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Old 11-04-2009, 11:59 PM   #273 (permalink)
MetroMPG
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,316

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

MPGiata - '90 Mazda Miata
90 day: 52.07 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

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