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Old 03-31-2008, 10:46 AM   #1 (permalink)
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Measure the drag coefficient of your car

iwilltry has write a tool, for measure the drag coefficient of cars.
available at http://www.instructables.com/id/Meas...t-of-your-car/

After to have watched this one, I have modified it in order to greater precision.
The change, it is in the air density parameter: in origin, this value was fixed to 1.22 Kg/m^3, now, it is calculated in order of air density based on the climatic condition, and isnt' required to adjust it for altitude

for example, now, in Rome, with 15C temperature, 48% of humidity and 1012mBar pressure, the air density is of 1.1735Kg/m^3.

New version, is avaible :http://forum.ecomodder.com/attachmen...1&d=1206974721

Attached Files
File Type: zip stima cx fabrio.zip (82.1 KB, 360 views)
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Old 03-31-2008, 12:33 PM   #2 (permalink)
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no comment?
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Old 04-05-2008, 06:37 PM   #3 (permalink)
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I am thinking: but the inertia moment of the wheels?
and the gearbox (or only the differential) losses?
The inertia wheels, isnt' a big problem: it is possible to add an ipotetical additional mass at the car, this system, translate up the total curve of deceleration.
But the losses from gearbox?
I have serch in the internet for to look the course of curve of loss for to know if it is liear or exponential.

ah, after indagation, I discovered that the rolling resistence isnt' constant, but in functio of speed:
Rr= F+K*Speed
F= Crr*Mass
K=???
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Old 04-05-2008, 08:39 PM   #4 (permalink)
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I've seen this spreadsheet before, but haven't used it.

I think AndrewJ used it to estimate the change in Cd of his Civic.
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Old 04-05-2008, 08:45 PM   #5 (permalink)
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Yep, I used it to calculate my Cd a few times.

I thought about changing the air density value, but I looked it up and it's based at sea level, Since I'm right at sea level, I didn't mess with it.
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Old 04-05-2008, 09:29 PM   #6 (permalink)
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Not 100% sure, but air density values pulled out of books tend to be at STP. Standard temperature and pressure are not common values one would see at sea level. For example, I believe standard temperature is 0C...

I don't know how big a difference the true air density would make on Cd accuracy, but if the value was pulled out of a book it is probably off.

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Old 04-05-2008, 09:32 PM   #7 (permalink)
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Quote:
Originally Posted by LostCause View Post
Not 100% sure, but air density values pulled out of books tend to be at STP. Standard temperature and pressure are not common values one would see at sea level. For example, I believe standard temperature is 0C...

I don't know how big a difference the true air density would make on Cd accuracy, but if the value was pulled out of a book it is probably off.

- LostCause
In the aviation world standard temperature and pressure is 29.92 @15 C at seal level.
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Old 04-05-2008, 09:52 PM   #8 (permalink)
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Quote:
Originally Posted by Lazarus View Post
In the aviation world standard temperature and pressure is 29.92 @15 C at seal level.
Well, in my world...

You are right, but the idea of inaccuracy is still there (albeit, lessened...).

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Old 04-05-2008, 09:55 PM   #9 (permalink)
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Quote:
Originally Posted by Lazarus View Post
....at seal level.
I imagine that's usually pretty close to sea level?
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Old 04-06-2008, 04:02 PM   #10 (permalink)
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boys, I have big esteem for you, but if you answer therefore, I take care myself !

the file "stima Cx fabrio.zip" is a my modified version from original by Iwilltry.
Added new function for evaluating correct air density it is an improvement.
You could try sone variation in climatic condition for know itse impact.
This is, argument of my first post and you answers only after my second post when, I have exposed a new problem.
I must cry or laugh?

ah, Basjous has extimated drag coefficient of aerocivic using the original spreadsheet

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