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Old 05-13-2013, 12:30 PM   #11 (permalink)
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I dunno. Looks pretty good to me:



(Different lens distortion in the images, so I aligned the roof and the top of the rear side window.)

My Kammback ( http://ecomodder.com/forum/showthrea...sion-3518.html ) is more aggressive than "template", and it was good for about a 5% improvement in fuel economy @ 55 mph (88.5 km/h) -- A-B testing only, though, so take with a grain of salt.

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Old 05-13-2013, 01:15 PM   #12 (permalink)
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Who else has done a similar Kammback?
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Old 05-13-2013, 03:37 PM   #13 (permalink)
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Nice animation Metro!
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Old 05-17-2013, 03:24 PM   #14 (permalink)
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Thanks metrompg... a lot of what I've done I did going off of your car.... I may go ahead and lessen the angle some and see if there's any improvement....
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Old 05-17-2013, 08:59 PM   #15 (permalink)
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It's less than 22 degrees, gradual incline and generously rounded corners.

Looks good to me.
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Old 05-18-2013, 03:56 PM   #16 (permalink)
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Well now I'm confused as to wether I move it up while I extend it(planned for a couple weeks from now).... Is it Worth moving up???
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Old 05-18-2013, 04:12 PM   #17 (permalink)
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whether to move

Quote:
Originally Posted by blowncopcar View Post
Well now I'm confused as to wether I move it up while I extend it(planned for a couple weeks from now).... Is it Worth moving up???
When Hucho was doing the aerodynamic optimization on the VW Golf/Rabbit,his team happened to do one iteration (of many) of the roofline at 11-degrees of downslope,which happens to coincide with the Aerodynamic Streamlining Template.
The lowest drag recorded for the car happened to occur with this roofline.
This roof did not make it into production as it would have violated a contractual agreement with Giorgetto Giugiaro,designer of Golf, but VW finally offers this contour with their 1-Liter Car.
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Old 05-18-2013, 04:15 PM   #18 (permalink)
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The angle is important whether or not it is short or longer. Longer at the "wrong" angle is not nearly as helpful; so I'd suggest tuft testing what you have now, and if they are not stable, then correcting the angle will improve your FE with the short(er) Kamm back. Then you can extend it if you want to.
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Old 05-18-2013, 04:54 PM   #19 (permalink)
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Yes! It will be worth it.

To achieve the optimal effect it is best to stay with the Template no matter where you cut it off.

The 22° limit is not EVER intended to be looked at until you are like 5 feet back behind your car. That is the gross absolute limit the air will remain optimally attached PROVIDED you have followed the template to that point. If you dive down to 22° when you should be at only 8°. it will not be a good thing.

Think of it like this, the front of you car has lifted the air up and out of the way, it now wants to drop back down to where it started, and the more gently you can let it slide down the back of your car, for as long as possible with the template as a guide, this will achieve the best result. If you drop the angle too abruptly, the air will not follow the shape of your car, and the pressure will drop too much.

So, That said, I think it OK to cheat it down a little bit for a short Kamm like you have, this is only based on what I see with the Honda Fit & Toyota Prius. It kinda make sense to minimize the wake size as opposed to following the template and having the highest pressure return.





Use my post #6 if you want to optimize the design, drastic angle changes will unhinge some of the benefit you are seeking to gain. Besides, I'm asking you to keep it higher which has the added benefit of improved visibility in addition to cracking good aero performance.

Tuft testing only tells you if you really screwed up, air will be attached with your current design, and certainly with my proposal. Tuft tests also only show what the air is doing in the 1/4 inch layer of air on the car, the air being moved about drastically by your vehicle goes out several feet, and the air being affected in total is out to several yards. It is hugely complicated how it all interacts to the point where even with supercomputers and teams of engineers, we still build multi million dollar wind tunnels to test things in real life.

We're really talking about optimization here, not whether it "Works". Clearly your MPG results show what you have currently offers some improvement, raising it, going to an easier transition, will only improve it that much more. In my opinion.
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Old 05-18-2013, 05:35 PM   #20 (permalink)
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Quote:
Originally Posted by aerohead View Post
... the roofline at 11-degrees of downslope,which happens to coincide with the Aerodynamic Streamlining Template.
The lowest drag recorded for the car happened to occur with this roofline.
.....violated a contractual agreement with Giorgetto Giugiaro


Here's an 11° Golf roof with the template overlay.




And a Bigger Version of the same

They shoulda listened to Hucho! Who names their kid Giorgetto anyway?

This is an interesting idea though.


Last edited by ChazInMT; 05-18-2013 at 11:06 PM..
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angle, degrees, kammback, metro, plexiglass

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