Car & Driver Aerodynamics Test
 

This is great! Mainstream car magazines are picking up on what we already know: aerodynamic drag is critical to car efficiency.

http://www.teslamotors.com/sites/def...n-the-road.pdf

Aerodynamic Tesla Model S Electric Car Wins The Wind-Tunnel Wars

Wind tunnel contests! :D

Bring 'em on!

More proof we shouldn't get too hung up on small published Cd variations; who knows if they're accurate anyway?

How is the Insight worse than the Prius?

I'm more inclined to think the "differences" could be the result of 'static' (no tires rotation) versus 'dynamic' (dynamometer) wind tunnel testing techiques. Spinning wheels create eddy currents which create a LOT of drag!

The guys at Green Car Reports should read Road & Track once in a while--they were the only publication I know of to accurately report Mercedes' claimed drag coefficient for the CLA250 (the only version available in the US, in addition to the AMG) of .28, not .23 (straight from the horse's mouth here).

.30 for the 2001 insight? uh think again people.

That CLA number looked bogus from the beginning. Nice to see a test of it. .30 instead of .23! I wonder how much could be innocent variation between testing facilities? Any of it?

What's with the .30 Cd on the Gen 1 Insight?

They name the cars but they decline to name the tunnel? What are the odds it's A2?

The car that costs 3x as much has better aero? I am shocked.

The *information* that I thought MOST USEFUL was their listing of aero power at 70 & 100 MPH:

14 HP @ 70 MPH; 42 HP @ 100 MPH - Tesla S P85
14 HP @ 70 MPH; 42 HP @ 100 MPH - Toyota Prius (GenIII)
16 HP @ 70 MPH; 45 HP @ 100 MPH - Chevrolet Volt
16 HP @ 70 MPH; 48 HP @ 100 MPH - Mercedes-Benz CLA-250
18 HP @ 70 MPH; 53 HP @ 100 MPH - Nissan Leaf SL

...that's two KNOWN points on their respective aero curves!

I don't know about that Cd of the Insight, if they got one off the road, I am guessing it is missing chunks. Most G1s are missing underbodies, wheel skirts, every one I have seen has destroyed tire spats/wheel wells.

The thing that still keeps it ahead is the FA. Even a better CD Prius will have a worse CDa with how giant (in comparison) it is. Same concept of the Elio, as it has poor CD but a tiny FA.

the quoted 0.23 is for the special bluemotion edition in europe with extra features such a narrow LRR tires, grill shutters, etc.

http://www.teslamotors.com/sites/def...ind-tunnel.jpg

This isn't the same test is it ? ( Note the color of the car and the wheels )

So why not email the guys at the A2 and ask about what condition the Insight G1 was in ? Or as i have mentioned in the past, ask if they would join us for a VIP Q&A session ?

( I think you forum moderators would have more clout than I would .)

117 Godspeed Ln, Mooresville, NC 28115

(704) 799-1001

A2 Wind Tunnel

variations
 

I'm not current on this sort of thing,but in Hucho's first English translation book of the mid-80s he reported that a VW 1600 was taken to all the pertinent wind tunnels and measured.There was no agreement between any of the tunnels,but their was a standard deviation of around 2.5% or so,and many tunnels' Cds were within this standard deviation.
The GM,Lockheed Marietta,Pininfarina,and one other tunnel in Europe had the 'best' results.
There exists a standard wind tunnel calibration model which all tunnels are supposed to use.
I suppose that its a GOOD thing that automakers think Cds are important enough to 'fudge.'

2.5% is a pretty tight deviation considering the published and measured values being stated here.

spinning wheels
 

One SAE Paper from General Motors in the 1960s reported that the difference in Cd between spinning and stationary wheels was of such a low statistical significance that it didn't warrant the time and expense to test; and spinning tire effects could be easily simulated with trip strips in the tire/wheel area.
Exposed-wheel race cars MUST be tested with spinning wheels.

Road Horsepower
 

Their was a time when Csaba Csere (MIT graduate engineer) was the Technical Director at CAR and DRIVER and he made sure that at least some of the road tests included Road Horsepower figures based upon coastdown testing at the Chrysler Proving Grounds,just a stones toss from Hogback Road where C&D had their offices.
I was so impressed by this capability that I arranged to have the CRX fee-tested by them in 1991.
I'm delighted to see the article,and that Don Sherman is still beating the aerodynamic drum.It was his "CRISIS FIGHTER PINTO" article in May,1974 which got me into streamlining.:D

same?
 

The curved test section walls and overhead lighting are identical to those in the C&D photos.A2 is a good guess!

tight
 

If we could duplicate the test at different tunnels we'd have our answer.

I call phooey on the whole thing! Maybe i'll go pick up some 19inch turbine wheels and call it a day.:confused::mad::p

Back to la la land i go---->:turtle::)

Correct; Mercedes claims .28 for the CLA250, and .22 for the CLA180 BlueEfficiency. Just goes to show how much Cd can be influenced by the details.

G1 Insight drivers pretty upset at Car & Driver! :eek:

That's interesting, of course. The article speculates on the same theme. But .28 is still lower than the C&D tunnel offers (.30). So, I'm still wondering if the testing method makes its own noise. Did Mercedes test without the wheels spinning? That kinda thing.

;)

I wonder how Cd is actually calculated in a wind tunnel? Is it derived from total drag force and calculated frontal area? If so, that leaves a lot of room for fudging--er, estimating--numbers.

Fore/aft force gauges under each wheel measure drag resistance.

