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Old 10-11-2020, 10:38 PM   #11 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
I don't think so.

There's no justification for the 0.19 Cd figure - it's not the result of any measurement and is just a guess. As far as I know, the 7-10 people figure is wrong.

As I say, this is how misinformation starts and then spreads!
The justification is based on the book. Did you read the thread?

Look at the space between the front seat and the reeear seat. This vintage tugboat-looking car looks like it could have a seat added => 7-10 people.

The point: How good is this general shape/strategy? Make a smaller version, streamline the wheels, and maybe you're at 0.20 cD (or lower)? And it looks to have a relatively small frontal area.

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Old 10-11-2020, 10:56 PM   #12 (permalink)
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Originally Posted by sgtlethargic View Post
The justification is based on the book. Did you read the thread?

Look at the space between the front seat and the reeear seat. This vintage tugboat-looking car looks like it could have a seat added => 7-10 people.

The point: How good is this general shape/strategy? Make a smaller version, streamline the wheels, and maybe you're at 0.20 cD (or lower)? And it looks to have a relatively small frontal area.
There is a guess in the book - you seemed to have taken that guess as if it is gospel. And, if the wheels were enclosed by bodywork, how much do you think the frontal area would have grown?

Looking at cars like this as role models for making a modern low drag shape is, I am afraid, just silly. It was a wonderful car 100 years ago.

I do not not know of even one old production car that has a good Cd in the context of 2020 - not when the car has been tested in a modern wind tunnel, anyway. With very few exceptions, even the one-offs and research vehicles look pretty ordinary in a modern context - eg Kamm's only K series car that has been tested in a modern wind tunnel had a Cd of 0.37, the lovely Tatra T87 production car had a Cd of 0.36 - and so on.
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Old 10-12-2020, 12:27 AM   #13 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
There is a guess in the book - you seemed to have taken that guess as if it is gospel.
No, I asked a question about the claim in the book.
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And, if the wheels were enclosed by bodywork, how much do you think the frontal area would have grown?
I'll guess 10%.

Quote:
Looking at cars like this as role models for making a modern low drag shape is, I am afraid, just silly.
I'm looking at aerodynamics and considering strategies.

Why did you put modern in there? Are there no not-modern low drag shapes?

Quote:
I do not not know of even one old production car that has a good Cd in the context of 2020 - not when the car has been tested in a modern wind tunnel, anyway.
I'm not looking for an old production car with a good-for-2020 cD.
Quote:
With very few exceptions, even the one-offs and research vehicles look pretty ordinary in a modern context - eg Kamm's only K series car that has been tested in a modern wind tunnel had a Cd of 0.37, the lovely Tatra T87 production car had a Cd of 0.36 - and so on.
I'm looking for the exceptions, whatever the age, to copy.
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Old 10-12-2020, 01:14 AM   #14 (permalink)
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One of the misconceptions that has developed on this group is that there are plenty of old, low drag cars eg from the 1930s. In fact, when they are tested in modern wind tunnels, I don't know of any at all. (The claims of the time are notoriously optimistic.)

There are old low-drag shapes - but not cars.

Modern cars are the ones to look at if you want to gain lessons in low-drag design - today's cars are lower in drag and lift than any production cars that have come before.

The Tropfenwagen is one of my all-time 'hero' cars but I wouldn't copy any of its technology if I were building a car today.

The Tropfenwagen's Cd was just stunning - for its time. It isn't stunning today. Taking a guess made in a book as to what the Cd would have been if the wheels had been enclosed, and then somehow extrapolating even further as to what total drag would be of a modern version of that car, seems about a useful as doing the same for the design of its 2.58 litre W6 engine.
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Old 10-12-2020, 01:26 AM   #15 (permalink)
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Didn't the Volkhart-Sagitta V2 retest from 0.17 to 0.21 in a modern tunnel?

I gave it my best shot.





This could probably drop the Beetle from ~0.41 to ~0.30. The open bulkhead is at the stock rear bumper location. The solid bulkhead a reasonable truncation*.

