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Old 04-18-2007, 08:45 AM   #1
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brake down of coefficient of drag on a car.

The book Build Your Own Electric Vehicle (by Bob Brant) has a chart (page 138) with the amount of drag of each area of a car (done in the 1970's) it is as fallows:
body-rear 33.3%
Wheel wells 21.4%
Body-under 14.3%
Body-front 11.9%
Projections and indentations 7.1%
engine compartment 6%
Skin Friction 6%

yes, these numbers are a bit dated, but look, wheel wells are 21% of your drag, and under body is next, of course the rear is the biggest, and hopfully in the last 30+ years that has been improved a bit.
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Old 04-18-2007, 09:02 AM   #2
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Think my windshield wipers are really worth the full 7.1% ? I almost hope so. I've been dying to get a coroplast cover that mounts at the hood hinges and rides just upto the height of the wipers. Anyone had luck w/this? I'd rather not remove the wipers.
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Old 04-18-2007, 09:55 AM   #3
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Interesting that the body-rear drag is more than 2.5X the body-front drag.
I wonder if that's still true today.

Body-rear is about 1.5X the wheel wells. So there's more to be gained by addressing the rear's drag. Assuming you actually can do something about it.

However the comparison becomes more extreme because you would likely address only the rear wheel wells (due to the fronts needing room for steering).

What do we know about reducing drag at the back end? I know a nice big bulbous taper would be nice. Or use of a wind tunnel. What about us mere mortals who don't want to appear too odd on the road?
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Old 04-18-2007, 11:26 AM   #4
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What do we know about reducing drag at the back end? I know a nice big bulbous taper would be nice. Or use of a wind tunnel. What about us mere mortals who don't want to appear too odd on the road?
Then you mere mortals will get merely mortal mileage Along the lines of extraordinary mileage requires extraordinary measures, if you want to drastically reduce rear drag then a rolling science experiment might be needed. I know I'll receive a lot of snickers and doubletakes when I get my slope back and extension built but I don't care. They aren't paying for my gasoline. Besides, I like being the geek.

John David
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Old 04-18-2007, 11:26 AM   #5
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What do we know about reducing drag at the back end? I know a nice big bulbous taper would be nice. Or use of a wind tunnel. What about us mere mortals who don't want to appear too odd on the road?
Then you mere mortals will get merely mortal mileage Along the lines of extraordinary mileage requires extraordinary measures, if you want to drastically reduce rear drag then a rolling science experiment might be needed. I know I'll receive a lot of snickers and doubletakes when I get my slope back and extension built but I don't care. They aren't paying for my gasoline. Besides, I like being the geek.

John David
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Old 04-18-2007, 11:30 AM   #6
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Then you mere mortals will get merely mortal mileage Along the lines of extraordinary mileage requires extraordinary measures, if you want to drastically reduce rear drag then a rolling science experiment might be needed. I know I'll receive a lot of snickers and doubletakes when I get my slope back and extension built but I don't care. They aren't paying for my gasoline. Besides, I like being the geek.

John David
Ask basjoos about extraordinary measures.....
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Old 04-18-2007, 01:00 PM   #7
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All right, you guys.
You're both extraordinary.

Anyway, my daily driver is a wagon.
How about a spoiler? I've seen them on some current wagons and hatchbacks.

And - If a roof spoiler is effective, then maybe a vertical spoiler would smooth the air flow around the vertical rear corners??
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 04-18-2007, 02:10 PM   #8
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Interesting that the body-rear drag is more than 2.5X the body-front drag.
I wonder if that's still true today.
Yes -- absolutely My professors frequently mention the importance of the back end of structures with respect to aero

Take a cylinder for instance.... The drag force of a cylinder is almost 20X greater than the drag force of a foil - a cylinder front edge with a nice tapered rear

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What do we know about reducing drag at the back end? I know a nice big bulbous taper would be nice. Or use of a wind tunnel.
The wind tunnel will only confirm that your "bulbous" rear end is best.

If you want to see the shape nature intended -- look at the shape of a rain drop. Falling liquids organize themselves into the most efficient configuration (for their viscosity - but that shape still applies).

Ever wonder why the front edge of a tear drop shape is blunt like it is? Because that shape minimizes stagnation pressure (kinetic energy lost to slow down/stop a fluid) by allowing flow to move over it in any direction - so it can find the easiest direction Look at the front end of a Toyota yaris and you'll see a much more "defined" blunt shape Or even look at the front (and rear) end of my avatar

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Anyway, my daily driver is a wagon.
How about a spoiler? I've seen them on some current wagons and hatchbacks.

And - If a roof spoiler is effective, then maybe a vertical spoiler would smooth the air flow around the vertical rear corners??
That's the idea Keep adding little bits and pieces all the way around -- eventually you'll get the teardrop shape :P Spoilers are typically more "acceptable" for other people on the road Following the teardrop shape without completing the tear drop also works in the aero department
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Old 04-18-2007, 07:17 PM   #9
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trebuchet03,
I see you know more about this than I do.

I want to pipedream on this item but I'm still playing catch up on projects I've already started or thought out. Still keeping my eye out for ideas though.

Does somebody make a generic spoiler like some kind of extrusion that you can attach as a trailing edge to any corner?
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 04-18-2007, 07:40 PM   #10
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trebuchet03,
I see you know more about this than I do.
Naw - just passing information along

Quote:
...I'm still playing catch up on projects I've already started or thought out.
I believe that is the problem that all tinkerers have
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