Chevy Volt Coefficient of Drag (aerodynamics)
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Thread: Chevy Volt Coefficient of Drag (aerodynamics)

  1. #1
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    Default Chevy Volt Coefficient of Drag (aerodynamics)

    I recently stumbled on Wikipedia's page for 'Automobile drag coefficient', and was amazed to see that, while the Toyota Prius is currently listed as the fifth most aerodynamic vehicle (with a Cd of 0.25) - no surprise there - the Volt is quite far down the list (with a Cd of 0.281)!

    Notable vehicles that are more aerodynamic than the Volt include practically every Hyundai (Sonata & Sonata Hybrid - at #6! -, Genesis, Elantra) and Lexus car (IS, LS400 <1988 model>, ES, GS, LS430, LS460), as well as many cars I didn't realize have excellent aerodynamics (Audi A2, Mazda3 sedan & hatchback, Mitsubishi Diamante, Toyota Camry/Camry Hybrid, Infiniti G35, Mazda6 sedan & hatchback, Volkswagen Passat <1997 model>, Mercedes-Benz B,E & S-class), and some that aren't quite as surprising (Tesla Model S at #3, with a Cd of 0.24; Honda Insight, Porsche 997 Carrera, Nissan Leaf with Cd of 0.28).

    Perhaps most baffling to me however, is the car that's rated #1: the GM EV1 (at 0.195 Cd)!

    My question to GM therefore, is "Why does the Volt compare so poorly to so many other production vehicles, including GM's own previous attempt at a maximum fuel-efficiency car?" (Not sure if that qualifies as a suggestion; moderators please feel free to move this thread if there's a more appropriate forum!)

    Not to say that a coefficient of drag rated at 0.281 is a bad thing, but why are so many 'normal' (ie. non-hybrid or EV) vehicles rated better? Doesn't that suggest there's lots of room for improvement, especially in an EREV where aerodynamics are of particular importance?
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    I think the numbers depend on what criteria and methods used to test for the drag. I think this post answers your question: http://gm-volt.com/2009/12/04/chevy-...s-and-insight/
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    As the article says, the Prius number is just complete BS hype. Point whatever is easy to say not so easy to do. Note that in normalized testing the EV-1 would be .21.

    The takeaway is that there is room for big improvements is outlandish claims about Cd numbers.

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    I remember the article on this site that stated GM tested the Prius and could not get the Cd measurements that Toyota has quoted.

    http://gm-volt.com/2009/12/04/chevy-...s-and-insight/

    "GM has talked tirelessly abut how aerodynamic the Volt is, how its shape was born from the wind tunnel, and how important aerodynamics are for the car to achieve its 40 miles all electric range. Yet for all that talk, the company has never released any official figure for the vehicle’s coefficient of drag (CD). This is the numerical measurement that indicates how slippery a car is, and unlikely to be slowed by wind resistance.
    Bob Boniface who is chief of Voltec design finally provided us the details that were obtained when GM measured the Volt and its competitors on its own wind tunnel.
    “We had the comparably-equipped 2010 Prius with 17 inch wheels, and the new Insight,” he said.
    “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.”
    “The Insight was 0.32, and the Volt was .28,” he said.
    “I’d like to test the Volt in the same tunnel where Toyota got their 0.25 value,” he teased.
    He expanded:
    This number is for the Volt IVer which is representative of our production car. We were resistant to give out the number earlier for two reasons. One we wanted to wait until we tested our IVer. Also, we didn’t want to report it out until we saw where the competition was because we know that those numbers depend on how the tunnels are calibrated.
    If I quoted 0.28 a year ago people would have said ‘aha’ the competition got 0.25. But its really all relative to what tunnel it was tested on.
    The base Prius with the smaller wheels may come in lower, but we don’t offer 15 inch wheels.
    EV-1
    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.
    “But that vehicle didn’t have a rollover and frontal impact structure of today’s standards,” said Boniface. “You could not sell that car today. That’s not to say we wont get to .21 again with another car. We always try to get drag down but we have to protect our styling flexibility.”
    Corvette
    The current base Corvette is the most slippery and its a 0.29, slightly less aerodynamic than the Volt."