Vertical force gauges under each wheel measure LIFT & DOWN forces.

Nissan would beg to differ. They test with wheels spining and the engine running to maximise under hood airflow management too.

What was considered adequate by GM in the 60's isn't nessesarily best practise today. It's unlikely we'd have fairly conventional looking 0.22CD sedans with that attitude.

M-B wheels spinning?
 

Mercedes-Benz uses the FKFS wind tunnel that Kamm built in the late 1930s.It does not have a moving floor,or provision to roll the wheels unless it's been recently modified.

how calculated
 

From: Drag Force = ( 1/2 X rho X Cd X A X V-squared ),
they measure the actual axial drag force,then using actual station pressure,calculate the air density at test time,then using the projected frontal area,and actual test section air velocity, squared,the coefficient falls out of the equation.
To complicate matters,the EPA does not recognize the frontal area of the side-view mirror(s).
The drag reflects the actual frontal area,including the mirrors.
If the drag is mandated to reflect frontal area sans mirror(s),then the coefficient of aerodynamic drag will be arbitrarily high.
A marketing statistician could 'cook' the numbers.
It's regulated ambiguity.:p

differ
 

I'm not certain what Nissan's tunnel specifications are,and how their quanta compare to other facilities.
General Motors research was conducted at Lockheed's Marietta,Georgia facility.It is considered a world-class wind tunnel,good enough for the Skunk Works and all major NASCAR teams.
GM has produced Cd 0.089 vehicles.I'm not sure what Nissan's claim to fame is.
If you have some technical information from NISSAN it would be most appreciated.Hopefully it is not from the same tunnel that their 280-ZX was developed in.

The GT-R has a 0.26CD which is great for what it is.

The aero section starts about 10mins in:

https://www.youtube.com/watch?v=52Yyo5s9HgQ

And of course Mercedes have a rolling road wind tunnel, nothing 1930's about it:

https://www.youtube.com/watch?v=AH-vZrpBxus

The Mercedes video shows rollers under the wheels, which is part of it - they get the wheels spinning. But having the whole "road" surface rolling past the car sets up things under the car that cannot be accounted for otherwise.

Actually there is a 5 belt system to simulate the interaction with the ground plane.

The whole thing can be pivoted for cross wind testing too.

...it must be nice...

The video of the Merc didn't get to show the really important stuff. Who cares about what's happening 2ft above you... what about closer to the ground???

I find that .26 very hard to believe. The down force of the GTR is IMMENSE. Formula 1 cars (per wiki) range from .7 to over 1, how can it have such an incredible Cd, yet be held down so powerfully?

This article is exactly what I mean: 2009 Nissan GT-R - First Drive Review - Car Reviews - Car and Driver

"Nissan claims the GT-R generates more than 176 pounds of downforce at 186 mph. At both ends. According to wind-tunnel tests by the German magazine Sport Auto, few production cars are better, and those few—the Bugatti Veyron, the Mercedes SLR McLaren, and the Porsche Carrera GT—cost a lot more than the $70,475 base price the GT-R will command when it reaches the U.S. this spring."

So which is it, Datsun? ( I do love the GTR, but these two claims together don't pan out. Is there something more to this?)

Let's stick to road cars, not sure what F1 has to do with anything.

A McLaren P1 produces 1323 pounds of downforce at 161mph. Is the GT-R's 186 at 186mph still immense? If you build a car with low drag, it won't have much lift, add a good under tray and diffuser and you should have modest downforce with no drag penalty (compared to an unclad underside at least).

ANALOGY - imagine this airplane WING airfoil as a car (lefthand pix):


http://forums.x-plane.org/uploads/mo...1357577231.jpg

...or, think about this being UPSIDE down, and think what the inverse of LIFT-INDUCED drag would be (DOWN-FORCE drag!):

http://2.bp.blogspot.com/_2JggT2EnsU...stration+2.bmp

Sure, let's stick to road cars. So you know, the purpose of bringing up the F1 car was the correlation between downforce and drag. So the drag of a stationary F1 car may not be much, but when air is forcing it down (part of the GTRs superb handling, stability, and track performance) it increases it's grip (downforce). That's the point. More force pushing down takes more force to accelerate or maintain speed.

About road cars, can we stick with reasonable road cars and not multimillion dollar hyper car hybrids with fluctuating aerodynamics that change depending on speeds and conditions? Especially since at certain speeds, the downforce lets up because it will destroy the rear suspension. Or it's used as a rear brake. Either way, factors the Insight and GTR do not deal with.

Unfortunately, I can't find anything on the downforce of a G1 Insight.

Oh, and comparing to somewhat ordinary cars: Corvette Wind-Tunnel Testing - Is Corvette Styling Just For Looks? - Corvette Fever Magazine - View All Page

Only the C3 was able to actually create downforce. The rest all created lift. Now the Corvette is no GTR (although Car and Driver rates the Vette #1, and GTR #5) it also isn't a Focus. I compared the 2009 model GTR to a 2009 model Corvette, because after the "Black" (or is it a V Spec? I forget what they name them) GTR came out, it started getting more and more downforce. Then the regular GTR essentially was as good as the previous year "Black."

The reason Corvettes create lift is that in profile, they're basically an aerofoil.

Squared off cars don't have this problem.

Honda Quoted the 1981 City as having zero lift (written just under the blue car):

http://ecomodder.com/forum/attachmen...1&d=1401704457

If you start with zero lift, you won't have to induce much drag to create the modest downforce that the GT-R has. That's part of it's genius, staying away from swoopy high lift curves.