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Old 03-26-2006, 02:08 AM   #1
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Aerodynamic auto designs, past and present, and their possible implications

A little article I wrote, in which I attempted to find a reasonably accurate relationship between aerodynamics and fuel economy:

http://www.evworld.com/blogs/index.c...d=87&archive=0

A brief summary:

Fuel economy can be improved by roughly 30% in today's cars just by adressing aerodynamic drag. This is without making the engines smaller or less powerful, without shrinking the car itself, without going to a hybrid-electric or diesel powertrain, without low rolling resistance tires, without synthetic transmission oils or changed gear ratios, without CVTs, without composite body materials or major weight reductions. Just by changing the physical shape of the car. Further, the most aerodynamic cars sold in America today(ie. Toyota Prius) aren't near as aerodynamic as a handful of similarly sized cars built 70+ years ago!

For those interested, read the whole thing.

It's not perfect, as it doesn't delve into any serious automotive technical analysis and likely has a 10% margin of error given other unacounted for characteristics, but I do believe my estimations to be reasonable.




There are also few downsides to addressing drag.

Styling? There are lots of different styles possible! The automakers just need a little imagination.

If stability becomes too much of a problem at high speeds, then why not gear/govern the cars for 90 mph or so? The speed limits in the U.S. usually aren't higher than 70 mph(with a few exceptions).

Storage space? Why not elongate the rear of the car more to increase it? The examples listed had more than adequate storage space, infact improved over today's autos.

But then the oilies and auto industry wouldn't be making as much money. Oilies from decreased fuel use, auto monopolies from decreased maintenance due to less horsepower at speed... Then there's the issue of slowly rationing out technology to maximize profits, especially in the case where the industry refused Buckminster Fuller's Dymaxion, execs explicitly yelling him because they wouldn't have no more advancements to sell!
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Old 03-26-2006, 03:47 AM   #2
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Welcome Toecutter! I'm glad

Welcome Toecutter! I'm glad to see another aero advocate here. Between you and MetroMPG, hopefully I can get my CD down to something more reasonable. I have seen two CD numbers for the del sol, 0.378 and 0.42. I don't know which, if either is right, but my CD is ridiculously high in either case.

So far I have lowered the car by a net 3/4". By net I mean that the ride height increased by 1" when I installed the 2" over stock diameter tires. Then I dropped the height by 1 3/4" with adjustable coilovers. The oversized tires have only a marginally wider track but give me a bit taller final drive. They also fill the wheel well much better which according to one article that I read, reduces drag in the wheel well area. Metro has suggested rear wheel covers and a tonneau cover for the trunk lid and others have experimented with underbody panels. I am considering all of these options. Do you have any opinions or suggestions about what would have the most dramatic impact on my CD?

My drive is mostly at 25-45 MPH so I didn't think that a reduction in CD would help that much. But looking at the spreadsheet that I put together for calculating total drag, using the same formulas in your article, indeed if i reduced my CD I can see similar benefits as what was stated in your article.

Great article by the way. But I think you might want to look at the CD of the Insight. IIRC it is 0.25, which would make it the lowest CD of any production car, not the Prius. BTW, I also have a 2004 Prius. Well actually, now the wife has it.
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Old 03-26-2006, 04:34 AM   #3
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Quote:Welcome Toecutter! I'm

Quote:
Welcome Toecutter! I'm glad to see another aero advocate here. Between you and MetroMPG, hopefully I can get my CD down to something more reasonable.
One thing to note, MetroMPG has more experience than I do. He slapped carboard on his metro and experimented, I never have(yet).

But yes, given what others have accomplished, especially Phil Knox and his T100 pickup(going from .44 to .25 Cd increased mpg by 32%), you can get your Cd down to a fairly low level.

Quote:
I have seen two CD numbers for the del sol, 0.378 and 0.42. I don't know which, if either is right, but my CD is ridiculously high in either case.
I remember reading .378, from back when I was looking for EV conversion candidates.

Quote:
So far I have lowered the car by a net 3/4". By net I mean that the ride height increased by 1" when I installed the 2" over stock diameter tires. Then I dropped the height by 1 3/4" with adjustable coilovers.
I've read that optimum ride height for reduced drag is 2.9". The closer you arrive towards this, the better.

Quote:
The oversized tires have only a marginally wider track but give me a bit taller final drive. They also fill the wheel well much better which according to one article that I read, reduces drag in the wheel well area.
Wider tires also increase frontal area(slightly), and tend to induce more wheel well turbulence.

