View Single Post
Old 11-04-2009, 10:59 PM   #273 (permalink)
MetroMPG
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
Quote:
Originally Posted by NeilBlanchard View Post
Right, we cannot forget the 3-dimensionality of the situation. I remember reading in Hucho about the Schlör "Pillbug", that they predicted a Cd of 0.18 theoretically based on the 2D profile, but after they built it they measured 0.13 (they had to tweak things like the height).

My best guess is that the air flow around the sides is how it ended up being better than the 2D profile had predicted.
I've always thought this is the case with GM's EV1 as well (Cd 0.195). The rear glass angle is quite a lot steeper than the generally accepted angles on current aerodynamic production cars. It's something like 18 degrees, if I recall (hoping that's right).

The major difference being that the EV1 from the driver's door back to its trailing edge has a pronounced conical shape which interestingly isn't always apparent in profile photos. The plan taper is quite dramatic, and the car lacks the typical defined "shoulders" at the transition from "side" to "top" surfaces. All of which likely lets it get away with a steeper profile taper (roof through to trunk) because proportionately more pressure recovery is happening along the side of the vehicle.

Of course, this is all topside related.

Back to the bottom taper issue: personally, I'll be sticking closer to 4 degrees.
__________________
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