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Old 05-02-2020, 10:59 AM   #12 (permalink)
Cyborg ECU
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Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,272

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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Originally Posted by JulianEdgar View Post
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.
I like this test idea you've had and developed and thanks for posting it here.

"The Template" doesn't have much evidence for it? Have you read the dedicated "thread" templates here? Have you read the studies or rear box cavities? Some of that research has been posted here before, too. Use the search function to find it. I see elements of box cavity research incorporated into low drag road vehicles these days, such as the Prius. I think your dismissal that these are things that "get a lot of coverage here" at EM but don't "have much evidence" is not correct.

The flat decklid spoiler is not just for the attached flow at the separation point. It is also intended to capture vortices better, and the test results from El Mirage suggested the mod had an effect on my vehicle. The test was not precise enough for some conclusions, but the observed effects were quite a lot greater than the noise of 0.75-1.9%.

Now, what could be fun this summer, if I get the chance, would be a combination of the throttle stop and coast down to zero method. El Mirage offers 1.5 miles of open flat "road" ... I could observe a throttle stopped max speed, quickly return to a predetermined speed, and then coast down to zero in each pass.
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

Last edited by California98Civic; 05-02-2020 at 11:14 AM..
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