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

[superchow]

...but we have to protect our styling flexibility.

Basjoos had some styling flexibility - not this Viridian Joule colored greenwashing-mobile. The more I hear about this car, the more I am tempted to seriously mod my Civic.

[DieselHybrid]

Hi- great site!

I question the validity of the statement regarding the Volt's superior aerodynamics.

CdA= Cd * wetted area

Dubious claims of Cd aside... let's focus on frontal area.

Without taking into account side mirror area (btw GM's claim of "drag neutral" mirrors is ridiculous)- the Volt is supposedly 70.8 wide by 56.3 inches high while the Prius is 68.7 inches wide by 58.7 inches high. However, the Prius does not have as deep an airdam as that which is installed on the Volt.

Assuming an average ground clearance of 5.5 inches for the Prius and 3.0 inches for the Volt- frontal area would be 25.4ft^2 and 26.2ft^2 respectively. Advantage: Prius.

Yes, the airdam serves to direct high speed flow away from the vehicle's relatively "dirty" underbody. It's an easy/cheap fix that GM is exploiting to help earn impressive EPA hwy fuel economy ratings with their latest crossovers and SUV's.

However, deep airdams also increase frontal area. (Cd of an object is measured as the fraction of its drag when compared to the drag from an equivalent flat plate area.) Of course, with a larger equivalent flat plate area, GM should possess a lower Cd.

This would help explain GM's claimed low Cd of 0.28, vs. the Prius Cd of 0.30.

Careful shaping of a vehicle's underside or underside fairings can reduce drag better than an airdam- whose increase in frontal area may mask an overall potential drag reduction. Reference Aptera's lack of an airdam as an example.

An early morning coast-down test with each vehicle would help dispell all the aerodynamic conjecture.

Finally, in the absence of independent testing, I would not place much faith on any manufacturer's claims.

[superchow]

Ah... a refreshing approach: A coast-down test. My mind is reeling with the excuses Chevy could come up with to discredit cold hard evidence...

[JackMcCornack]

I'm with you in concept, DieselHybrid (though I differ on a few details). The Cd thing can indeed be 'spun' to give higher drag vehicles nicer numbers than they deserve--if one is manufacturing cars, the easiest way to get a lower Cd is to make the car bigger. Hucho has a drawing showing how cambering a roof can increase drag, because the frontal area goes up faster than the Cd comes down. Open cockpit racers nearly always have higher Cd than their closed cockpit counterparts, but don't always have less drag (the Bristol 450 is a case in point) because they have more frontal area.

[aerohead]

there is an SAE paper which deals with "scatter" based on a single vehicle or standardized calibration models when tested in 6-or 8 different wind tunnels.
GM and Lockheed ( closed return ) results are the most conservative as far as Cd.
Ford publishes a yaw-averaged Cd ( they'll yaw the car up to 8-degrees in the tunnel )when they claim a Cd.
Is anyone saying if a uniform procedure was followed,to arrive at these latest numbers?
The mean standard deviation between GM,Ford,Chrysler,JARI,MIRA,Pininfarina,Canadian National Labs,etc., was only about 2.2%.

[tjts1]

I'm willing to bet the total drag (CD x frontal area) is lowest for the Insight, then the Prius then the Volt. The devil is always in the details.

[tasdrouille]

You might very well be right.

Volt: ((70.8 * 56.3 * .85) + 240 (airdam)) / 144 = 25.19 sq ft -> * 0.28 = 7.055 CdA
Prius: (68.7 * 58.7 * .85) / 144 = 23.8 sq ft -> * 0.3 = 7.141 CdA
Insight: (66.7 * 56.1 * .85) / 144 = 22.09 sq ft -> * 0.32 = 7.068 CdA

This is all just approximations, but in the end it looks like the total drag for all those cars is going to be within a 2-3% range.

[NeilBlanchard]

We did the math on the Aptera 2e PP6, and the CdA came in just under 3. The "production" version lowers the Cd from 0.15 down by ~9%, and the frontal area increases a bit, so we'll find out (hopefully) later what it is.

[jamesqf]

Originally Posted by aerohead

Ford publishes a yaw-averaged Cd ( they'll yaw the car up to 8-degrees in the tunnel )when they claim a Cd.

This is a good point. In these parts, it's not at all unusual to have a 40 mph or so crosswind - and anyone who's flown small planes into tight fields knows that a bit of slip is a darned good way to bleed off altitude/airspeed in a hurry. So I wonder what that 0.28 Cd straight into the wind becomes when the effective wind's at 30 degrees?

[thatguitarguy]

The EV1 had a nice clean underside, but since the wheels weren't inline, it had 4 wheeltracks instead of 2. The narrowing of the rear must have aerodynamically outweighed this penalty in frontal area and drag, but what if it had 3 wheels and an even narrower stern?