Quote:
Originally Posted by Nautilus
I've also used the downloadable Excel Drag Coefficient calculator.
Frontal area for a Mk I Seat Leon has been published as 2.011, 2.20 or 2.28 sq m. Used an average, 2.16 sq m.
Weight of the car, with all bits and bobs as of Oct 2020, falls around 1364kg. Added driver weight and fuel weight (gasoline density average 0.748 kg/liter) for a total weight of 1471kg.
Tire rolling resistance for passenger car tires may be from 0.007 to 0.015. Tires were rated "E" in fuel economy, so I've assumed the worst and used 0.015.
After filling in the speeds with car coasting down in neutral, Excel formula gave a Cd figure of 0.29 (actually 0.290031, but sixth decimal place figures are too small to matter in Real Life).
As we can see, I had been too optimistic pulling a 0.28 figure from the hat, but my labors over the last few years had not been in vain.
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I think that coast-down tests are completely inaccurate - it's one of the two things that Aerohead and I agree on.
Putting invalid data into an Excel spreadsheet suddenly doesn't make the data correct.
At minimum do some testing windows up / windows down and see if the difference makes sense in terms of calculated drag. It doesn't when I do this with multiple runs and averaged results.