Decimal places

[freebeard]

Compounded error bars?

[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.

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.

[aerohead]

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.

[Piotrsko]

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.

[aerohead]

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?

[AeroMcAeroFace]

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.

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.

[freebeard]

Thanks. I gave it a shot at Permalink #2.

[aerohead]

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

[aerohead]

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.

please see my # 28 permalink below

[AeroMcAeroFace]

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.

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.

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