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Old 08-26-2019, 09:11 PM   #11 (permalink)
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Originally Posted by freebeard View Post
The factory wind tunnels vary between themselves by a similar amount.
Thanks for the response.

I still find value in the coast down results. I can use those to compare the next one I do if I change something else on the car. If my speed is above 20 mph at the 70 second mark then I did something right.

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Old 08-26-2019, 11:19 PM   #12 (permalink)
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Agreed. Good for comparisons, but don't get official figure envy.
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Old 08-27-2019, 02:06 PM   #13 (permalink)
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Agreed. Good for comparisons, but don't get official figure envy.
If the variables at the bottom of the speadsheet is changed, shouldn't the CD and CRR numbers change also?
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Old 08-27-2019, 02:18 PM   #14 (permalink)
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I don't know. It would depend on the structure of the spreadsheet and the definitions of the variables.
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Old 08-28-2019, 10:59 AM   #15 (permalink)
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shouldn't

Quote:
Originally Posted by modified metro View Post
If the variables at the bottom of the speadsheet is changed, shouldn't the CD and CRR numbers change also?
Since you're testing on a different day,the only way that you could isolate a difference,is to do back-to-back coastdowns,with and without the mods,so you'd always have a baseline reference from the same day/conditions to compare to.
You could have a dozen unaccounted for variables,any one of,which could affect your coastdown times.It would be impossible to sort it out.
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Old 08-28-2019, 11:22 AM   #16 (permalink)
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Quote:
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Since you're testing on a different day,the only way that you could isolate a difference,is to do back-to-back coastdowns,with and without the mods,so you'd always have a baseline reference from the same day/conditions to compare to.
You could have a dozen unaccounted for variables,any one of,which could affect your coastdown times.It would be impossible to sort it out.
Thank you for the response.

I would totally agree with that. Not only environment changes but the possibility of things beings different on the car, tire air pressure, dragging brakes, wheel bearings, anything can make a difference.
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Old 01-07-2020, 07:50 AM   #17 (permalink)
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Hi everyone.

Can anybody share with me this SAE norm?

Actually I have used the Excel sheet to calculate my values also, but I am not satisfied with the sheet as I find the math in here too basic.

I have done a prototype version of a version (on Scilab) with least square adjustment that would be able to give a more robust answer, with least square adjustment.

At the time, I get 0.590m2 as a CdA for my car, with an estimated accuracy of +/-0.037m2 (1sigma). I did only 4 runs, 2 in each direction.

I am planning to integrate in the model some different values, including slope (this would allow to do the testing on a non-flat road) and wind speed relative to the car direction.

I am happy to share with you the results, and would be happy to get some raw data from you to test on the algorythm.

Best Regards

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