My new way of measuring on-road drag

[JulianEdgar]

Originally Posted by California98Civic

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?

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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.

[California98Civic]

Originally Posted by JulianEdgar

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.

[JulianEdgar]

Originally Posted by California98Civic

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.

Rather than my having to wade through lots of anecdote-style evidence about The Template and box cavities, can you just direct me to some current peer reviewed tech papers (or published textbook material) on them? And what on earth does “capturing vortices better” mean?

(And I have had a look on here for the source material that supports The Template and box cavities but cannot find it. I don't remember either a 'perfect shape style template' or 'box cavities' being mentioned in any of my car aero textbooks - but I am very happy to follow up if someone can nominate some references.)

[JulianEdgar]

Not strictly on using the throttle-stop method, but this testing from today shows why I won't be testing rear diffuser strakes, side skirts or underfloor vortex generators when I do the throttle stop testing (probably tomorrow).

https://www.youtube.com/watch?v=mHrDp0PJhrs

[freebeard]

Rather than my having to wade through lots of anecdote-style evidence about The Template and box cavities, can you just direct me to some current peer reviewed tech papers (or published textbook material) on them?

From first principles: The Holy Template, based on a half-circular cross section, is commonly applied to vehicles with a rectangular cross section. Mair is more useful.

And what on earth does “capturing vortices better” mean?

Reattachment.

[nemo]

Just some thought, Ideas and questions.

Pot for throttle on fly by wire cars.

Would engine load be a reliable indicator of drag?

Wouldn't you want to testing drag and downforce at the same time if your goal is FE. One can certainly create downforce at the cost of drag.

[JulianEdgar]

Originally Posted by freebeard

From first principles: The Holy Template, based on a half-circular cross section, is commonly applied to vehicles with a rectangular cross section. Mair is more useful.

Reattachment.

I am not sure I understand you, but whenever anyone says anything in automotive engineering like 'it's from first principles' as a justification, I worry. To me, it's a bit like saying I am doing engine management tuning from first principles - stoichiometric, flame front speed and Ohms Law. It doesn't take you far in the real world of practical tuning!

[JulianEdgar]

Originally Posted by nemo

Just some thought, Ideas and questions.

Pot for throttle on fly by wire cars.

Would engine load be a reliable indicator of drag?

Wouldn't you want to testing drag and downforce at the same time if your goal is FE. One can certainly create downforce at the cost of drag.

I thought of electronic throttle control (and diesel engine management, often with no throttle) when I was developing the idea. In both cases, the accelerator pedal position determines, for a given rpm, the torque output. So, no problem.

There is no need to test for downforce at the same time: either lift or downforce will cause induced drag, and so measuring drag already includes it.

Of course, you can choose to measure lift/downforce alone if you wish to, and that can be done quite accurately and fairly easily on the road.

[freebeard]

I am not sure I understand you...

You may no be alone.

I'm not sure (obviously) how to put it in words, but it is the mind-set that had Elon Musk produce the Cybertruck. Buckminster Fuller did as well as anyone:

The wellspring of reality is the family of weightless generalized principles.

Lack of knowledge concerning all the factors and the failure to include them in our integral imposes false conclusions.

There is an inherently minimum set of essential concepts and current information, cognizance of which could lead to our operating our planet Earth to the lasting satisfaction and health of all humanity.

All I was saying was that that Template is not always fit for purpose.

[j-c-c]

Originally Posted by JulianEdgar

Not strictly on using the throttle-stop method, but this testing from today shows why I won't be testing rear diffuser strakes, side skirts or underfloor vortex generators when I do the throttle stop testing (probably tomorrow).

I didn't see enough info in the the video to agree with the stated conclusion of the test.

For example, the strakes seemed to have much wider spacing then is the norm, I am not convinced measuring pressure at a single point is reliable to ascertain what I think is really happening with the entire system, which is the goal (?), rather then a single point, and the two PW taped squares purpose was not explained. My assumption, they were the bases that held the strakes being tested, so they had an unknown thickness, and they likely disrupted the under airflow, like a spoiler would, and to what to degree, I don't know, but a test with no strakes and PW only, would have resolved that question, for that single point of air pressure test measurement.