: February 02, 2010
Posts: 206
Location: NW Surrey, UK
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| Posted: Wed Jun 22, 2011 5:37 am 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... |
Someone tell me if I've got this right?
Based on this ^^^ from page 1, Cd is a multiplier in the equation; so as that figure is reduced, so the overall drag (air resistance) trends towards zero.
Cd = 0 is impossible as that would imply either infinite frontal area or infinite velocity to be possible...or progress through an infinitely dense medium!
For a given amount of drag (read as specific fuel consumption at constant load and speed) a more efficient shape will allow a greater frontal area.
As I write this, I'm not sure if it proves that the Dehler is a more aerodynamic shape or not!
Feel free to correct me if I'm wrong, as I'm happy to be educated further!
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1990 SA Microbus - Supercharged wbx going in with UN-1 trans.
www.025motorsport.com |
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IdahoDoug
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Location: N. Idaho
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| Posted: Tue Jun 21, 2011 9:53 pm Post subject: |
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Someone mentioned adding a Dehler top reduces Vanagon drag by 10% "if that's how you read CD" (which is an auto industry term Coefficient of Drag). Coefficient of Drag is a ratio of the vehicle's frontal area, to its total drag if I recall correctly. So you can put a taller top on and reduce the CD. But you have much higher total fuel-sucking drag despite a CD "improvement". So don't use that as a measure of aerodynamic improvement that will reduce drag. I cannot accept that putting a heavy object atop a vehicle that makes the vehicle taller can cause the vehicle to move through the air more easily. No way. It is total drag, plus or minus any mass you've changed that will determine which way you move the fuel consumption needle when making body changes.
DougM
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randywebb
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Location: Greater Metropolitan Nimrod, Orygun
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| Posted: Tue Jun 21, 2011 12:32 pm Post subject: |
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| Classicvibe wrote: |
| randywebb wrote: |
| somebody should custom-make a short luggage bag to just fill that space -- it's seems like a good way to cut down on the noise |
I just read on another thread where someone took the time to re-vamp their luggage area while spraying the top (recent thread). It reminded me that when I spray this visco elastic sound deadening product, I plan to remove the luggage part as well, bring it back to life, and spray the back-side with the product. I think this will have a heavy hand in controlling resonance. |
stick a microphone under there (tape it down on some foam) and see what you get while on the hwy
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silverbulletuk
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Location: NW Surrey, UK
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| Posted: Tue Jun 21, 2011 1:46 am Post subject: |
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Apologies for reviving a year-old thread almost to the day, but it looks like it's still worthy of some attention...
My interest in the subject comes from my dormant "tin-pop" concept, that centres on a salvaged roof skin panel that's hanging from the garage rafters. I was/am going to make a full-length, fully elevating roof with slide-out sides (think original Viking Spacemaker meets roof tent) to sleep 3 upstairs. For when the kids get taller but won't leave home...
When I get the decks clear I'll get back to it!
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1986 syncro twin-slider rhd, 3.2S Oettinger wbx6
1990 SA Microbus - Supercharged wbx going in with UN-1 trans.
www.025motorsport.com |
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