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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 04-09-2008, 10:29 AM   #11 (permalink)
Batman Junior
 
MetroMPG's Avatar
 
Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,513

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 52.71 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)

Appliance car - '14 Mitsubishi Mirage ES (base)
90 day: 60.16 mpg (US)
Thanks: 4,058
Thanked 6,957 Times in 3,602 Posts
So a big part of the "it depends" issue would seem to be quantifying what counts as a "smooth" vs. "rough" underbody.

Based on the 5 production examples I linked to above, it would seem those vehicles may fall into the "rough" underbody class, since lowering aided each one.

Also, we can't forget that setting ride height optimally for miniumum drag in production vehicles is probably in competition with practical considerations (e.g. speed bumps), and handling considerations.

__________________
Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown



EcoModder
has launched a forum for the efficient new Mitsubishi Mirage
www.MetroMPG.com - fuel efficiency info for Geo Metro owners
www.ForkenSwift.com - electric car conversion on a beer budget
  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 04-09-2008, 11:12 AM   #12 (permalink)
A madman
 
brucey's Avatar
 
Join Date: Mar 2008
Location: WV
Posts: 1,018

Pequod - '17 Subaru Outback
90 day: 22.79 mpg (US)
Thanks: 73
Thanked 183 Times in 98 Posts
Send a message via AIM to brucey
Sooo, putting a smooth undertray on lesbaru (9 inches under the lowest part) might actually hurt drag? Seems like its worth testing.
  Reply With Quote
Old 04-09-2008, 12:42 PM   #13 (permalink)
Banned
 
Join Date: Feb 2008
Location: california
Posts: 1,329
Thanks: 24
Thanked 161 Times in 107 Posts
Everybody missed the point. Lowering the car reduces the frontal area by hiding more of the tires inside wheel wells. Read "under car aerodynamics, part 1" on autospeed. Lowering the car at highway speeds is a trick used by luxury car makers all the time.
  Reply With Quote
Old 04-09-2008, 01:37 PM   #14 (permalink)
EcoModding Lurker
 
Join Date: Apr 2008
Location: SF Bay Area
Posts: 35
Thanks: 0
Thanked 3 Times in 3 Posts
tjts1,

With all due respect (and speaking only for myself), I do in fact realize that lowering the ride height does hide more of the wheel. However, as I've said above, there is a lot going on under the car and I think people like Hucho and others who have actually *measured* the Cd for various ride heights probably know what they're talking about.

As I said above, many times the underlying assumptions are not stated explicitly, so it is very hard for us novices (and I'm assuming none of us are professionals in the auto aerodynamics field) to make judgments about the conditions under which a particular change produces a particular result.

As before, my plea is mostly for additional research into the literature to try to find out what's really going on. Speaking of which, I am not sure which autospeed article you are referring to. I've read all the articles I could find on that site and none (that I found) were entitled "under car aerodynamics, part 1". I did find (and quickly reread) "Modifying undercar airflow" parts 1 and 2 and saw nothing in there that referred to ride height (though maybe on my quick re-read I missed it). Got a link?

--Steve
  Reply With Quote
Old 04-09-2008, 01:38 PM   #15 (permalink)
Master EcoModder
 
tasdrouille's Avatar
 
Join Date: Jan 2008
Location: Mirabel, QC
Posts: 1,672

The Guzzler - '08 Hyundai Elantra GL
90 day: 33.12 mpg (US)

Got Soul? - '11 Kia Soul 2U
Thanks: 35
Thanked 86 Times in 57 Posts
The rougher the underside of the vehicule, the more benefit there will be from a reduction in ride height. On the other hand, if the underside of the vehicule is a very good aerodynamic shape, increasing ride height will reduce the drag coefficient. If I remember right, an airfoil can have a Cd of roughly .05 up high in the air. Bring this airfoil in ground proximity and the Cd now jumps to .1 or the like due to ground effect. There is a reference of this in Hucho's book. The aptera is also a good example of this IMHO.

Generally, your average car will benefit from ride height reduction.

The frontal area argument is not really relevant. Say you drop 2 inches in a 22 square feet car, the net result is a reduction in frontal area of roughly 0.75%.
__________________



www.HyperKilometreur.com - Quand chaque goutte compte...
  Reply With Quote
Old 04-09-2008, 01:41 PM   #16 (permalink)
Cd
Ultimate Fail
 
Cd's Avatar
 
Join Date: Feb 2008
Location: Austin,Texas
Posts: 3,585
Thanks: 2,872
Thanked 1,121 Times in 679 Posts
It sure helped for the Ford Fusion Bonneville team.

( Plus, it makes the car look cool ! )
  Reply With Quote
Old 04-09-2008, 03:22 PM   #17 (permalink)
EcoModding Lurker
 
Join Date: Apr 2008
Location: SF Bay Area
Posts: 35
Thanks: 0
Thanked 3 Times in 3 Posts
Re the salt flats racing, here is an excellent example of what I am referring to when I mention "unstated assumptions". It may well be that Hucho et al are assuming "normal highway speeds" (or maybe even "normal around-town speeds") whereas the folks out on the salt flats are running at much higher speeds and may therefore be getting different results.

