Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 05-02-2020, 01:56 AM   #11 (permalink)
Banned
 
Join Date: Nov 2017
Location: Australia
Posts: 2,060
Thanks: 107
Thanked 1,605 Times in 1,136 Posts
Quote:
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?

[/url]
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.

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 05-02-2020, 10:59 AM   #12 (permalink)
Cyborg ECU
 
California98Civic's Avatar
 
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,299

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)
Thanks: 2,373
Thanked 2,172 Times in 1,469 Posts
Quote:
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..
  Reply With Quote
Old 05-02-2020, 11:26 AM   #13 (permalink)
Banned
 
Join Date: Nov 2017
Location: Australia
Posts: 2,060
Thanks: 107
Thanked 1,605 Times in 1,136 Posts
Quote:
Originally Posted by California98Civic View Post
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.)

Last edited by JulianEdgar; 05-02-2020 at 05:50 PM..
  Reply With Quote
The Following User Says Thank You to JulianEdgar For This Useful Post:
aerohead (06-17-2020)
Old 05-03-2020, 01:55 AM   #14 (permalink)
Banned
 
Join Date: Nov 2017
Location: Australia
Posts: 2,060
Thanks: 107
Thanked 1,605 Times in 1,136 Posts
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).

  Reply With Quote
Old 05-03-2020, 01:47 PM   #15 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 27,561
Thanks: 7,736
Thanked 8,554 Times in 7,041 Posts
Quote:
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.
Quote:
And what on earth does “capturing vortices better” mean?
Reattachment.
__________________
.
.
Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster

____________________
  Reply With Quote
The Following User Says Thank You to freebeard For This Useful Post:
aerohead (06-17-2020)
Old 05-03-2020, 02:48 PM   #16 (permalink)
Master EcoModder
 
nemo's Avatar
 
Join Date: Nov 2010
Location: US
Posts: 1,015

Chief - '06 Pontiac Grand Prix
90 day: 26.7 mpg (US)

SF1 - '12 Ford Fiesta S
90 day: 30.95 mpg (US)
Thanks: 195
Thanked 247 Times in 190 Posts
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.
  Reply With Quote
The Following User Says Thank You to nemo For This Useful Post:
aerohead (06-17-2020)
Old 05-03-2020, 05:57 PM   #17 (permalink)
Banned
 
Join Date: Nov 2017
Location: Australia
Posts: 2,060
Thanks: 107
Thanked 1,605 Times in 1,136 Posts
Quote:
Originally Posted by freebeard View Post
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!
  Reply With Quote
Old 05-03-2020, 06:01 PM   #18 (permalink)
Banned
 
Join Date: Nov 2017
Location: Australia
Posts: 2,060
Thanks: 107
Thanked 1,605 Times in 1,136 Posts
Quote:
Originally Posted by nemo View Post
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.

Last edited by JulianEdgar; 05-03-2020 at 06:47 PM..
  Reply With Quote
The Following 2 Users Say Thank You to JulianEdgar For This Useful Post:
aerohead (06-17-2020), nemo (06-19-2020)
Old 05-03-2020, 08:15 PM   #19 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 27,561
Thanks: 7,736
Thanked 8,554 Times in 7,041 Posts
Quote:
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:

Quote:
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.
__________________
.
.
Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster

____________________
  Reply With Quote
The Following 3 Users Say Thank You to freebeard For This Useful Post:
aerohead (06-17-2020), California98Civic (05-04-2020), JulianEdgar (05-03-2020)
Old 05-04-2020, 12:01 AM   #20 (permalink)
Master EcoModder
 
Join Date: Dec 2016
Location: Florida
Posts: 380
Thanks: 64
Thanked 160 Times in 135 Posts
Quote:
Originally Posted by JulianEdgar View Post
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.

  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com