Cars compared in wind tunnel
 

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

Conclusions:

• Already at 50 to 80 km/h (depending on the car), aerodynamic drag becomes the greatest loss factor
• It's much easier to reduce drag than weight-related losses: reducing the Cd from 0.29 to 0.28 is like eliminating 100 kg vehicle weight
• It's currently feasible to achieve a Cd of 0.22 in production cars, but with compromises in styling
• You can't tell how aerodynamic a car is just by looking
• Taller vehicles (e.g. vans) usually have higher consumption because of larger frontal area, but in the case of the VW Golf vs. Golf Plus vs. Touran this is partially compensated by a better drag coefficient.
• Top-of-the-line model (wider tires, more grill airflow) has greater consumption by up to 0.3 L/100km (see Mercedes E500 vs. E200)
• Longer cars often have better Cd, so that they have better aerodynamics than a small, short car despite larger A (see Peugeot 407 vs. 207)
• Important details are: A pillars, underbody covering, lowering by 20 mm reduces Cd by 0.01, long front/rear overhangs are advantageous, rearview mirrors, separation area at the rear (small is better), small wheel wells (little air dams in front of the wheels help prevent air from entering them)
• Convertible top down is really bad, folding hardtop is somewhat better than softtop (see Mazda MX-5)
• Crossover cars (4x4) are worse than their standard counterparts (see Audi A6 Allroad vs. A6 Avant)
• Flat sportscars are not always better (see Lamborghini Gallardo vs. Porsche 911 Carrera)
• Carrying bicycles & ski boxes on the roof or on the rear can wreak havoc on the aerodynamics; the amount of worsening is greater on sedans than on station wagons (see Mercedes C 200 and C 200 T)
• SUV aerodynamics suck (see BMW 530d Touring vs. X5 3.0d)
• Station wagons are worse than sedans because the turbulent region at the rear is larger; so-called "lifestyle wagons" are not quite as bad, but have less cargo space (see Mercedes C 200 T vs. C 200)
• Formula 1 racing cars have a terrible drag coefficient: 1.20
• Mean drag coefficient has decreased over the years (1900: 0.95; 1920: 0.75; 1940: 0.58; 1960: 0.49; 1980: 0.42; 2000: 0.30). Some of the best: 1921 Rumpler Tropfenwagen 0.28; 1957 Citroen DS 0.37; 1966 NSU Ro 80 0.36
• For current cars, the best Cd is 0.26 and the worst is 0.60 .

Here are the vehicle data: (Sorry, I don't know how to format the table here, so this is just a semicolon-delimited list. But you should be able to copy and paste this into a text editor, save it as a text file, and then open the text file with Excel, specifying ";" as the delimiter character. There must be a better way...)

