aerodynamics

[DavidPallister]

Shape has far more influence over drag than actual cross sectional area. A teardrop shaped element will have far less drag than a square element of half the cross sectional area, purely because of its aerodynamic shape.

A hightop with a smooth, gradually changing shape will have less drag than a pop top with an abrupt, sharp leading edge (as most have), as this sharp edge causes the airflow to separate off the surface, creating turbulence and a big pressure differential, resulting in lots of drag


Dave

[nevill3]

Does any body have an explanation of the different numbers in the image posted earlier, or even a translation of some of the text?

[kevtherev]

It mainly discusses the drag coefficient values of each shape

[Mocki]

Proves what most of us have been saying for years, pop tops catch more wind ...... And that without the wind that gets between the pop top and the van roof .

[Allanw]

Like oil viscosity, drag co-efficicents are something that is quite misunderstood.

Those images do not necesarily prove that the high top has less drag - the drag co-efficient is based on the frontal area (extreme measurements as viewed from the front), and the cd figure is the drag as a percentage of a brick with the same outer dimensions.

The high top is a much larger brick, so could still potentially have more drag. (Though I think it actually doesn't in this case )

SO there is actually the potential to have a car that is 3 times the size, with the same drag co-efficient - it's not a direct comparison between differing body styles. This is shown by the bottom image, which shows a cd figure only slightly worse than the standard van in the top image. The frontal area is considerably higher, but the cd figures are similar - the drag is still a LOT higher, it's just that it is a relatively efficient bigger size

I don't know why cd isn't an absolute measurement, which would allow for comparison. The relative figure used is only really helpful when looking at different specs of the exact same body style

[boxer]

The poptop turbulence is probably due to the roofrack thing at the front .

[silverbullet]

I shouldn't even be logged in but you gotta sit down for a cuppa sometime

There was a good article in the SAE Auto design mag last month. Lots of design work being done on this subject:
Flat floors and air dams/splitters
Ducting cooling pack (radiator to you and me) flow into front arches to fill low pressure area and reduce drag there
Ducting rear arch into LP zone behind vehicle
Door mirror drag reduction (why don't new cars have side rear view cameras and screens instead of mirrors now?), Smooth/passive/active wheel trims, the list goes on.

All of which have to be balanced with cooling system efficiency for both engine, brakes, cabin ventilation etc etc

I know of at least two highly-modded T3 buses fitted with undertrays and owners report very good results for fuel economy and wind noise.

[ghost123uk]

Did you know that despite the "look" of it, a T25 van has about the same drag coefficient as an E Type Jag

[keith]

Proves what most of us have been saying for years, pop tops catch more wind ...... And that without the wind that gets between the pop top and the van roof .

i dont think it says that.

it takes the drag over the surface area to provide a co-efficient.

a big surface area will catch more wind....which is why the comment about the etype doesnt really stack up to how most people would view it.

the etype has a small surface area....so its drag co-efficient might not appear great.....BUT it catches less wind than a t25

[Mocki]

fact remains pop tops catch more wind........ try gaffer taping the joins over so the draught dont get in and see how much quieter it is for a start......if you can hear it for the tractor engine rattling!! lol!

[AdrianC]

Now I know a t25 is about as aerodynamic as a brick but I flew in a beeckcraft twin engined thing today and noticed that as it taxied faster so the damping of bumps in the runway go much better as the aerodynamics reduced the apparent weight of the plane.

Aeroplanes are generally designed to develop as much aerodynamic lift as possible. It's considered "fairly useful" in stopping them falling out of the sky. This doesn't apply to vans in quite the same way.

Those images do not necesarily prove that the high top has less drag - the drag co-efficient is based on the frontal area (extreme measurements as viewed from the front), and the cd figure is the drag as a percentage of a brick with the same outer dimensions.

The high top is a much larger brick, so could still potentially have more drag. (Though I think it actually doesn't in this case )

There's a similar page from a German mag somewhere, which includes the Westfalia hightop.

The Westy poptop has a Cd of 0.51 and a frontal area of 3.17m2, giving a CdA of 1.62. The Westy hightop has a Cd of 0.42 and a frontal area of 3.8m2, giving a CdA of 1.60. Both of them had the little chin spoiler fitted.

Ah, here we go...
http://www.vwpix.org/berichte/deutschla ... Seite5.JPG" onclick="window.open(this.href);return false;
(Sorry, too big to embed, so click to view.)

[boxer]

So the pop top is noiser similar drag 'ratio' and better mpg? So what do you think wouldbe the effect of filling the roof rack so that the roof was smoother. I don't know a lot about aeodynamics but I'm pretty sure that air pupping up over the roof edge would be less turbulent if it didn't have a lp trough under it. Think i'll make ply cover for it to see what it does to the noise levels at least.

[boxer]

http://www.autospeed.com/cms/article.ht ... g&A=110351" onclick="window.open(this.href);return false;

Interesting article.

Seems that our biggest problem is the x sectional area of the busses a***! regarding drag.

A spitter will still help keep some weight on the front wheels though.

[AdrianC]

A spitter will still help keep some weight on the front wheels though.

I'm unconvinced, and that's having driven the same (standard height/wheel/tyre) van with and without. Now, OK, the reason it's now without is that it was shagged at the corners - the filler fell out, so it was just gaffer tape. But, also, the reason it was shagged was that it grounded REGULARLY. I don't miss it one bit.

[Titus A Duxass]

[quote="boxer"]

Seems that our biggest problem is the x sectional area of the busses a***! ...[/quote

Correct, how the air leaves a body is more critical than how the air meets the body.
that's why streamliners (Bonneville) are teardrop shaped.
There is a formula, IIRC the tail length should be 2 or 3 times the diameter of the nose.

So we want a T3 without front shaped like a trout's head with a 6 meter long tapering tailcone and semiconformal wheels - now where's me isopon....