Be careful here. There is a tradeoff between optimizing the tire size and integrating the wheel itself with the side profile of the car in minimizing drag.

Quote:
Metro has suggested rear wheel covers and a tonneau cover for the trunk lid and others have experimented with underbody panels.
They work. Make sure the tonneau cover does not vibrate at speed, or more turbulence will be generated.

Quote:
Do you have any opinions or suggestions about what would have the most dramatic impact on my CD?
You can do the following, in decreasing order of impact starting with the largest impact(my best guess, anyway):

1) Smooth Underbelly
2) Rear wheel well covers
3) Rear spoiler specifically designed for reduced drag(you'll likely have to design and build this yourself!)
4) Sealing off the grille
5) Rear tonneau cover
6) Front air dam
7) Side skirts
8) Wheel spoilers
9) Smooth wheel covers
10) Shaved door handle kit
11) Remove passenger side mirror
12) More aero driver side mirror(or replace with camera setup)

Doing all this, if executed properly, could reduce your Cd to the mid .2 region.

Quote:
My drive is mostly at 25-45 MPH so I didn't think that a reduction in CD would help that much.
Not much. Although, about 35-40 mph or so is when aero drag and rolling drag start to balance out.

You'll probably see a city fuel economy increase around 5-10% if you could cut Cd from .38 to .25. Highway fuel economy would increase around 30% though.

Quote:
Great article by the way. But I think you might want to look at the CD of the Insight. IIRC it is 0.25, which would make it the lowest CD of any production car, not the Prius.
I thought the Insight was no longer produced. Otherwise I would have mentioned it. But looking on the fueleconomy.gov website, it's listed for 2006. I'll have to make a minor correction.

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Old 03-26-2006, 05:24 AM   #4
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Re: Quote:Welcome Toecutter! I'm

Quote:
Originally Posted by The Toecutter
You can do the following, in decreasing order of impact starting with the largest impact(my best guess, anyway):

1) Smooth Underbelly
2) Rear wheel well covers
3) Rear spoiler specifically designed for reduced drag(you'll likely have to design and build this yourself!)
4) Sealing off the grille
5) Rear tonneau cover
6) Front air dam
7) Side skirts
8) Wheel spoilers
9) Smooth wheel covers
10) Shaved door handle kit
11) Remove passenger side mirror
12) More aero driver side mirror(or replace with camera setup)

Doing all this, if executed properly, could reduce your Cd to the mid .2 region.
Glad to see the smooth underbelly is first on the list. I like that approach because it does not change the appearance of the car. Now if I could only find some coroplast!

I'd consider a rear spoiler but wouldnt have th slightest idea what it should look like.

Im wondering about the front air dam. That would add to the frontal area right? But the benefit would be less turbulence under the car? What should the dam look like. How close to the ground?

Also I have the OEM mud guards, since they are behind each wheel I figured that they would not effect aero drag. What do you think?
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Old 03-26-2006, 06:50 AM   #5
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welcome,

welcome, toecutter!

wonderful article. as you know, the piece about phil's knox's toyota pick-up is one of the most accessed articles at EVworld. i should think yours will be similarly popular (at 89 hits as of my reading... watch it climb) - there's clearly a real hunger for this kind of information out there. thanks for writing it.

(how come your article isn't listed on the EVworld home page? i usually check in there every couple of days, and didn't see yours linked from the "blog" section.)

coincidentally, just yesterday i was also reading some of your posts in the EV list archive. toecutter is also working on an EV conversion - one that will be far more functional and interesting than the forkenswift i'm working on with my friend.

Quote:
One thing to note, MetroMPG has more experience than I do. He slapped carboard on his metro and experimented, I never have(yet).
i may have played with some cardboard and a scangauge, but i'm *far* from expert on the subject. i'm very much a student, and still devouring whatever useful information i can get my eyes on.

the EVworld article about phil knox's pickup is a good red pill article like toecutter's. it makes it clear that modern mainstream cars, "as delivered" from the manufacturers, basically suck aerodynamically. with a basic understanding of why they suck and some examples of cars that don't suck, it's not hard to go from there with duct tape and cardboard to improve Cd.

speaking of which - yesterday i de-slapped the cardboard wheel skirts off my car in order to hose off the winter accumulation of crud & salt from the wheel wells, suspension bits and underbody. i won't be slapping the cardboard back on; in the next week or 2 i'll make a "good" set of plastic skirts.

