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Chevy Volt Coefficient of Drag (aerodynamics)

21K views 28 replies 23 participants last post by  Upper5Percent 
#1 ·
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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#3 ·
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.
 
#4 ·
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-volts-coefficient-of-drag-is-0-28-beats-prius-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).

 
#5 ·
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.
 
#6 ·
That's some really good information that someone should update this chart with:

http://en.wikipedia.org/wiki/Automobile_drag_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.
 
#7 ·
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.
 
#8 · (Edited)
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...?
 
#9 · (Edited)
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.
 
#10 ·
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.
 
#11 ·
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.
 
#14 ·
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.
 
#17 ·
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.
 
#18 ·
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.
 
#19 ·
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).
 
#20 ·
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?
 
#21 ·
By definition, the Cd is supposed to be a convention. But keep in mind that Cd only refers to how aerodynamic the shape of the car is. What is actually important is the drag area, which is the Cd times the frontal area. So if a car with a slightly lower Cd than the Volt were twice as large in frontal area, the overall drag force at any given speed would likely be higher.

That said, MBZ is on a mission to actually target very low Cd numbers. Its likely they are throwing more money at such research so it would make sense that they can hit lower numbers than GM can with the Volt. And MBZ would likely have more incentive in doing so given the higher HP average of their fleet, which translates into poorer CAFE performance. Reducing the Cd in their cars would help alleviate some of that. GM likely isn't struggling with CAFE numbers so much so there isn't a huge need to invest heavily in it.
 
#22 ·
The comment I'm making is not a scientific measurement, but a subjective one: I found my 2001 Lincoln LS to feel more "eerodynamic" than the Volt. What I mean by that is that once I got it to highway cruising speeds (which of course required significant gas) it seemed to just not want to stop rolling. The Volt acts differently: when you are at a higher speed, it seems very susceptible to sideways wind buffeting and disturbances from larger passing vehicles, and there is some other type of resistance I can't put my finger on, but it is there. When you start to decelerate to a stop, *then* it definitely doesn't want to stop rolling-- it will roll and roll when it is slowing down, so if you are in situation where you can coast to a stop you get regen for days: those situations are the exception not the rule.

But I don't get a feeling of really being able to coast as well at higher speeds. Having said that, i'm sure it is better than the vast majority of vehicles, and I do see as I'm going along with speed control on that regen occurs on the slight dips and so on.
 
#23 ·
But I don't get a feeling of really being able to coast as well at higher speeds. Having said that, i'm sure it is better than the vast majority of vehicles, and I do see as I'm going along with speed control on that regen occurs on the slight dips and so on.
Were you attempting to coast in neutral? If not, your point is moot. Even in D, the Volt sends enough energy back through regenerative braking to more than compensate for its better-than-2001-Lincoln LS aerodynamics.

Also, I do not share your experience with the wind. I've been in 30-50 mph gusting crosswinds, and the Volt handled it okay. The Volt doesn't handle high cross winds as well as my last car, but then again, very few cars do. I'm sure being shaped like a mako shark had something to do with that.
 
#25 ·
A few things to consider:

While the aero drag goes up as the square of the speed, the power required to maintain that speed goes up by the cube.

The bigger the vehicle (especially in length) the easier it is to get low Cd number.

EU pedestrian safety rules pretty much eliminate pointy front ends on new cars. (as well as pop-up headlights).

Cars shapes are natural wings, curved on top, flat on bottom. The original Insight, while notable for low Cd, was notorious for being blown around on the highway. Many aero features on cars are not designed for lower drag, but to reduce undesirable lift, which costs in terms of Cd.

Coast-downs can be useful when making mods on the same model of car, but variations between different models in their inertia of rotating mass can influence results. For the same tire outside diameter, for example, the larger rotational inertia of a 18" wheel will show less slowing in a coast down than that from a 15" one.

The most realistic car-based wind tunnels are of the "moving road" type. A car sitting on a non-moving surface with a 70 mph wind blowing across it does not generate the same flow patterns as either reality or a tunnel with the road "rolling" under the car at that 70 mph. Such tunnels are rare and expensive.

YMMV
 
#26 ·
Fairly silly. These fractional differences in CD are insignificant compared to the effect of slight differences in altitude, temperature, and velocity on range. How does an air cooled battery impact range? A 20 mph crosswind? etc.
 
#29 · (Edited)
Want to throw these numbers for a loop?
Try a real world application...:)
Put a bike rack with bike on any of these cars...and then look at the numbers...:rolleyes:
 
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