Chevy Volt Coefficient of Drag (aerodynamics)

[tboult]

Wheels impact drag, just like an airdam.. the more air flowing under the car the greater (in general) the drag.

What I don't get is that drag must be known in a standard calibration process. Since EPA testing is done using Dynonometers it need the drag as input.. somewhere there must be data from the EPA calibrations. It may not be CD (which is pointless without also considering area), but that would be the more meaningful, and hopefully more standard, comparison.

[Overdrive]

The Prius is available with two different wheel sizes. Most people have the small, 15" wheels, but there is a 17" alloy available. This is the wheel option that GM used in it's tunnel test of the Prius to compare apples to apples.

[rubley]

Tesla is claiming 0.24 for the Model S, which I also find suspiciously low.

Personally, I'd like to see an option to ditch the passenger side mirror and add wheel skirts.

[Ziv]

Overdrive, I didn't know that. I have looked up Prius options several times and never saw the 17" wheels, nor have I noticed them out in the wild but that is me being less than observant. So I was casting unwarranted aspersions on Boniface, which makes me feel better about GM and worse about my own ability to analyze PR articles.

[pjk447]

this factor does not equate to nearly 1mpg.... it only matters so much in the total design of the car... to go any further would take away from other designs of the car ...like practicality....

[supermachoman]

Someone at GM was saying that the EV1 couldn't even be produced today with its ridiculous .21 drag coefficient. Apparently safety standards have changed now and the sacrifices wouldn't be possible.

[Ladogaboy]

A few comments:

First, I'd be interested in the comparison of CdA as well. That is a better indicator of how much energy it takes to move the vehicle through the air.

Second, the wheel size and design can have a huge impact on the overall aerodynamics of the car. If Toyota tested the Prius under ideal circumstances, it might beat out the Volt. I'd prefer to see an objective wind tunnel test before I take any sides.

Third, the reason the pure EVs (not PHEVs or hybrids) can achieve such low Cd is because they can have a completely closed front facia and an almost unobstructed, smooth under belly. Those two factors alone can account for upwards of 20% improvement in Cd. As a result, the Tesla S as an EV could achieve a .24 Cd, but if it were a hybrid, the number would be closer to .29 Cd.

[daryllafferty]

The 17" alloy wheels are a Prius option that comes with the 'Advanced Technology Package".

It seems to me that if the 17" wheels result in the car sitting higher off the road, then the Cd might be higher, for the same reason that the taller air dam on the Volt reduces drag by reducing the amount of air passing under the car.

[Ladogaboy]

It seems to me that if the 17" wheels result in the car sitting higher off the road, then the Cd might be higher, for the same reason that the taller air dam on the Volt reduces drag by reducing the amount of air passing under the car.

This would be moot because, even though the 17" wheels are a larger diameter, the outside diameter of the tire would remain the same. When transitioning to a larger rim, they maintain the outside diameter of the tire by lowering the tire's aspect ratio (profile).

[Aussie Pete]

Bing this thread up again because we now see cars like Mercedes Benz A Class getting low 20s for coefficient of drag. Given how much detailing and effort is in theVolt, and the compromises, why is a conventional car so much better in numbers?

It is surely strange that there is no SAE or other standard for calibrating cd testing so we get like for like numbers?