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Old 05-02-2020, 02:56 AM   #11 (permalink)
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Originally Posted by California98Civic View Post

My coast down method was different than the one you describe. Rather than measuring time between two speeds, I coasted from X speed to a full stop, measuring distance with GPS. But I think I need better GPS. Mine was reporting accuracy of only to within 20 to 50 feet. I was coasting 2600 to 3500 feet to stop, so that's roughly 0.75 to 1.9 % inaccuracy in my calcs from just the GPS. Not awful but not great. What GPS are you using to confirm speed?

Coastdowns should never be done coming to a standstill, in my opinion. Why? Because you are then spending too long at speeds where aero drag is basically not relevant - but it potentially adds to the error.

But I have never had any luck with coast-downs - of any sort. And Rob Palin (ex Tesla aerodynamicist) was scathing about them in a recent email to me.

The GPS I am using to show speed is my car's speedo - MoTeC 10Hz GPS.

On your tests and their results. I'd just call all those designs 'separation edges'. (I think 'box cavities' is another thing - like 'The Template' - that gets a lot of coverage here but doesn't have much evidence for it.)

Top and side separation edges will add a small drag benefit (assuming attached flow to the back of the car) but I very much doubt it's measurable by either a coastdown or my throttle-stop technique. If you wished to, you could directly measure the pressures in the base area of the car (ie the part exposed to the wake) but I doubt there'd be any measurable change, except perhaps in a reduction in suction peaks where the separation edges themselves are changing the flow pattern ie less wraparound.

The dust patterns on the top of the bumper cover are interesting but I cannot draw any conclusions from them.

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