Cars compared in wind tunnel

[NeilBlanchard]

Hello,

Originally Posted by lunarhighway

yes sure, but that's the catch, total aero drag is frontal area (in square feet or meter) multiplied Cd

this is gives you a CdA

a tiny car with a 0.32 Cd might have less overall drag than a prius, just because the prius has more frontal area...

frontal area determins how much air will have to be pushed around the car, drag quoeficient how smooth this will go

Right, I understand that. My comment was towards what some posts seem to be saying: that just because a vehicle is taller, means that it must necessarily therefore have a higher drag coefficient.

The OP does list an item saying that "...lowering by 20 mm reduces Cd by 0.01" which implies that this is a linear affect. I can't see how this could be a constant? All else being equal, lowering a vehicle only reduces the frontal area of the tires only.

Both the Boxfish and the Aptera are relatively tall, and they have much lower Cd than almost any other vehicle; so height is only one possible factor in the Cd.

[lunarhighway]

alright...that's true, there's no connection between the dimentions and the Cd of something, just like i think it's not a good idea to generalize about hight reduction and Cd reduction. a lot of these numbers that get bounced around on the web seen to be lifted from studies made on a particular car, thats sometimes not even mentioned. and while these figures may apply to a comparable car, they sure don't apply to all cars.

i think reducing height might work well for cars with a very messy underside... proximity to the road might eliminate turbulence to some extend... or creates certain pressure effects which are favorable.

if the bottom you a car is virtually as aerodynamic and smooth as the top, this won't crate to much additional turbulence and allowing more air under the car will produce less drag than forcing the same volume of air to travel over and around the car, wich is a longer path and thus would take more energy

[mwpiper]

Another way to define Cd is as a measure of the efficiency of the shape. Lower is more efficient. Cd is dimensionless. It is independent of size.

[Cd]

Does anyone know how to translate the web page ?
Also, where is the actual article located ? I see lots of pretty pictures, but no article.
I found it very interesting, and would love to read more.

Thank you very much for posting this !

[lunarhighway]

try Yahoo! Babel Fish - Text Translation and Web Page Translation

it seems to brake up the navigation sp perhaps you can keep the original article open in another window for that, and it'll give some "creative" translations, but it should give you an idea what they're talking about

[gary]

after a seriously good cup of coffee it ocurred to me that lowering a car can have an impact on cd via reducing the gap between tyre and fender alone. The othe benefits that occured to m where ithat it had the same effect as adding side skirts and a front air dam of the same amount it was lowered by. Make sense? I am a novice so I accept that I may be on the wrong track

[lunarhighway]

well in theory if no other forces are at work lowering a car does decrease the frontal area slightly

if you lower a car by 2.5cm about 1 inch

and your tires are 15cm wide

than you'd reduce the frontal area by

2*15cm*2.5cm=75 square cm

if the standard car has a frontal area of 2square meter and a Cd of 0.30

than it's Cda would be 2*0.3= 0.6
ofter lowering the fontal area is

2-0.0075=1,9925
the new Cda would be 1.9925*0.3=0,59775

this would have the same effect as lowering the Cd from 0.3 to 0,298875

that's roughly a 0,001 decrease in Cd

of course this is just an example and once again perhaps being closer to the ground has other benefits wich outweight the reduction of frontal area.

on some cars frontal area might be even further decreased by suspention linkages being more retracted.

[PA32R]

Originally Posted by Dradus

Yeah, I was surprised too, thats 220lbs per .01 of Cd. Thats a huge savings when you look at it like that, especially when you're building a lead acid EV conversion.

Is there any data out there showing different weight reduction equivalents?

Yeah, that's cool all right. That means if I have a 3000 pound car and perform aeromods on it to reduce the Cd from 0.42 to 0.28, I should be able to leave the ground like a blimp!

[kach22i]

Originally Posted by Bearleener

The German car magazine "Auto Bild" (2008-04-11 issue) tested the aerodynamics of cars in the Daimler wind tunnel.
www.autobild.de/mmg/mm_Bildergalerie_668619.html?tab=0&page=0
Drag factor = Cd*A, where Cd= drag coefficient, A= frontal area

All I see is the slide show.

Did they move the data?

[gone-ot]

...I would assume the "text" was from a article that has since been moved into archives.