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Old 03-04-2013, 07:39 PM   #81
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Originally Posted by mx5autoxer View Post
I figured Matt's car would be up there. Why is it beneficial to ditch the back glass? I would think that glass would smooth the flow of the air going over the roof, down the glass, and spilling over the rear quarters and under the wing.
With a rear window the cabin is a big parachute. Taking out the back window gives all the air coming in thru the windows somewhere to go.
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Old 03-04-2013, 10:52 PM   #82
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i'm not so sure about that. not having the rear glass definitely affects some of the flow coming down off the roof.
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Old 03-04-2013, 10:55 PM   #83
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Also notice the windows are also open/no windows.
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Old 03-05-2013, 01:00 AM   #84
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Originally Posted by M.Adamovits View Post
With a rear window the cabin is a big parachute. Taking out the back window gives all the air coming in thru the windows somewhere to go.
Not really the reason.. Cabin pressure builds up the moment you are travelling at speed. Then air doesn't 'parachute' into the cabin much at all.

The reason for guys removing the rear window in the hardtop has to do with the fact that they found that the wing was more effective this way. No way without a wind tunnel to say if it's because it alters the air's flow as it comes off the top towards the wing, or if the wing is actually getting airflow that is coming out of the rear window hole, but either way, the whole reason is because it aids the wing.
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Old 03-05-2013, 01:59 AM   #85
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Not really the reason.. Cabin pressure builds up the moment you are travelling at speed. Then air doesn't 'parachute' into the cabin much at all.
By that theory an aerodynamicaly formed cone would form in a regular parachute and it would be ineffective at slowing anything down. Although some density change happens the air doesnt just form a perfect shape. You get a ton of turbulence.
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Old 03-05-2013, 01:05 PM   #86
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Yeah I understand that's it's not a nice pretty bubble of pressure inside like having the windows up, it's very turbulent, but the 'parachute' effect is an exaggeration - I don't think the guys who pulled the rear window out were seeing significant MPH changes at the end of straights, from what I have heard it was much more differences in wing performance.

I run a rear lexan window though, so I cannot comment from personal testing. I've considered doing some sort of quick disconnects on it such as Dzus fasteners so that I could do back to back tests.

How high the wing is mounted plays a big role in that too though. The APR GTC-200 for example mounts pretty low, and IIRC even with the 2.5" risers still isn't at the roofline, so the no-rear-window configuration might benefit those wings much more than it would for someone who has gotten their wing up higher..

-Ryan
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Old 03-05-2013, 01:50 PM   #87
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It seems to me that with the back window removed,

a) there is high-velocity air coming off of the rear roof lid meeting slower air
b) this is going to tend to pull air out the rear windshield opening, thus pulling air through the cabin windows.
c) this is also going to form a vortex with the axis of the vortex along the pitch axis of the car. The downward side of the vortex is going to hit the rear wing, effectively increasing its angle of attack.

This is from my 5 minutes of coffee & M&M's fueled cereberal CFD. Feel free to discuss.
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Old 03-05-2013, 03:24 PM   #88
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I think you're right, without the rear window much more air is going to flow thru the cabin. Poor man's AC.

Though I agree it may have more air hitting the wing, it's going to be highly turbulent and likely not beneficial. That's my guess anyways.

Why doesn't someone windtunnel Miatas already?
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Old 03-05-2013, 10:31 PM   #89
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Where is plucas when you need him? lol
he can model up a rear windowless miata and use his CFD program.
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Old 03-05-2013, 11:16 PM   #90
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With the rear window removed, the air moves forward through the hole and into the cabin from the rear, then exits out of the side windows. Anybody who has ever had a zip-down plastic rear window can attest to this.
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Old 03-05-2013, 11:25 PM   #91
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Where is plucas when you need him? lol
he can model up a rear windowless miata and use his CFD program.
I'm here. I am busy at the moment with paying customers

Maybe when my schedule clears up, or if somebody wants to pay to have it done (and allow me to share), it can be moved up the list.
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Old 03-06-2013, 12:07 AM   #92
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Quote:
Originally Posted by sixshooter View Post
With the rear window removed, the air moves forward through the hole and into the cabin from the rear, then exits out of the side windows. Anybody who has ever had a zip-down plastic rear window can attest to this.
True, I had a zip-down plastic rear window, and I always got air coming in from the rear.

Quote:
Originally Posted by Mobius View Post
It seems to me that with the back window removed,

a) there is high-velocity air coming off of the rear roof lid meeting slower air
b) this is going to tend to pull air out the rear windshield opening, thus pulling air through the cabin windows.
c) this is also going to form a vortex with the axis of the vortex along the pitch axis of the car. The downward side of the vortex is going to hit the rear wing, effectively increasing its angle of attack.

This is from my 5 minutes of coffee & M&M's fueled cereberal CFD. Feel free to discuss.
The pulling only works with small openings. Example: windows cracked a little, air is pulled from inside the cabin by the boundary layer around the car. Windows all the way down, air enters the cabin, circles around, exits at the rear of the window opening.
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Old 03-06-2013, 02:00 AM   #93
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So then the vortex is forming and moving air through the cabin and out the windows.

Never been in an NA with the window open. Interesting.
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Old 03-06-2013, 02:39 AM   #94
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Quote:
Originally Posted by plucas View Post
I'm here. I am busy at the moment with paying customers

Maybe when my schedule clears up, or if somebody wants to pay to have it done (and allow me to share), it can be moved up the list.
If I paid you, I wouldn't share it
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Old 03-06-2013, 06:21 AM   #95
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HATE the idea of removing rear windows.

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Old 03-06-2013, 09:15 AM   #96
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Ya Tage Evanson, I was following him after this inspiring article "Anatomy of A Giant Killer"

1995 Honda Civic - 11 second Civic - Import Tuner Magazine

Sad what happened to his car, but now the car is a Phoenix
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Old 03-06-2013, 01:07 PM   #97
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If I paid you, I wouldn't share it
Most wouldn't want to share it since they are paying for the knowledge. This is why I can't share more information on some of my findings.
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Old 03-06-2013, 01:11 PM   #98
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Originally Posted by sixshooter View Post
With the rear window removed, the air moves forward through the hole and into the cabin from the rear, then exits out of the side windows. Anybody who has ever had a zip-down plastic rear window can attest to this.
I'm not try to contradict the hive-mind here.. But my zip down rear window clearly blows air out. pulling it thru the windows..
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Old 03-06-2013, 04:29 PM   #99
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Quote:
Originally Posted by M.Adamovits View Post
I'm not try to contradict the hive-mind here.. But my zip down rear window clearly blows air out. pulling it thru the windows..
I have a zip-down in my NB and I can definitely feel it circulating around behind my head, I wouldn't be surprised if it was in fact coming in through the rear..
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Old 03-06-2013, 07:09 PM   #100
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Time for some tuft testing to see where the air comes from.
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