EcoModder Forum Decimal places

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 04-21-2021, 08:57 PM #21 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 18,936 Thanks: 5,271 Thanked 6,259 Times in 5,015 Posts Compounded error bars? __________________ . Cold hearted orb that rules the night, Removes the colours from our sight, Red is grey and yellow white, But we decide which is right, And which is an illusion. The Day Begins -- Moody Blues_________________
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Margin of error

Car manufacturers almost always release drag coefficient data rounded to two decimal places, so the 0.31 is actually 0.31 +/- 0.005.

Maximum possible value:
0.315x0.937=0.295155

Minimum possible value:
0.305x0.937=0.285785

The error in the measurements even after the 6.3% drag decrease is thus 0.01, there is no way that 6 decimal places is in any way scientifically appropriate.

It is completely irrelevant whether it is 6%, 6.3% or 6.3000%, the margin of error from the original Cd is way too large to get anything remotely accurate.

Quote:
 derived from this literature might offend only those who've never attempted the voyage.
I think the people it may offend are scientist and engineers who know the limitations of margin of error.

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everyone else

Quote:
 Originally Posted by JulianEdgar Um, to you it might need psychic powers. To everyone else it's pretty obvious that the change in drag wasn't 6.300000%.
Speculation?
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Here's an example of a reverse-engineering challenge involving Tesla's Model S
* Julian Edgar reports that the Model S has a 8% cooling-drag system.
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The Tesla Model S has been reported with:
Cd 0.26
Cd 0.247
Cd 0.24
Cd 0.225
Cd 0.208
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SAE 2012-01-0178 provides us with a frontal area of 2.4-meters-squared ( 25.8333 sq-ft )
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This gives us a range for the Model S drag factor, from CdA 6.7166 sq-ft ( 0.6239 meters-squared ), to, 5.3733 sq-ft ( 0.49919 meters-square )
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This range offers a spectrum of at least five-different cooling-system drags.
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Since Julian Edgar has not provided any specificity as to what ' 8% ' actually means, ' 8%' remains an unknown quantity to any of the 136,000 EcoModder.com members who might wish to 'engineer' cooling systems.
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I'm often uncertain what underlies the motivation for some of Julian Edgar's posts. Certainly he must understand that members actually have a desire to 'engineer.'
And since the 'signal-to-noise ratio' of some mods complicates the ability to even discern an actual 'change' in drag, the need of highest specificity cannot be overemphasized.
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Quote:
 Originally Posted by AeroMcAeroFace Car manufacturers almost always release drag coefficient data rounded to two decimal places, so the 0.31 is actually 0.31 +/- 0.005. Maximum possible value: 0.315x0.937=0.295155 Minimum possible value: 0.305x0.937=0.285785 .
If someone publishes a number, it is universally agreed that number is the observed data point. What you should have said is that actually the number observed could be between these two points, which would be odd unless the sensors were not calibrated to a standard.

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scientifically appropriate

Quote:
 Originally Posted by AeroMcAeroFace Car manufacturers almost always release drag coefficient data rounded to two decimal places, so the 0.31 is actually 0.31 +/- 0.005. Maximum possible value: 0.315x0.937=0.295155 Minimum possible value: 0.305x0.937=0.285785 The error in the measurements even after the 6.3% drag decrease is thus 0.01, there is no way that 6 decimal places is in any way scientifically appropriate. It is completely irrelevant whether it is 6%, 6.3% or 6.3000%, the margin of error from the original Cd is way too large to get anything remotely accurate. I think the people it may offend are scientist and engineers who know the limitations of margin of error.
Probably depends upon the type of science one is doing.
When I'm 'noodling', I simply leave whatever is on the display, or move it into memory, for future re-insertion, for the duration of the calculations.
It would be 'more' work, and less time-efficient to do otherwise. Not to mention accuracy issues.
When I began formal studies, rolling force coefficients for tires easily ran to ten decimal places. Observed wind tunnel values, up to 4-places.
If you're looking for that 0.005 difference from a radiator shutter, you want as 'tight' an accounting as possible.
You know that it requires extremely advanced statistical analysis tools to even identify some 'trends' which are under scientific examination?
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Quote:
 Originally Posted by aerohead Probably depends upon the type of science one is doing. When I'm 'noodling', I simply leave whatever is on the display, or move it into memory, for future re-insertion, for the duration of the calculations. It would be 'more' work, and less time-efficient to do otherwise. Not to mention accuracy issues. When I began formal studies, rolling force coefficients for tires easily ran to ten decimal places. Observed wind tunnel values, up to 4-places. If you're looking for that 0.005 difference from a radiator shutter, you want as 'tight' an accounting as possible. You know that it requires extremely advanced statistical analysis tools to even identify some 'trends' which are under scientific examination?
I have no doubt that the measured value in the wind tunnel was to 4 or even 5 decimal places. However, the published number was to two decimal places.

