GM disputed Prius Aerodynamics - any follow up?

Knowing GM .... I wouldn't believe a word of it ... everything else relating to the dolt has been a lie ... why would they start telling the truth now ....




REV

 

I vaguely recall something about this, that GM tested the Prius with 17" wheels because the Volt uses 17" wheels?? Toyota's Prius numbers may have come from measurements with 15" wheels.

It's also important to keep in mind that what matters is the TOTAL aero drag, which is the Cd multiplied by the frontal area. I have no idea how the frontal areas compare. But the differences in Cd talked about here are rather small, so a small difference in frontal area could compensate in terms of final drag.

Also important to keep in mind that these drag figures refer only to the simple situation of stable airflow from the front. Aero drag when there are side or swirling winds is probably too difficult to calculate theoretically.

And of course aero drag is only part of the resistance the car must work against, and depends (at least in the simplified airflow-from-the-front model) on the square of the (air) speed.

 

Perhaps standing still. Not sure what good that is. When it is moving is when gas is being consumed; that is when I want it measured.

All that really matters is $ into the tank and battery (and pollutants out). Since the Volt doesn't beat the Prius when both are running on gas at 65 mph, it seems unlikely that the Volt's Cd is lower (or matter much if it is).

 

I just watched these 2 videos!

http://dai.ly/wvVPYe

 

The biggest differences 17" wheels and tires will make is increased rolling resistance, but the wider tires will also increase drag. These wind tunnels are not standardized, which means I would not trust numbers as being interchangeable. The spinning wheels of toyota's 17s may make the side more aero, gm tries hard to move the air around the tires.

Here's a thread of speculation.
GM: Chevy Volt's Cd is 0.28, Prius is 0.30 (tested in multiple tunnels), EV1 is 0.21 - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

I'm sure toyota would have tested a volt in their wind tunnel and trumpetted the difference if the 17s really were .25 versus the volts .28 thought.

Certainly this only matters if it improves fuel economy or reduces wind noise. Key drag is CdA, and the prius has a more efficient ice drive train. GMs epa highway is 40, IIRC the prius 5 with 17s should do arround 46mpg, and the one with 15s 48 under the same testing.

 

I am not so sure that a sane company would go to that level just to trumpet a difference in Cd numbers, and to disprove fuel economy?

 

Wind tunnel testing is not standardized. You can test three ways:
1. Car stationary, wheels stationary.
2. Car stationary, wheels rotating.

And then there's the way the people who really, really need to know
exactly what's going on with/in the airmass streaming around the car,
the Formula 1 racers:

3. Car stationary relative to the wind tunnel, ground plane (a conveyor
belt-like affair) moving at appropriate test MPH, wheels rotating:
[ame=http://www.youtube.com/watch?v=o-0BWcOcw8w]Understanding Formula 1: Factory Hinwil. Wind Tunnel. - YouTube[/ame]

 

Drag coefficient, schmag schmoeff... or something like that. Which performs better in the real world re: fuel efficiency?

GM trying to catch Toyota in a "lie"? As much as they lied about the Volt, and admitted it, it wouldn't seem worth quibbling over methodology, not unless you want to degrade into a rational constructionist fallacy, which I think GM is insinuating.

 

Well, I'm not sure why you are calling Toyota insane, but they do similar things. They do certainly do such things all the time, and proudly proclaimed that the Gen III had a lower coefficient of drag than the gen II. GM also chose to compare to the 17s not the 15s, because we can assume the 15s have lower drag.

 

Prius sales in 2011:
in the US: 136,500
in Japan: 252,500
TOTAL worldwide: 415,400 (including Europe)

Volt sales in 2011:
TOTAL: 7,671

So Toyota sold 54 Priuses for each Volt sold. Why bother responding?

sources:
[ame=http://en.wikipedia.org/wiki/Toyota_Prius]Toyota Prius - Wikipedia, the free encyclopedia[/ame]
December 2011 Dashboard: Sales Still Climbing | Hybrid Cars

 

I suppose this sadly is probably addressed to me. Toyota normally responds to these things if they are false. If the prius 5 had the same cd as the ones (1-4) with 15 inch wheels, you can bet they would say so.

 

I can't think of a good way to market that. "The Prius has an awesome low 0.25 coefficient of drag, even the Prius Five!"

 

I can't see where this issue is worth getting all wrapped around the axle over.

The contribution of the wheels alone is quite small, 0.09 for the wheel and wheel well together.



Above from, with lots to read on drag/Cd:
Road loads

Is it too early to call Bullsh!t on this?

 

Yes, different wind tunnels will give different drag, its up to the testing methodology. volt and prius are both aerodynamic cars. Mitsubishi eclipse because of its lower frontal area was even lower I think they pump the numbers into the computer. Going from 15s to 17s is rarely reported on the euro or us tests even though they change l/100km or mpg.

Probably why its a footnote. GM can't really use the volt has lower drag than the prius v, or five or what ever we call your car now, but higher coefficient than prius II. I'm sure my 17s are slightly less aero than the toyota oems, but they were lighter and less expensive

Is it really far fetched that wheels and tires change the aerodynamics of a car? I'm sure my higher rolling resistance tires hurt my mpg more than the slight loss of aerodynamics. YEs there is no santa clause. GM even said their testing made the ev1 get a .21 instead of a .19

 

GM is so far behind the 8 Ball in competition with the Prius they are
grasping at straws to make the Volt abortion look better than it isn't.

GM=Gross Management and it continues
GM=Gross Marketing and it continues.
GM=Gone Missing from the world of quality automotive product.

 

Agree! This reminds me of the "Most common car traded for a Volt" propaganda. It sounded like Volt sales were on fire (No pun intended). Why are claims like these more important than sale numbers.

 

OK, I'm stupid. Something was bothering about the part in the YouTube video where she refers to discovering that they could close up all of the upper grille opening and most of the lower one. Then I looked at the Cruze on the Chevy website, compared photos with the Volt, and realized that they were just modifying the existing Cruze bodywork. Not a clean-sheet-of-paper design. I knew the Volt was based on the Cruze but because I never paid much attention to the Cruze I didn't realize that it was so much more than sharing a common floorpan and reference points.

If the opening is to be closed up anyway, it would seem to be more aero (and save weight) to eliminate the grille and reshape that area???