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Old 04-02-2009, 09:12 PM   #1 (permalink)
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Difference between .17 and .11? (Aerocivic mention)

I've seen a few of the threads about basjoos Aerocivic, and it's gotten me to thinking about my design a little. I was originally planning to get as close to a tear drop as possible, by placing the windshield out far ahead of the cabin, but I'm wondering if the effort would be worth it, or if I could even achieve .11. I'd probably end up at more like .14 or .15, given the wheels and imperfect tear drop shape, I'm thinking.

This is on a ground-up build, so I can really do it either way, but I'm thinking the necessary glass may be very costly and/or heavy.

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Old 04-02-2009, 09:46 PM   #2 (permalink)
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Are you talking about a custom glass windshield? If you find a place, let me know. I probably won't like the price, but I'm wondering what the approximate cost is. All I can find is guys who'll modify OEM windshields for chop-tops and such. Though, at that, if you've got some real skill, a Saab 900 windshield, raked back further, might be a nice efficiency improvement or a start to a complete custom canopy.
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Old 04-02-2009, 10:03 PM   #3 (permalink)
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Quote:
Originally Posted by The Atomic Ass View Post
I've seen a few of the threads about basjoos Aerocivic, and it's gotten me to thinking about my design a little. I was originally planning to get as close to a tear drop as possible, by placing the windshield out far ahead of the cabin, but I'm wondering if the effort would be worth it, or if I could even achieve .11. I'd probably end up at more like .14 or .15, given the wheels and imperfect tear drop shape, I'm thinking.

This is on a ground-up build, so I can really do it either way, but I'm thinking the necessary glass may be very costly and/or heavy.

What do you mean worth it? Here is what might be gained from Aero mods
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Old 04-04-2009, 10:01 AM   #4 (permalink)
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Originally Posted by evolutionmovement View Post
Are you talking about a custom glass windshield? If you find a place, let me know. I probably won't like the price, but I'm wondering what the approximate cost is. All I can find is guys who'll modify OEM windshields for chop-tops and such. Though, at that, if you've got some real skill, a Saab 900 windshield, raked back further, might be a nice efficiency improvement or a start to a complete custom canopy.
I'm talking about a completely custom vehicle. With a custom body. I'm wondering whether to make it into as perfect a tear-drop shape as is possible for a car, or go a route similar to basjoos, where there is still a recognizable front compartment.

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What do you mean worth it? Here is what might be gained from Aero mods
Certainly there's some to be gained... But I'm wondering if I can even achieve .11, and if I can't, whether it would be a good compromise to go with a design known to achieve .17.
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Old 04-04-2009, 11:13 AM   #5 (permalink)
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Well, it still sounds like you'll need a special windshield unless you could chop an OEM one and get it to work by reducing the cockpit's frontal area as well as leaning it back, but getting a rounded shape to blend with the sides is the hard part. It occurred to me after, but a Lancia Stratos windshield would probably be amazing. There is a company that builds exact replicas with parts so accurate they're often used as replacements for damaged originals. Cost is unknown, but it has an extreme rake and wraps around quite dramatically, very much like an aircraft. That should allow you to build a cockpit behind it for up to 4 people. I'm going with an actual aircraft canopy myself, but the resulting car will likely be only a single seater, possibly two in tandem depending on the car I choose to start with. FOr the windshield, I'm compromising aero a little by making a 3-piece front like a WWII fighter, with a flat safety glass center panel. Perhaps a similar thing could be done for a wider cockpit with a split windshield like a pre-war car, having two flat panels angled towards the sides (something like a De Havilland Mosquito).
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Old 04-04-2009, 05:23 PM   #6 (permalink)
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Going off the figures in your table -
Cd = 0.17 vs Cd = 0.11 burns an additional gallon of fuel every 807.7 miles or 124 additional gallons every 100,000.

Aside from any concerns about time until pay-off, I wonder if there are any additional eco-penalties/benefits associated with a custom built windshield vs a mass produced windshield???
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Old 04-04-2009, 06:27 PM   #7 (permalink)
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The front end shape is not critical for wind resistance unless you are going for true laminar flow. Lifting the air to the roof in two steps is not much harder than doing it all at once, and the resulting flow can still be resolved gracefully. If you want a light, fast car, crosswind stability is a major concern, and having a thin, heavy front end is a great benefit.
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Old 04-04-2009, 10:30 PM   #8 (permalink)
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Originally Posted by Bicycle Bob View Post
The front end shape is not critical for wind resistance unless you are going for true laminar flow. Lifting the air to the roof in two steps is not much harder than doing it all at once, and the resulting flow can still be resolved gracefully. If you want a light, fast car, crosswind stability is a major concern, and having a thin, heavy front end is a great benefit.
I believe that's the point that was probably nagging the back of my mind. Aerocivic copy it is then.

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