Coast down testing

[modified metro]

Originally Posted by freebeard

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.

[freebeard]

Agreed. Good for comparisons, but don't get official figure envy.

[modified metro]

Originally Posted by freebeard

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?

[freebeard]

I don't know. It would depend on the structure of the spreadsheet and the definitions of the variables.

[aerohead]

Originally Posted by modified metro

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.

[modified metro]

Originally Posted by aerohead

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.

[ptitviet]

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