The Tropfenwagen had a couple of other things going for it. The wheels and tires are exceedingly thin by modern standards (the chassis exceedingly thick), and the fenders/running boards are well in tune with modern time attack standards.

*That truncation on the left with a Coanda nozzle pumped by engine cooling air. Breers on the right.

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Old 10-12-2020, 01:42 AM   #16 (permalink)
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Originally Posted by JulianEdgar View Post
One of the misconceptions that has developed on this group is that there are plenty of old, low drag cars eg from the 1930s.
I don't recall seeing anything like that, but I hadn't been here for years. Links for a thread or three?
Quote:
In fact, when they are tested in modern wind tunnels, I don't know of any at all. (The claims of the time are notoriously optimistic.)

There are old low-drag shapes - but not cars
Someone in this thread claimed the 1922 Rumpler yielded a 0.28 cD when tested in a modern wind tunnel!
Quote:
Modern cars are the ones to look at if you want to gain lessons in low-drag design - today's cars are lower in drag and lift than any production cars that have come before.
You mention "production" cars quite a bit- I don't know why.

Do you consider the 1954 Fiat Turbina, with its claimed 0.14 cD, to be a modern and/or production car?
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Old 10-12-2020, 02:04 AM   #17 (permalink)
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Originally Posted by sgtlethargic View Post
I don't recall seeing anything like that, but I hadn't been here for years. Links for a thread or three?
Aerohead loves quoting drag figures for old cars - there are probably hundreds of threads where he does it.

Quote:
Someone in this thread claimed the 1922 Rumpler yielded a 0.28 cD when tested in a modern wind tunnel!
That's right - and a Cd of 0.28 is pretty bad in a current context.

Quote:

You mention "production" cars quite a bit- I don't know why.

Do you consider the 1954 Fiat Turbina, with its claimed 0.14 cD, to be a modern and/or production car?
Well, we're talking about legal road cars, aren't we? I talk about a 'production car' to differentiate it from a concept car, a research car, a solar car, a one-off special, etc, etc.

The Fiat was a one-off research vehicle and didn't have a Cd of 0.14.

"That its drag would be low was suggested by tests of a one-fifth-size model in the Turin Polytechnic's tunnel that showed a drag coefficient of only Cd=0.14."

A model of the car had a Cd of 0.14 - see how carefully the writer covers themselves!
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Old 10-12-2020, 02:23 AM   #18 (permalink)
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If it doesn't matter if it is legal / a production car / etc, have a look at the Ford Probe concept cars for proper passenger-carrying cars with very low drag. I like this one particularly:



And I think its 0.153 Cd and 1.904m^2 frontal area are believable. (See SAE paper 831000)
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Old 10-12-2020, 02:48 AM   #19 (permalink)
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Originally Posted by JulianEdgar View Post
That's right - and a Cd of 0.28 is pretty bad in a current context.
How so, when the Gen1 Honda Insight is 0.25?
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Well, we're talking about legal road cars, aren't we? I talk about a 'production car' to differentiate it from a concept car, a research car, a solar car, a one-off special, etc, etc.
For me, the idea is to have a street-legal car that's likely patterned after one of the said types of cars that has a verified drag coefficient that's lower than any production cars'.
Quote:
The Fiat was a one-off research vehicle and didn't have a Cd of 0.14.

"That its drag would be low was suggested by tests of a one-fifth-size model in the Turin Polytechnic's tunnel that showed a drag coefficient of only Cd=0.14."
Would you say the car necessarily has a higher drag coefficient? I could see a scale model missing some drag that the fully functioning car has. But are scale models inaccurate due to size?

The Wikipedia page claimed it was the lowest drag car for 30 years. It seems that low number would've been challenged in that time, but then again, maybe not.
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Old 10-12-2020, 03:02 AM   #20 (permalink)
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I don't recall seeing anything like that, but I hadn't been here for years. Links for a thread or three?
Search terms: Jaray, zeppelin, Koenig-Fachsenfeld, etc.



The FIAT Turbina is a good call. A 'productized' version of the Redhead or Bluebird-style Bonneville cars.



https://www.jalopyjournal.com/forum/...1119314/page-3

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