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Old 06-16-2008, 04:59 PM   #6 (permalink)
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,410

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

MPGiata - '90 Mazda Miata
90 day: 52.07 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: 66.16 mpg (US)
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3 smartie car - I like it.

A good rule of thumb is that you can begin with a 10 degree angle, and if your transition is gentle enough, that angle/tangent (if a radius) can be increased beyond that and still retain attached flow.

You could do worse than simply look at the rooflines of existing relatively aero vehicles, eg:

(from Kardboard Kammback: testing a partial boat tail prototype )

Also, don't forget it's a 3D shape - the transition from "side" to "top" is important, and should be generously radiused to aid pressure recovery & discourage vortex formation. If you look at my kardboard prototype, you'll see I used too sharp an angle at that transition point. AndrewJ did much better with his finished work.
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

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
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