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Old 01-17-2013, 11:34 AM   #1 (permalink)
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Kammback size?

I am thinking about a small kammback on my van. Does anybody know how the effectiveness is altered by size? I am mostly trying to go as small as I can while having the highest, effect to size ratio. The reason I need it to be small is that it has to be a split design and I occasionally have to drive with the back doors open. Thanks in advance

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Old 01-17-2013, 12:12 PM   #2 (permalink)
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Make 'em as big as you can. The bigger you make them the more effective it'll be.
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Old 01-17-2013, 05:11 PM   #3 (permalink)
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Figure out a way to quickly detach them from the doors whenever necessary. I'm thinking brackets and wing nuts. Or maybe make each half into two parts and fold them over each other like a cardboard box.

Make them as big as possible and remember to put a wide radius on that top corner, especially with a van.
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Old 01-17-2013, 06:34 PM   #4 (permalink)
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Size does matter.
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Old 01-17-2013, 06:42 PM   #5 (permalink)
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effectiveness/size

Quote:
Originally Posted by chrisgerman1983 View Post
I am thinking about a small kammback on my van. Does anybody know how the effectiveness is altered by size? I am mostly trying to go as small as I can while having the highest, effect to size ratio. The reason I need it to be small is that it has to be a split design and I occasionally have to drive with the back doors open. Thanks in advance
If you'll do a Search EcoModder for the thread 'full-boat-tail trailer',go to page 25 of that thread,and down to permalink#248,the 12th and 15th image from the page top have pictorial drag tables for two different types of boat tails.
You can look at the lengths and compare drag coefficients.
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Old 01-17-2013, 08:51 PM   #6 (permalink)
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Quote:
Originally Posted by aerohead View Post
If you'll do a Search EcoModder for the thread 'full-boat-tail trailer',go to page 25 of that thread,and down to permalink#248,the 12th and 15th image from the page top have pictorial drag tables for two different types of boat tails.
You can look at the lengths and compare drag coefficients.
http://ecomodder.com/forum/showthrea...tml#post294074







(I find it interesting that the bus with the rounded end boat tail (0.263) has lower drag than one with edges (0.274), since we usually say edges are better on the rear end.)
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Old 01-17-2013, 09:59 PM   #7 (permalink)
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Quote:
Originally Posted by Sven7 View Post
Figure out a way to quickly detach them from the doors whenever necessary. I'm thinking brackets and wing nuts. Or maybe make each half into two parts and fold them over each other like a cardboard box.

Make them as big as possible and remember to put a wide radius on that top corner, especially with a van.
I have been stewing on this for a while, as I want to be able to open my bed topper with my soon-to-be Kammback. So, one of my ideas was mounting brackets to the topper, and mounting hinges to them and the Kammback so I could open it. I don't think I'm going to do that, as I've since mulled over attaching the whole thing to the topper door, but it might give you an idea.
Or, perhaps you could leave a seam in the middle where the doors open, and bolt each half to each door...

-->To quote MetroMPG "I've spent a silly amount of time staring at the back of the truck." (used truck instead of car)
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Old 01-17-2013, 10:21 PM   #8 (permalink)
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Great images thanks for posting.
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Old 01-18-2013, 11:49 AM   #9 (permalink)
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First, let me say, my brain hurts! Lots of thinking going on lately.

I had read the entire thread mentioned above. Saw the images on page #25. I was having a little trouble with the second batch of images quoted above, the piggy bank looking box. If the Dryden van and all the boattails out there were right, surely the the boattails shown there hurting or barely improving the drag couldn't be right. What was wrong? I think it's in the last piggybank. the shaded area I assume is the sides being angled in, duh, addressing on the top wasn't doing too much by itself. Right?

And something else I have been playing with. The shape of the tail, that's where this thread kind of started. I'm still a bit stuck on the Dryden van, but I have also now read threads about Cinderella, and Tiger Woods. I'm an engineer, there must be a spreadsheet, right? There is:


(X and Y scales are in inches, based on the Dryden van that was 80" wide.)

I was doing OK until the Cinderella threads. 4 seconds, 5 seconds, 22, I'm still processing. A simple "4 second" tail is much longer than the template. Easy to make the following observations about end of the clockface tail:

3.66 seconds - 22
4 seconds - 24
5 seconds - 30

I get that a simple clock doesn't a template make, but thought I'd be closer. A "5 second" clock starts to get close to the template, but has 30 of taper in it. It ain't so simple! But you all knew that. I guess my question is, just what clock is worthy of a basic template? One at something like 5.5 seconds, limited to 22 would be close to the template. I may narrow that down, but haven't seen it here, I must be off on an unreasonable path?

And back to my graph. Both the Dryden van, which was tuft tested, and the Trailer Tail, which was optimized for angle, are very close. They both have limited lengths (80" and 48"). Both seem to work very well. Just what is a good model for a practical tail? (Recall I'm thinking about large vehicles, 100" wide, so the template tail is like 178", not practical at all.)

Last edited by ennored; 01-18-2013 at 12:38 PM..
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Old 01-18-2013, 01:06 PM   #10 (permalink)
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Quote:
Originally Posted by Sven7 View Post
http://ecomodder.com/forum/showthrea...tml#post294074





(I find it interesting that the bus with the rounded end boat tail (0.263) has lower drag than one with edges (0.274), since we usually say edges are better on the rear end.)
Good observation. That's not all.
* Comparing #4 & 5 Cd 0.314/0.325 => 3.4% reduction.
* Comparing #6 & 7 Cd 0.263/0.274 => 4.0% reduction.
IMHO two taller sides play a larger role than one narrower top edge.

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