Vehicle;Cd;A (m^2);Cd*A (m^2);increased consumption @ 120 km/h (L/100km);increased consumption @ 150 km/h (L/100km)
Taller vehicle:;;;;;
VW Golf;0.33;2.21;0.73;;
VW Golf Plus;0.32;2.38;0.76;0.1;0.2
VW Touran;0.31;2.55;0.79;0.2;0.3
Top-of -the-line:;;;;;
Mercedes E 200;0.27;2.22;0.60;;
Mercedes E 500;0.28;2.25;0.63;0.2;0.3
Small vs. midsized car:;;;;;
Peugeot 207;0.31;2.13;0.66;;
Peugeot 407;0.29;2.24;0.65;-0.1;
Convertible:;;;;;
Mazda MX-5 CC (folding hardtop);0.37;1.78;0.66;;
Mazda MX-5 Roadster (softtop);0.39;1.82;0.71;0.2;0.3
Mazda MX-5 Roadster (top down);0.45;1.80;0.81;0.7;1.4
Crossover:;;;;;
Audi A6 Avant;0.31;2.26;0.70;;
Audi A6 Allroad (normal setting 0.33, offroad 0.34);0.32;2.38;0.76;0.2;0.3
Sportscar:;;;;;
Porsche 911 Carrera;0.27;2.00;0.54;;
Lamborghini Gallardo (15 cm flatter than Porsche);0.33;1.91;0.63;;0.6
Bicycles & ski boxes (Mercedes C 200 & C 200 T):;;;;;
Sedan;0.27;2.22;0.60;;
Sedan, ski box on top;0.33;2.36;0.78;0.5;0.8
Sedan, bicycle on top;0.36;2.42;0.87;0.7;1.2
Sedan, bicycle on rear;0.37;2.22;0.82;0.8;1.4
Wagon;0.30;2.20;0.66;;
Wagon, ski box on top;0.32;2.41;0.77;0.2;0.3
Wagon, bicycle on top;0.37;2.43;0.90;0.6;1.0
Wagon, bicycle on rear;0.32;2.25;0.72;0.2;0.3
Wagon, ski box & bicycle on top + bicycle on rear;0.52;2.17;1.13;2.0;3,0
SUV:;;;;;
BMW 530d Touring (wagon);0.29;2.28;0.66;;
BMW X5 3.0d (SUV);0.33;2.88;0.95;;2.0
Station wagon:;;;;;
Mercedes C 200 (sedan);0.27;2.22;0.60;;
Mercedes C 200 T (wagon);0.30;2.20;0.66;;0.5
Ten best drag coefficient:;;;;;
BMW 5 series;0.26;;;;
Mercedes S class;0.26;;;;
Lexus LS 460;0.26;
Toyota Prius;0.26;
Audi A4;0.27;
BMW 3 series;0.27;
Mercedes E class;0.27;
Mercedes C class;0.27;
Mercedes CL;0.27;
Porsche 911;0.27;
Ten worst drag coefficient:;;
Formula 1 race car;1.20;
Mercedes Actros (aerodynamic cab-over-engine truck);0.81;
Morgan Roadster;0.60;
Land Rover Defender;0.59;
Hummer H2;0.53;
Mercedes G model;0.52;
Morgan Aero 8;0.51;
Jeep Wrangler;0.49;
Jeep Commander;0.41;
Chrysler PT Cruiser;0.38;

Nice find!

Very sweet stuff, thanks for the post, :)

Danke!

Excellent data and info. Interesting to note that aero improvements are significantly more effective than weight reduction.

RH77

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?

Great find!

Morgan Aero 8 @ 0.51!

So a bike on the back (compared to on top) is worse in a sedan but better in a wagon. Weird.

Yeah y'all can put your spares back in the trunk now.

Cool - thanks for posting :D

Cars compared in wind tunnel
 

It didn't say in the article, but I'm guessing they meant the rolling resistance at constant speed, which is proportional to the weight, and not accelerating and braking, where weight has a greater effect on fuel economy.

Back-of-the-envelope calculation (similar to Mercedes E200 @ 100 km/h):

Aerodynamic drag Fa = (1/2)*Cd*A*rho*v^2
= (1/2)*0.29*2.22*1.21*(100/3.6)^2 = 301 N
Reducing Cd to 0.28 would reduce Fa by 10.4 N

Rolling resistance Frr = Crr*W = Crr*m*g
= 0.01*1600*9.81 = 157 N
Reducing weight by 100 kg would reduce Frr by 9.8 N

Yup, that fits! So this weight equivalent applies only to constant-speed cruising. In the city weight matters much more.

Why?

Meh - if your driving without it to save weight..... keep it out..... Just because one thing overtakes another is no reason to say one force is insignificant (I mean, people buy tires with low rolling resistance :D). Yes, weight reduction only gets you so far... But, when half the pool is closed - do you swim in the other half or bake in the sun? :D

Exactly. Besides Its not like I'll be stranded when I get a flat.
http://www.billyandkelli.com/weblog/...lat-lineup.gif
Full size spare + jack and tools adds up to 55lb in a 2600lb car.

AAA :D I only need a card - less than a half ounce, maybe? :D
http://tbn0.google.com/images?q=tbn:...images/AAA.jpg

The fuel you save is going to be burned and more when you call a tow truck.