recently in the mirror thread, basjoos posted 2 links to some good quality aerodynamic information, some of which also explicitly answers the question: where can the largest aero improvement be made to a typical car? i'll repost it here fyi.

basjoos' links were from a series of reports written by a professor at chalmers university of technology in sweden. he apparently teaches (taught?) a course called "MTF235 Road Vehicle Aerodynamic Design" in the faculty of mechanical engineering (thermo and fluid dynamics section).

http://www.tfd.chalmers.se/~lelo/rvad/

click on "reports archive" in the left navigation area for the goods.
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Old 03-26-2006, 12:53 PM   #6
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Re: welcome,

Quote:
Glad to see the smooth underbelly is first on the list. I like that approach because it does not change the appearance of the car. Now if I could only find some coroplast!
Don't take its position on that list too literally. It was just my guess, and I'm no where near an aerodynamics expert! But yes, it will have sizable impact if properly done.

Quote:
I'd consider a rear spoiler but wouldnt have th slightest idea what it should look like.
When I get the time, I might draw one for you. Essentially, you would want to achieve a 6:1 finess ratio, but since your car is too short to really achieve that, cut it off prematurely. This spoiler would actually start at the very end of the car's roof, with the purpose of directing airflow.

Quote:
Im wondering about the front air dam. That would add to the frontal area right?
Slightly.

Quote:
But the benefit would be less turbulence under the car?
Usually yes, to the point where it lowers your overall Cd*A, even if it might add some A.

Quote:
What should the dam look like. How close to the ground?
The dam should be rounded at the front and flush against the car's bumper itself, with the bottom of the airdam slightly lower than the car's ride height.



Quote:
Also I have the OEM mud guards, since they are behind each wheel I figured that they would not effect aero drag. What do you think?
They likely add a lot to it. A flat panel has a Cd of about 1.9! To have any air flowing against it will add a lot of drag relative to what your vehicle experiences.

I found this interesting link that you might want to look at:

http://www.teamdelsol.com/technical/.../aerodynamics/


Quote:
how come your article isn't listed on the EVworld home page? i usually check in there every couple of days, and didn't see yours linked from the "blog" section.
Quite simple, really. I don't have a recent pic of myself to attach to my blog profile. It might be a while before I do.

Quote:
coincidentally, just yesterday i was also reading some of your posts in the EV list archive. toecutter is also working on an EV conversion - one that will be far more functional and interesting than the forkenswift i'm working on with my friend.
Eventually, yes. Mine is starting out much like your forkenswift due to budget contrainsts, on the otherhand. It won't be staying like that though, considering I have a lead foot.


I also read those links in the mirror topic and enjoyed them. They had actual examples to illustrate the drag reductions that could be made, which is very useful when actually designing additions to a car's body.

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Old 03-26-2006, 01:32 PM   #7
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Re: welcome,

Quote:
Originally Posted by The Toecutter
I found this interesting link that you might want to look at:

http://www.teamdelsol.com/technical/.../aerodynamics/
Great info! I'm gonna buy the civic front lip and remove the mud guards. Then the underbelly pan. I thought I would use corrugated plastic. Any reccomendations?
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Old 03-26-2006, 01:59 PM   #8
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there is much more you can

there is much more you can do to the car to remove airdonamic drag then all of the things he listed.

Making the air go around the tire will make a huge difference. Also on most cars where the foglights are it's very wallish. So when the air hits there it stays there.

I have so many ideas, just not enough metal and not enough dremel bits.
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Old 03-26-2006, 02:49 PM   #9
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Quote:I thought I would use

Quote:
I thought I would use corrugated plastic. Any reccomendations?
It will be difficult for me to recommend a material, as I'm still deciding what to use for my aero mods, but I have thought of linoleum, PVC, corrugated plastic, and polyurethane. The material needs to be stiff and thick enough to where it won't ripple, and my considerations for strength are especially important since my electric car will eventually be able to top 140!
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Old 03-26-2006, 03:53 PM   #10
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a thought about front air

a thought about front air dams (chin spoilers) and undertrays:

if the purpose of an air dam is to keep the flow away from the "dirty" underside, why would you need both if you have already cleaned up the bottom with a good tray(s)?

i've always thought it should be one or the other, not both. and i'd lean towards installing the trays because they improve Cd without increasing A. automakers prefer the dams primarily because they're a heck of a lot cheaper to design and add to a vehicle.
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