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Aerodynamics

I was hoping that the Model S would have "top-notch aerodynamics" (cited from ) but then there is this rumored number of Cd=.28. The upcoming Mercedes B(!) class has .26, the E class coupé has .24 (reportedly the best Cd of any mass production passenger car). That does not make the Model S' .28 stand out, at least not in a positive way. The difference between .28 and .24 is huge. Does anyone have the source where the .28 comes from? Or are there any other, newer, better numbers?

Sorry, do not enclose your links in </> otherwise they do not show... I meant: Cited from http://www.teslamotors.com/models/specs

Prius: .25
Volt: .27
Aptera 2e: .15
Hummer H2: .57
Source: http://green.autoblog.com/2009/03/26/greenlings-what-is-cd-and-why-does-...

Let's see now, for some reason there are concerns about coeff
of drag. And a CD that is simply "rumored" at that. Big question is why? And why no worry about rolling resistence? Or drivetrain frictional losses? The issue of concern (assuming one is concerned about such things) is miles per kilowatthour, not CD nor rolling resistance, nor weight. Notice that the Chevy Volt is a cramped, squashed down vehicle. You really want that? The original Volt had a very distinctive and attractive styling, which had "bad" aeros and got tossed. GM should have tossed their range goals instead. They ruined what would have been something unusual for GM : a desireable car. What they got was the Volt. Yuk!

Prius: .25
Volt: .27
Aptera 2e: .15
Hummer H2: .57
Frisbee: .01

Cessna 310 : 0.027
Eiffel tower : 1.9
Round pebble : 0.47
Golf ball : 0.3
Borg cube : 1.5

Tesla Roadster: 0.35
Porsche 918: 0.39 (what?!)
Loremo (Concept): 0.20
typical values for a Formula One car: 0.70-1.10 (wow!)
http://en.wikipedia.org/wiki/Automobile_drag_coefficient

F1 cars are different beasts. They have so much power that grip becomes the most important thing. They could run on the ceiling at a reasonable speed !

With settings for circuits that require high downforce that could be done as low speed as 50mph. F1 cars do not weight much.

Range with my Roadster takes a big hit at Interstate speed due to the poor aerodynamics...has anyone seen a range vs speed curve for the sedan?

@Thomas, there is no data on that. Not even educated guesses because nobody outside of Tesla actually knows the CdA or rolling resistance of the Model S. All they say that it is very aerodynamic, and I believe that. However it is also a lot bigger car than Roadster (A in that CdA) it might have worse result than Roadster for aerodynamic drag, and because it weights a lot more it will also have higher rolling resistance.

Thomas,

I assume nobody outside Tesla has seen that curve. Average energy consumption in the Roadster is specified with 250Wh to the mile, whereas Tesla states in the FAQ 300Wh/mile for the Model S. Aerodynamic drag should be comparable to the roadster (bigger area cancelled out by better drag coefficient). Thus, the 20% increase should stem mostly from increased rolling resistance (much higher vehicle weight) which is independent of velocity.
As a result, I assume that the peak in the range vs. speed plot in the blog gets shifted a little bit towards higher velocity, perhaps from 17mph to 22mph.
Again, that's all speculation. We will see.

The upcoming 3 series will have the same Cd as the upcoming B class: .26

Any news from reliable sources what the Cd of the Model S will eventually be?

To be fair though, shouldn't the comparison be more of an apples to apples? This is a relatively large four door sedan (about the length of a 5 series, and wider than any sedan I can think of). So any CD comparison should be made to cars of that size, not to the prius, or the 3 series IMHO.

@mscottring, that's precisely what the Cd value is for. The aerodynamic resistance is Cd x Area x (Speed^2). The Cd value is independent of the size of the object by definition, it merely describes how well the air can flow around it. The area is going to be much larger for the Model S than for Prius or 3 series (or Roadster), which is why the Cd is particularly important to keep aerodynamic resistance in check at higher speeds.

It looks like the 2011 5 series comes in at .29 and the 2011 E class sedan (based on E350) comes in at .25

In my mind that 0.25 is the number to beat. I could be wrong but I think that's the lowest CD in that class.

Actually, maybe I should back up. What is the actual length of the Model S, based on the Beta?

Volker - I understand, and agree with your point. The CD on the Model S should be much lower, in my opinion. I'm just trying to compare the CD of the model s to other vehicles of it's same (rough) size and configuration (4 doors). Based upon that is the model s the leader in it's class? That's the claim TM makes.

@mscottring, according to Mercedes, the E class coupé has .24 which reportedly is the best Cd of any mass production passenger car.

Model S has an approximate overall length of 16’4" (4973 mm), overall height of 4’8" (1426 mm), and overall width (with mirrors) of 7’2" (2189 mm). Model S has an unladen ground clearance of approximately 6" (154mm). Additional dimensions will be released this winter.
http://www.teslamotors.com/models/facts

@mscottring, I see your point now. Yes we should actually compare to 4-door sedans of a certain size, even though the Cd as such is size-independent. With this in mind, I agree that probably the E class sedan sets the bar.

Volker - What's interesting here (at least to me) is that the E class doesn't "look" as aerodynamic as it apparently is. By contrast, the Model S has a relatively aerodynamic appearance (and a flat underside), but still comes in with a higher number.

Whatever the Cd of the Model S ends up being, it has to be better than the Smart Car that looks like a rolling telephone boot. Some Googling finds the Smart at 0.345 or thereabouts.

That said, if it meets or exceeds 0.25, that would be a feather in the cap for Tesla!

Elon Musk said the Model S was 0.225.

He was happy of the fact it was less than the Prius at 0.25.

The guys working on aerodynamics at Tesla came from Formula 1.

I thought that I saw 86 kWh figure for battery somewhere and with the range of 320 it makes 269 Wh/mile. Basically what TM is saying is that wheels will affect 20 miles for the range.
Agreed that Cd should beat MB E, official MB pages says that Cd is between 0,25-0,29 depending on the model. S-model is over 10 cm or 4 inches wider than E Merc.

WhiteKnight
"Elon Musk said the Model S was 0.225."
If this is correct, that´s great. Should be around those figures.

Alpha was quoted as measured at .25 by the designer(Franz). Would expect production to be similar plus or minus a bit....

You would think that with the nose redesign and other refinements, that the beta would easily be better than the alpha.

Elon Musk said the Model S was 0.225. (WhiteKnight)

Incredible. If that's true then that's a historic achievement. Do you remember where you heard it? Any information on the source? To me, it sounds too good to be true (although I'd love to believe it).

Enlighten me. I was under the impression aerodynamics plays a modest role in speeds under 90 mph.

For highway driving conditions, it is estimated that driveline uses about 15% of the total energy to required to push your vehicle down the highway, tire rolling resistance represents about 25%, and air drag is about 60%! While the traditional sources advocate saving fuel by driving less or driving slower, there are greater gains that can be made by modifying the aerodynamics, engine, and rolling resistance of the vehicle.

http://www.recumbents.com/car_aerodynamics/

Petero - The two links above help explain a bit. Aerodynamics come into play above 90 mph when we're talking about down force or up lift. Drag comes into play at a much lower speed.


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