Agreed, on the tiny change from a radiator shutter, they will do measurements down to 3,4 or 5 decimal places. However, that doesn't necessarily reflect in the published data, if the cD was actually 0.304 and then a radiator shutter was added that reduced the drag to an actual 0.299, the published number is still going to be 0.30.

The difference may be 1.6%, but it would be erroneous to claim that because the initial figure was 0.30 and there is a 1.6% reduction, that the new drag coefficient is 0.295.

Because as we see in the above example the actual figure is 0.299.

Quote:
 What you should have said is that actually the number observed could be between these two points, which would be odd unless the sensors were not calibrated to a standard.
You are confusing accuracy of measurements with precision, read about it here https://en.wikipedia.org/wiki/Accuracy_and_precision.

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 04-23-2021, 12:21 PM #27 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 18,936 Thanks: 5,271 Thanked 6,259 Times in 5,015 Posts Thanks. I gave it a shot at Permalink #2. __________________ . Cold hearted orb that rules the night, Removes the colours from our sight, Red is grey and yellow white, But we decide which is right, And which is an illusion. The Day Begins -- Moody Blues_________________
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 04-23-2021, 12:27 PM #28 (permalink) Master EcoModder     Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 12,460 Thanks: 20,041 Thanked 6,224 Times in 3,843 Posts decimal places Here are some automotive entities who publish drag coefficients to three decimal places: Aston Martin Audi BMW Bochum University CALTY Cambridge University Chrysler Citroen Coventry University Robert Cumberford Daihatsu Daimler-Benz Mercedes-Benz Gary Eaker, A2 / Aerodyne Eindhoven Technical University Ferrari FIAT Ford Fuji Heavy Industries ( Subaru ) G.A.C. GALCIT ( Cal Tech ) General Motors Holden Honda Hoxan Hyundai Ital Design JEEP Konigsegg Lotus MIRA Mazda NASA Nissan Opel Penske Racing Peugeot Pininfarina Polestar Porsche Renault Shelby Super Cars ( SSC ) Tatra Tesla Trabant Triumph Toyota Antoine Volanis Volkswagen __________________ Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
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Quote:
 Originally Posted by AeroMcAeroFace I have no doubt that the measured value in the wind tunnel was to 4 or even 5 decimal places. However, the published number was to two decimal places. Agreed, on the tiny change from a radiator shutter, they will do measurements down to 3,4 or 5 decimal places. However, that doesn't necessarily reflect in the published data, if the cD was actually 0.304 and then a radiator shutter was added that reduced the drag to an actual 0.299, the published number is still going to be 0.30. The difference may be 1.6%, but it would be erroneous to claim that because the initial figure was 0.30 and there is a 1.6% reduction, that the new drag coefficient is 0.295. Because as we see in the above example the actual figure is 0.299. You are confusing accuracy of measurements with precision, read about it here https://en.wikipedia.org/wiki/Accuracy_and_precision.
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If the original data is three decimal places, then there is no issue deriving data to a higher level of precision (than you could with two decimal places), but even then it is still +/- 0.0005 and so no further precision can be gained.

Quote:
 Thanks. I gave it a shot at Permalink #2.
Indeed you did, I didn't re-read the thread. It is very important and one many people on here don't fully understand, see below for one example, perhaps unfairly cherry picked, but certainly, and unfortunately, representative of many people here on ecomodder.

Quote:
 I simply stated that, in order to satisfy 6.3% higher drag than the value which was provided, required 6-decimal place accuracy.

Last edited by AeroMcAeroFace; 04-23-2021 at 01:13 PM.. Reason: added item in brackets for clarity

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