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This is sort of a what if question for the older bloggers or possibly those with engineering credentials. A number of bloggers have made or suggested modifications for the Volt; some for convenience or personal preference but mostly for appearance. Does anyone know how much more fuel efficient the Volt would be with skirted rear wheels? This is common on cars raced on the Bonneville Salt Flats and occasionally on production cars such as the Honda Insight.

Are we talking a 5% increase or more like a 25% increase in fuel savings?
 

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GM has also stated that the design of the Volt's wheels contribute as well. effectively pumping air (like a fan?) to improve the efficiency of the under body airflow.
 

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Maybe skirting the rear wheels isn't it. Anything else to reduce drag? There are a few production cars below 0.28-0.29 ratings I see for the Volt, though not by much. It would take a fundamental shape change to get drastically lower -- and compromise cargo and people space in the process -- or look like the Mercedes Boxfish car :)

A few basics come to mind:

1. an even larger front air dam (may not be practical)
2. lower springs/ride height (reduces airflow below car, exposure to tires, etc.; also may not be practical)
3. narrower tires

More extreme steps are to remove things that disrupt smooth flow over the car: door handles, rear-view mirrors, antenna, etc. Practical limits can be quickly exceeded!

I would ponder removing or diminishing the rear wing. Change may help; it could hurt. There's a trade-off at highway speeds between stability and low drag. That would need experimentation (or input from GM engineers who no-doubt would have done a bunch of testing around the trailing edge of the car.)
 

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That reminds me of an article I read a while back about the 1939 Schlörwagen (73 years ago!)

"To demonstrate just how far the aerodynamic envelope was pushed in this golden decade of streamlining, this 1939 Schlörwagen prototype was tested originally at Cd .186, and a model of it was retested by VW in the seventies with a Cd of .15. Either of these values put the “pillbug” at or near the top of the list of the most aerodynamic concept cars ever built, like the Ford Probe V of 1985, with a Cd of .137. Built on the chassis of the rear-engine Mercedes 170H, it was substantially faster as well as 20% to 40% more fuel efficient than its donor car. The Russians took the Schlörwagen as war booty and conducted tests as a propeller driven vehicle. It represents a state of aerodynamic efficiency in league with the most aerodynamic cars being considered today, such as the Aptera."
Link




 

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-While filling the wheel well helps drag, I doubt it reduces the benefit of skirts by more than 30%. I also doubt any "fan" affect from the rims is a net positive for efficiency. IMO, GM should offer rear skirts as an option.

-The "rear wing" is actually designed for reduced drag and not high speed stability. It trips the airflow to reduce the flow separation drag. They could make the lower chamfer at the rear larger so the actual vertical rear surface is smaller thus reducing separation of flow. They could add a lower lip to help similarly as the "rear wing".

-The side view mirrors could be removed once regulations allow side view cameras. Until then, they could be made lower profile without compromising usable view. The upper part of their triangular shape doesn't help useful visibility and they could be made more of a trapezoid shape with the same bottom width as they currently have. I think this would actually improve the Volt's aesthetics too.

-The windshield wipers are exposed. Perhaps the hoods shape diverts the airflow enough so that hidden wipers wom't help much but perhaps not.

-The front could be smoothed out some more.

-As long as the Volt is a 4 seater the roof could be pinched down in the middle to reduce A (of CdA).

-The underbelly has some covering but not much. More could be added to create a smoother underbelly surface.

-Don't know if it could be done in an aesthetically pleasing manner but dimpled surfaces do reduce drag.

-An internal antenna to save drag from the current external unit.
 

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-While filling the wheel well helps drag, I doubt it reduces the benefit of skirts by more than 30%. I also doubt any "fan" affect from the rims is a net positive for efficiency. IMO, GM should offer rear skirts as an option.
http://gm-volt.com/forum/showthread...-Volt-Six-ways-it%92s-different-from-your-car

Reason No. 1: Aerodynamics.
<snip>
“The scoops in the wheels helps get rid of the extra material. They’re very low mass. The geometry of those wheels actually helps with the aerodynamics. They send air out or in as they rotate, and air going in under the car is very bad for aero.”
 

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http://gm-volt.com/forum/showthread...-Volt-Six-ways-it%92s-different-from-your-car

Reason No. 1: Aerodynamics.
<snip>
“The scoops in the wheels helps get rid of the extra material. They’re very low mass. The geometry of those wheels actually helps with the aerodynamics. They send air out or in as they rotate, and air going in under the car is very bad for aero.”
The Volt has a very low air dam that directs a lot of flow out from under the vehicle. The underside is flat but not very smooth since there isn't much (if any, I need to take another look) underbelly cladding. Any "fan" affect from the wheels may help the vehicle's aero number but hurts the wheel's rotational efficiency. The wheels rotational efficiency increases the parasitic load but does not affect the aero number. Also, the more air removed from under the car, which is good for the drag there, is bad for separation of flow at the rear. I would guess rear wheel skirts plus underbelly cladding and neutral flow (no fan affect) wheels would improve aero a bit and lower the wheels parasitic load a small amount. Of course, this is a lot of guessing to a very complex situation with minimal data.

The commentor, a designer not aerodynamicist, also later comments on the manual seat adjustments vs electric. While I personally have no problem with the compromise and I think the manual controls are by far the best I have experienced, they are one of the most cited disappointments with the Volt. The commentors party line that Volt buyers did not want to waste electricity is a joke. How much electricity does an electric seat control use in real world application? 100Whr/yr? There is some credibility to the weight savings but it's much. This was a cost tradeoff, pure and simple. Again, I don't have a problem with it at all for myself. I prefer the manual seat and the lower cost. I only point this out because this indicates the article reads more like a press release than a study.
 
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