However, the racing guys will often do things that increase drag in order to get better results in other areas like handling and traction so they may be lowering the car for reasons other than drag--reasons that would have nothing to do with improving mileage figures.

--Steve
  Reply With Quote
Old 04-09-2008, 04:39 PM   #18 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 15,861
Thanks: 23,922
Thanked 7,207 Times in 4,640 Posts
ride height

I'm going to also weigh in with the "it depends" bunch.Dr. Michael Seal's Western Washington University's Viking series of 100-mpg cars used an increased ride height to channel air under their reverse airfoil bodies.Ford Motor Company used active suspension to lower their Probe series hyper-milers.Generally,my thoughts as regards to members cars would be that lowering,as mentioned,veils part of the tire/wheel from the air stream,effectively reducing frontal area,which usually reduces drag,arithmetically,as a percentage of area reduction.Also,lowering increases the fineness-ratio,something Hucho's book also addresses in the boattail modifications to the Mercedes C-111 research vehicle.Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag.,and I believe there is no dispute with regards to this.Dr.Seal attempts to move up out of ground-effect into "clean" air,where fineness-ratios are doubled,Ford uses active suspension to lower cars on the highway,where ground clearance demands are lower than in an urban environment.A significant portion of Ford's low drag is attributed to "lowering".The recent article on the fuel cell Fusion also attributes "lowering" as a significant (0.08 off Cd ! ) to the cars drag reduction.With exception to W.W.University,all my accounts of drag reduction,aside from general coachwork,everyone lengthened the vehicle,lowered the vehicle,or did both.Since there are exceptions,we probably need to every vehicle on a case-specific basis.P.S.,also,in Hucho's book you'll find Dr. Morrelli's body which he developed at Pininfarina,which exhibits very low drag in "clean" air,and looks exactly like the Aptera,however suffers a drag increase as it is lowered into ground-effect.
__________________
Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
  Reply With Quote
The Following 2 Users Say Thank You to aerohead For This Useful Post:
ShadeTreeMech (12-04-2013), slowmover (12-02-2013)
Old 04-09-2008, 06:53 PM   #19 (permalink)
EcoModding Lurker
 
Join Date: Apr 2008
Location: SF Bay Area
Posts: 35
Thanks: 0
Thanked 3 Times in 3 Posts
Maybe you mean something different by "fineness ratio" than I understand it to mean, so perhaps you should provide a short definition. Mine is "width/length" (or, for a cylinder, diameter/length). I therefore don't understand how lowering a car can alter its fineness ratio.

Also, would you be so kind as to expand on your comment that "Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag". What does fineness ratio have to do with the ground effect? I could see it if you were *narrowing* the car, but lowering it doesn't do that.

Thanks,
--Steve
  Reply With Quote
Old 04-09-2008, 09:53 PM   #20 (permalink)
Pokémoderator
 
cfg83's Avatar
 
Join Date: Dec 2007
Location: Southern California
Posts: 5,864

1999 Saturn SW2 - '99 Saturn SW2 Wagon
Team Saturn
90 day: 40.49 mpg (US)
Thanks: 439
Thanked 530 Times in 356 Posts
tasdrouille -

Quote:
Originally Posted by tasdrouille View Post
The rougher the underside of the vehicule, the more benefit there will be from a reduction in ride height. On the other hand, if the underside of the vehicule is a very good aerodynamic shape, increasing ride height will reduce the drag coefficient. If I remember right, an airfoil can have a Cd of roughly .05 up high in the air. Bring this airfoil in ground proximity and the Cd now jumps to .1 or the like due to ground effect. There is a reference of this in Hucho's book. The aptera is also a good example of this IMHO.

Generally, your average car will benefit from ride height reduction.

The frontal area argument is not really relevant. Say you drop 2 inches in a 22 square feet car, the net result is a reduction in frontal area of roughly 0.75%.
I'm gonna stick with this definition as a general rule-of-thumb.

As far as I am concerned the Aptera is a "small-plane" body shape with fixed wheels. Since it doesn't fly (yet), there isn't any reason to make them retractable, .

We really really really really need open source wind tunnel software.

Orrrrrrrr, maybe DIY coin-op car washes could be adapted for wind tunnel testing. Get some big fans and have a "dirty mist" or "misty mud" mode on the car wash control dial. Use the dirty mist mode to "find" the bad aero spots on the car. Wash the car off (more coin for the car wash owner), make another aero mod, and repeat. Pay as you go in 1:1 scale !!!!!

CarloSW2

__________________

What's your EPA MPG? Go Here and find out!
American Solar Energy Society
  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to decide whether to remove rear spoiler to decrease drag, improve mileage MetroMPG Aerodynamics 130 02-10-2015 08:50 PM
Aerodynamics Seminar #2 - by Phil Knox MetroMPG Aerodynamics 2 05-10-2013 05:34 PM
EcoModding for Beginners: Getting great gas mileage. SVOboy EcoModding Central 55 08-20-2012 11:34 PM
How to reduce drag of surfboard roof racks? Palionu Aerodynamics 11 04-28-2012 02:15 PM
Sources of Aerodynamic Drag in Automobiles and Possible Solutions SVOboy Aerodynamics 12 02-17-2010 02:09 PM



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