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Old 04-29-2008, 07:58 PM   #1 (permalink)
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Kammbacks & tail aero info

The Swiss car magazine „Automobil Revue“ had an article on aerodynamics:
http://media.automobilrevue.ch/files...widerstand.pdf (pdf, 1.6 MB)

One reason why there’s been stagnation in aerodynamic progress in recent years is that the standard European drving cycle for reporting fuel economy, NEDC (see e.g. http://en.wikipedia.org/wiki/New_European_Driving_Cycle), has an unrealistically low average speed of 33 km/h, so there’s not much incentive for manufacturers to optimize the drag coefficient. Unlike the US EPA tests, NEDC is an artificial cycle not based on actual driving data. As a rule of thumb, on the NEDC every 10% reduction in aerodynamic drag results in 2.5% reduction in fuel consumption. In the real world, the improvement should be more dramatic.

A summary of aerodynamic improvements, expressed as change in drag coefficient, delta Cd (here a semicolon-delimited list), theoretically possible vs. actually realizable:

Measure;Theoretical;Actual
Rear;-0.100;-0.060
Smooth underbody;-0.030;-0.015
Rear diffusor;-0.025;-0.025
Wheels;-0.025;-0.020
Radiator;-0.020;-0.020
A-pillars & mirrors;-0.010;-0.010
Total;-0.210;-0.150

So if you were to max out, say, a VW Golf V, the drag coefficient would drop to (0.32 – 0.15) = 0.17; a 47% reduction!

Here’s a comparison of drag coefficient improvements with various changes to the blunt rear end of a cylindrical body:

The article’s text doesn’t specify, but here’s my take on this drawing: the T-shaped thing is a disk. When mounted too close to the end it has no effect, but placed farther back it improves the Cd by -0.053. Adding yet another, smaller disk farther back further improves the Cd by –0.082. A medium-length Kammback brings –0.099, while a full-length tapered tail (L/D ca. 3) brings –0.105.

In 1999 Morelli & Di Giusto modified a Fiat Punto, adding a Kammback and blower-type rear wheels, calling it the Fluid Tail:

Images of MetroMPG, Basjoos and others come to mind. The duct tape is familiar to all of us. Note that their Kammback has a clear plastic partial end cap. Don’t ask me how they opened the hatch, though.
The rear wheel rims were designed as radial blower fans, pumping air into the area behind the wheel, acting as a sort of air-pillow wheel fairing, as I understand it. Note also the Audi A2-like wheel well bulge (what for?).
Here’s their patent text: http://www.freepatentsonline.com/EP0937633.html .

More tail modifications:

Compared to the standard blunt end (a), adding what they call attikas (fascias or parapets; flat, parallel panels) (b) delays separation and moves the tail vortices back (reduces Cd by up to 10%); these can be set inward (c), further reducing the size of the „dead zone“ (reduces Cd by 0.06, which is about half as good as the ideal tapered tail); the open-ended Kammback made of flat panels (d); and „Fluid Tail" (e).

The French automaker Citroen seems to be dedicated to aerodynamics. The models C1, C2, and C3 either have a body-color above-bumper grill block or no above-bumper grill at all. And the C4 has some serious Kammback action (in person the look is even more dramatic):

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Old 04-29-2008, 09:50 PM   #2 (permalink)
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Thank you for posting this !
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Old 04-29-2008, 09:58 PM   #3 (permalink)
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Too bad it can't be translated.


( I wish I was tought more than one language in school. It's as if we here in America expect everyone to speak our language, but we won't speak theirs. )
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Old 04-30-2008, 09:08 PM   #4 (permalink)
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Quote:
Originally Posted by Bearleener View Post
Note also the Audi A2-like wheel well bulge (what for?).
I have Morelli's SAE paper (2000-01-0491) on this system and in the photo of the final car the bulge doesn't look nearly as pronounced. It looks like there's just enough room to clear the tire. Maybe they changed it or it's an optical illusion. He claims that the sytem saves 5 kW of power and consumes only 240 W, for a ratio of 20/1. Let's see, (5kW-240=4760W) / 746W/hp = 6.38 hp saved at speed. Not bad.
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Old 05-05-2008, 10:18 PM   #5 (permalink)
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I love the DIY kammback picture. Interesting stuff - thanks for posting.
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Old 05-05-2008, 10:53 PM   #6 (permalink)
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c) kind of looks like like my tail, stepped.

nice thing about steps is you can see right out thur them.

Last edited by diesel_john; 05-26-2008 at 11:44 PM..
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Old 05-05-2008, 11:06 PM   #7 (permalink)
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Quote:
( I wish I was tought more than one language in school. It's as if we here in America expect everyone to speak our language, but we won't speak theirs. )
Yeap, that's America for ya.

I wanna try that disk Idea... it'd be a lot easier for me I think.
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Old 05-06-2008, 12:23 PM   #8 (permalink)
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So why can't you put steps on the back window? I brought up this topic before and was shot down due to flow separation. It looks to me that anything that has more than a 12 degree slope should have a step or tail.
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Old 05-06-2008, 01:12 PM   #9 (permalink)
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Quote:
Originally Posted by apgrok1 View Post
So why can't you put steps on the back window? I brought up this topic before and was shot down due to flow separation. It looks to me that anything that has more than a 12 degree slope should have a step or tail.
I completely agree with you. Most cars can benefit from a properly designed lip spoiler / tail, and steps, it's all the same principle.

One thing that is based on the same idea as tail disks someone could try with a pickup is to place vertical plywood sheets every foot that would be shaped like the skeleton of a tonneau cover. At that point you might as well just cover it up, but it would be interesting to see the results.

I thought about the boat tail disk idea a while back when I was reading on the forward facing vertical splitters they put on big rig trailers, and the tailgate up for pickups, which create pockets of air the rest of the flow can slide on.
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Old 05-07-2008, 11:32 AM   #10 (permalink)
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diesel_john, what does your stepped tail look like? How is it fastened/reinforced? Thanks!

Has anyone thought about a retractable Kammback? Sure would be nice if it would fold together at low speeds, reducing vehicle length for easier parking and making it slightly more street-legal.

One possibility would be, say, 3 panels that fold out at the top and sides, perhaps deployed with a pull string arrangement through eyelets at the corners.

Or how about a clear plastic, inflatable, conical bag (something like those temporary bouys used for sailboat regattas), which would roll up into a little box when parked? Maybe you could even use the low-pressure area back there at speed to inflate it. Or exhaust gases at the tailpipe: pump directly to inflate, use a venturi with side tap to create vacuum to deflate.

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Last edited by Bearleener; 05-07-2008 at 11:42 AM.. Reason: Add exhaust idea
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