And no goo will help a total blowout. I've had two recently, one from a deep pothole at 45, almost went into a canal, and the other backing into a parking space, I hit the curb which had a old busted iron plate that went into the side like a knife. Both times I was back in business quicker than a tow truck would have arrived.

Weight is mostly a factor in acceleration, and more is better for long glides, but worse in pulses. If you drive over 40, weight (rolling resistance) is pretty negligible.

To each his own I suppose. I've never had a blow out or called a tow truck. Fix a flat got me home safely twice now.

2003 Jetta wagon vs sedan
 

I was researching the 2009 Jetta Sportwagon TDI and found it gets worse mileage than my 2003. Then I checked EPA's web page and found that the new EPA MPGs show the 2003 TDI wagon gets better mileage than the 2003 TDI sedan! In this unusual case there must be other factors in addition to Cd?
:confused:

newer cars tend to be much taller than older ones, this allows for a more upright seating position, wich eliminates the need for more legroom and thus allows for more usefull cargo space or a relative shorter vehicle although whitin the same model family sizes tend to increase.

this has the disadvantage that while drag coeficients might have slightly improved (although this is not always the case)
frontal area has increased quite a lot withing the same model range over the years giveing more overall drag.

also weight increased because of more and more safety and comfort addons some of wich take a lot of energy wich has to be supplied by the engine, creating aditional electric and mechanical load ,direct and indirect cooling requirements.

this would explain why newer models have worse FE than older ones, but not why a wagon could have better FE than a sedan.
even if they have a comparable if not a more refined engine.

it's unlikely but not impossible that it has a slightly better drag quoeficient.

perhaps certain standard features of the sedan are an option on the wagon, tire size, climate controle, airbag configuration, etc etc, might differ between the base models, since the wagon is more practical oriented, some customers might perfer a cheaper more stripped version. this might make a difference in weight and electric load and in case of the tires frontal area and Rolling resistance... this could produce the FE difference for both "standard configurations"... it's not entirely unimaginable that carmakers produce a stripped out version as stock wich would get better FE figures on paper, and than offer cheap or even free upgrade packs wich give the cusommer the impression they're getting a bargain, wihile their actual FE goes down.

also it is possible engine management en the gearbox ratio is different, since the wagon would be expected to haul more weight than the sedan, wich might be configgered more for a more sporty driveing experience.

that's about all rational explanations i can think of

Hello,

The Mercedes Bionic car (aka the Boxfish) is quite tall (it is similar to the Honda Element's dimensions); and yet it achieves a Cd of 0.19.

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

The comments that wagons have higher drag because of the larger wake area assumes that in the equivalent sedan the airflow separates at the rear of the trunk lid. We all know that this isn't necessarily the case and lots of sedans can have flow separate at the rear of the roof, giving you a wake area equally as large as a wagon plus a ton of rear lift. The airflow on a wagon more than likely stays attached all the way to the rear of the roof, negating some rear lift but still giving you the large wake. Since the wake vortices alternate (ever feel that side-to-side buffeting in the draft of a tractor trailer?) a wagon has one pair of vortices alternating behind the hatch, where a sedan could have a pair of vortices behind the backlight and another pair behind the trunk that interfere with one another and increase the pressure drag.

Pure speculation of course, but it just goes to show that the article goes into gross generalizations and the 2003 Jettas appear to be an exception to the generalization.

I agree with MechEngVT, who I think also just defined the "Kamm" effect.

Hello,

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.

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

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.

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 !

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

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

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.

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!

All I see is the slide show.

Did they move the data?

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

20mm
 

Something that comes to mind when thinking of 'dropping' a car,is the affect on fineness ratio,which Hucho very much associates with low drag potential.
Lowering,essentially 'lengthens' the car and the air likes that.
The new Leaf,and it's sister racer,Nismo Leaf both have "template" rooflines,however the Nismo is remarkably lower,giving it a fineness ratio closer to a high-mpg concept car.Nissan seems to be exploiting the benefit for performance.
This might be a safe general assumption.Many of the highest mpg cars use active suspension to lower the car at speed for even better savings than at static height.