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Old 10-11-2017, 04:20 PM   #76 (permalink)
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Blackfly - '98 Geo Metro
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Coastdown results: good news, bad news

Well, I spent over 3 hours gathering & analysing coastdown test numbers yesterday & today... And I've got good news and bad news.

Bad news first: the data were too noisy to conclude ANYTHING about either the skirts OR this fancy "prototype" decklid extension/spoiler (and little side spoilers):

Here are the variations tested, 3 runs per:

1) top up, windows closed, skirts on, rear spoiler on

The rest were all with the top & windows down (except windows up for one run, where noted):

2) skirts, rear spoiler
3) skirts only (removed spoiler)
4) skirts, windows UP
5) removed skirts (windows down again)

The differences between runs 2, 3 & 5 were smaller than the deviation within those sets. So they unfortunately showed nothing conclusive.

But there's some good news. A couple of things emerged conclusively above the noise, one of which we already knew, and one that I wasn't sure about.

The one we already knew: the best aero was with the top up. Yes, even that lumpy, bumpy top dragging a separation bubble behind it. This is well known in the Miata racing community (and fitting a hard top is even better than running with the soft top up).

The other thing that stood out above the noise was the answer to this question: is it more aerodynamic to have the side windows up or down when the top is open?

Answer = windows up is better.

If I wanted to be optimistic about the skirts & the spoilers, I'd look at single worst (highest drag) result of all yesterday's runs was in the last set with everything removed from the car. But the amount of deviation of that whole set really makes it impossible to say that conclusively.

Need moar datas!

I really need to re-test. More runs per set. Or get a wind tunnel. I dunno if I'll get to it this year or not -- I'm running out of time. Making stuff is fun. Actually figuring out if that stuff helps, hurts or makes no difference is time consuming and sometimes frustrating.
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