    Also, the EV1 was an extreme design (and also just a two seater).

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    I was lucky enough to be able to speak to one of the aero engineers during one of the Volt demo rides. She said the Cd reported by different manufactures varied according to which wind tunnel used for the measurement. According to her, the Prius measured several points higher than the Volt in the GM tunnel. She said with the exception of the EV1, the Volt had the lowest Cd of all cars measured at that facility.

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    That's some really good information that someone should update this chart with:

    http://en.wikipedia.org/wiki/Automob...ag_coefficient

    That's the first time I've seen the 0.30 number for the Prius, and updating it there would help stop the misinformation about the Volt's supposedly "poor" aerodynamics.
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    Here's a (kinda silly) idea...

    Since I'm sure there are quite a few people on this forum that own both a Volt and a Prius, if you wanted to geek out on this aerodynamics question you could have two drivers take both cars out on a highway and drive them side-to-side up to a high speed (80mph+) and then throw them both into nuetral at the same time and coast down to see which one has less drag (goes further/faster). Rolling resistance would also factor into this, so it would give a more complete picture than just the aerodynamics.

    Also, while considering Cd's for different vehicles, remember that the Cd is independent of the vehicles' size / frontal area. Some SUV's have surprisingly low Cd's, but you have to multiply the Cd by the frontal area to get a more relevant figure to determine the amount of aerodynamic drag it will experience. I believe the frontal areas for the Volt and Prius are similar though, so comparing the Cd's of these 2 cars is probably a good comparison.

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    Really interesting replies! I had no idea about the different wind tunnels being calibrated differently - maybe I didn't read the Wikipedia article closely enough, but you'd think that would be worth mentioning somewhere, as Jedi2155 said.

    It seems odd to me that the measurements given include a number of vehicles with Cd 0.28, and the Volt suspiciously just a hair worse at 0.281. The sources for most of the referenced numbers are all second-hand as far as I saw, mostly from automotive magazines, so how authoritative are their sources for the vehicles like the Volt which aren't referenced? As per the quote in MTN Ranger's post, GM's own testing suggests a Cd of 0.28 for the Volt - I wonder why Wikipedia didn't use that number?

    Also interesting is the CdA (drag area) as jsmay311 pointed out - does that mean we'll be seeing needle-shaped front ends in super-efficient cars of the future...?
    Last edited by TeV; 07-10-2012 at 04:48 AM.
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    The thing that really irks me is that GM actually put their own set of 17" wheels on the Prius and then were "Shocked, Shocked, mind you" that the Prius got worse aero results that with the stock 15" wheels the Prius is sold with. You don't fake up the competitions results and then say "The base Prius with the smaller wheels may come in lower, but we don’t offer 15 inch wheels."
    Sure the Prius .25 number is fake, but you don't lower yourself to their level.
    On edit: I think Hamtramck must be open. GM just closed the part of their website that allows you to see how many cars are within a stated distance of whatever zip code you enter. It also had the VINs which I used to use to track the production rate, but no mas... Maybe it is a glitch and the link will come back later this week. Too bad, I won't pay what GM is charging right now but it let the numbers geek in me run riot until I have saved enough to buy a Volt.
    Last edited by Ziv; 07-10-2012 at 08:09 AM.

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    I don't really understand the wheels commentary. Does a different size wheel affect drag significantly? Or perhaps it was more about wheel shape? It's possible at the time of testing, GM was only working on and comparing body shape, so they used nearly (or exactly) the same wheels on each car tested. But from a whole package perspective, it would be more interesting to know the numbers with each car using base factory wheels.

    I can't imagine GM would have changed the overall diameter of the wheel and tire on the Prius, so I am running with the assumption that they used 17 inch wheels with lower profile tires to keep the same overall diameter on the Prius.

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