GM: Chevy Volt's Cd is 0.28, Prius is 0.30 (tested in multiple tunnels), EV1 is 0.21

[JonnyG]

Originally Posted by MetroMPG

Interesting! I wonder what they did to the car to compensate for the increased CdA of the wider tires. (195 mm base vs 215 mm with the 17 inch wheels).

http://ecomodder.com/forum/showthrea...g-cd-7475.html

I'm not giving Toyota a free pass on marketing fudge either!

Yeah, as much as I love Toyota, something seems wrong there. I think they probably didn't test each version of the car. Instead, they probably thought that since the windshield was the same and the body was the same, the Cd must be the same. Hard to believe their engineers would allow that, but that's all I can come up with.

Regardless, the .30 that GM measured is probably wrong because that's way too much of a difference for just wide tires.

[MetroMPG]

GM is saying that the Cd depends on the wind tunnel (which is apparently true from, what I've read).

The point they're trying to make is that regardless of the absolute figure, their car is relatively better than the Prius*

(* with 17 inch wheels. Gotta love asterixes!)

[NeilBlanchard]

Hi,

They retested an EV1? The Cd number I have always heard is 0.19. it sounds like GM is also trying to downplay how good the EV1 was/is.

They should just start producing the EV1 again, with lithium batteries this time! And they had a 4 seat version of the EV1 ready to go, I understand.

[orange4boy]

GM just cant wean themselves from the BS.

According to GM’s aerodynamics engineer Nina Tortosa, the old method of testing yielded .19, but with current testing methodology it would equate to .21.

So they didn't actually test it again, just assigned it a number to make the volt look better.

GM's current testing methodology: Utter garbage.

Put the widest, non stock rims and tires you can find on the competition but leave yours stock. What kind of methodology is that?!!1!. Did they also leave the windows open on the Prius? FERCHRISSAKE!

GM is still full of CRAP!

Wheels account for around 30% of the drag in vehicle design so this "methodology" could easily have produced the few points they needed to come up with their preferred result.

[Cd]

I wish they would have explained more on their 'updated' test methodology.

Probably the first 'test' that anyone here on this site will do of the Volt will be an A-B-A coast down test to see how those Cd numbers match up.

[tasdrouille]

Lets not forget the Cd is only half the equation.

The Volt is supposedly 70.8 by 56.3 inches while the Prius is 68.7 by 58.7 inches. But, the Prius does not have the airdam we've seen on the most recent Volt pictures. So overall frontal area might be on the Prius side.

I am assuming the way they measured the Cd is different than the way it's normally done. Maybe they tested in a rolling wind tunnel, that can make a significant difference. But they never said anything so it's not worth anything anyway. The only thing I'll keep from that is that GM says the Volt has a 0.28 Cd.

The new airdam probably helps the Volt do better with 17 inches wheel than the Prius, since it's keeping a lot of air from going underneath and interacting with the draggy wheels.



Oh and btw am I the only one thinking the "aero neutral mirrors" claim is BS?

[cfg83]

Hello -

Sounds like a job for an independent wind tunnel facility.

The top-of-the-line Prius V does come with P215/45R17 tires standard. The Toyota website rates all models of Prius at 0.25 Cd, but that doesn't mean they tested each model separately. It's still a cheap shot because it weakens their argument. I'll bet they *did* do the 15" test, came out closer to even, slapped on the 17s, and found their loophole. If he wanted to be more sneaky, he could have said a "Prius V" was used in the test, and left it up to us to figure out the tires were 17s.

I thought the Cd for cars was calculated for all cars, not actually tested in an "official EPA" wind tunnel.

CarloSW2

[gone-ot]

...regarding the statement: “The Prius came in at .30,” said Boniface. “That was a number that was verified in our tunnel, in Chrysler’s tunnel, and in Ford’s tunnel.”

...it's simple, ask Chrysler and Ford what numbers "they" found for the Prius!

[Sorrow Six Star]

Originally Posted by tasdrouille

Oh and btw am I the only one thinking the "aero neutral mirrors" claim is BS?

the "stems" on the mirror seperate them from the body making their impact less adverse to the overall vehicle

the same thing Subaru did in the 80s.



they would be much better if they were smaller... is there a legal guideline on the size of the mirrors

[tasdrouille]

Well, those stems cause interference drag between the body and the stems and between the stems and mirrors. Just like interference drag is created between struts and wings of a biplane.

Also lets not forget drag equals Cd times frontal area. So in order for the mirrors to be "aero neutral" they'd need to reduce the Cd by as much percentage as they represent frontal area of the total car. Looking at the picture I previously posted, That would be far fetched to say those mirrors are completely enclosed in vortices and do not stick out at all in airflow that would otherwise be mostly undisturbed.

An SAE paper on the Volt aerodynamics would probably shut me up on this subject, and if the mirrors are really aero neutral I'm pretty sure there would be such a paper published. But until then I'll keep on doubting.