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Old 09-04-2010, 02:06 PM   #31 (permalink)
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Neil,

I might be mis-remembering but the boxfish work I read about said that the Cd of .095 was for just moving the ideal shape close to the ground. Wheels made the Cd come up quite a lot more.

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Old 09-04-2010, 02:12 PM   #32 (permalink)
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Neil,

I want to also say that this seems like a very good way to approach the problem, even though we might differ a little on the numbers.
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Old 09-04-2010, 03:28 PM   #33 (permalink)
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Hi Jim,

I'm pretty sure that the free space number on the boxfish shape was 0.06 and the 0.095 number applies to the blue model with wheels. There is a later blue model with open wheels (no mention of it's Cd) and the final "real" Bionic car with diesel engine and open wheels (partial rear skirts only) with a Cd of 0.19.

The 0.095 is the lowest number I have ever seen for a wheeled car -- I have confirmed that I am remembering it correctly. Since it already has wheels, that issue is removed... Unfortunately, that is the only image of the model I have ever found, so extrapolating all the key bits is not easy.

And as I said, since the section is basically square-ish, the volume inside is very practical and usable.
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Old 09-04-2010, 04:35 PM   #34 (permalink)
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OT - here's a video on the Bionic Car:
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Old 09-04-2010, 06:02 PM   #35 (permalink)
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...cute illustration.
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Old 09-04-2010, 09:16 PM   #36 (permalink)
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Patrick, you are a scholar and a gentleman! That video had an all too brief glimpse of the earliest Bionic/Boxfish clay model, and I've done a series of screen captures (low rez but much better than nothing!), and posted them here:

http://ecomodder.com/forum/showthrea...tml#post192455
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Old 09-05-2010, 10:10 AM   #37 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hi Jim,

I'm pretty sure that the free space number on the boxfish shape was 0.06 and the 0.095 number applies to the blue model with wheels. There is a later blue model with open wheels (no mention of it's Cd) and the final "real" Bionic car with diesel engine and open wheels (partial rear skirts only) with a Cd of 0.19.

The 0.095 is the lowest number I have ever seen for a wheeled car -- I have confirmed that I am remembering it correctly. Since it already has wheels, that issue is removed... Unfortunately, that is the only image of the model I have ever found, so extrapolating all the key bits is not easy.

And as I said, since the section is basically square-ish, the volume inside is very practical and usable.
You can't compare the Cd of full-size cars against clay models. In the translation from model to reality, M-B added 0.10 to the Cd. Some of that was from altering the basic body shape for more usable interior space, but they also added panel gaps and dozens of necessary features like door handles and brakes.

Anyway, you've seen lower drag shapes: the Goldenrod salt flats car, most EcoMarathon cars, some early 80's concepts from Detroit, and probably even the AeroCivic and Dolphin.
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Old 09-05-2010, 01:40 PM   #38 (permalink)
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Hi Matt,

Mercedes may have accounted for the scale difference/Reynolds Number by increasing the velocity; we don't know? I tend to think that they know enough to have the right number. The final Bionic is a real car with a Cd of 0.19, and the progression up from the fish, fish model, car model, to the built car, all fit.

The Goldenrod has a Cd of 0.11-0.12. You could be right about the Dolphin, but the AeroCivic is ~0.17.
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Old 09-05-2010, 04:47 PM   #39 (permalink)
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Neil and all,

I like to think it is possible to get beyond the limits of the standard car. It will never be possible to actualy achieve .04, but that should be the target.

Daimler Benz started out with the boxfish, but then reality set in, which means the bankers took control. And of course MB is not willing to break out of the 4 wheeled box that has been set as the standard for motor vehicles.

If a car could actually have an overall Cd of .07 and a frontal area half that of the conventional car with double wide seating, we might see energy needed for cars cut by 75% or so. That would start to change things.

It can be fun to put different moldings on production cars, and some stuff can be learned, but for real innovation we need to really think about getting free of the ground effect and conventional wheel arrangements.
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Old 09-05-2010, 11:38 PM   #40 (permalink)
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Jim,

I truly think that 0.1 is the lower limit for a street going car; no matter how many seats it might have. Even the 0.095 car had no cooling or circulation intakes. The best Aptera did was 0.11.

I just don't think you can take a Cd 0.04-0.06 "ideal shape" and put wheels on it, and put the wheels on the ground, and do it without hitting 0.1 or so. It just can't be physically done.

The fact that the 0.095 car has wheels and is that low Cd is amazing enough. The wheel skirts are unrealistically low clearance -- change that just a smidge, and you're up over 0.1.

I think it is pretty cool that nature and evolution has fine tuned things so well. Also, check out the structural "innovation" of the boxfish; with it's hex structure within the skin.

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