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Old 08-21-2012, 10:05 PM   #1 (permalink)
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Rounded corners vs. Straight edges on kammback

I am really considering a much more dramatic, full kammback for my Neon. Allowing the sides to taper in as much as possible without interfering with opening the trunk. Also, I am considering making a hinge system on the top, to allow me to fold it back and use the trunk. My issue is with building rounded corners on the top-to-side transition.

I know that rounded corners are preferred aerodynamically speaking, ease of construction is another story. To the best of my understanding, this is because with just a straight, creased edge, the airflow "trips" over the edge, causing vortices which will increase drag and severely limit the drag reduction potential, am I correct?

So how drastic of a rounded corner does it need to be? I assume the more rounded the better, but is there a known minimum amount of curve that is required?

Finally, I know people can compare the amount of roundness by talking about the size of it, for example a 3/4" radius. So does that mean that if the curve was a whole circle, it would be 1.5" in diameter? Is that sufficient for avoiding air tripping over the edge? I am just trying to get my head around what kind of curve I need, to figure out the best way to build it.

Thanks for any input, you guys have already helped me out so much in building my Neon to where it is, and I appreciate it. This site is great!


Last edited by 2000neon; 08-23-2012 at 12:20 AM.. Reason: Updated title to include more infor
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Old 08-21-2012, 10:36 PM   #2 (permalink)
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No, we're talkin 2" radius minimum. That's just a shoot from the hip thing, and it assumes a fairly close aero design. 3" would do you for sure.

Keep in mind though that rarely do panels come together at a 90° angle, it's usually more obtuse making the actual part the needs to be radiused smaller.
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Old 08-21-2012, 10:40 PM   #3 (permalink)
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Thanks, so as far as a 2" radius, the curve of a 4" diameter circle? Hmm i actually have some 4" pipe laying around as a nice template. Lol.
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Old 08-21-2012, 10:41 PM   #4 (permalink)
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Make the structure out of aluminum bar and bend a couple transverse ribs to shape- 6" should work quite well. Then when you put the coroplast on it will conform to the curves you want. No, it won't have compound curvature, but it'll be "good enough" without going into fiberglass.

Of course you could get close to compound curvature by making really slim V slits on the sides and curving it in just slightly like a lobster tail. That would take a lot of work, though. Go for good enough now and if it works well, make version 2 even better.

Try it at 20 degrees from level and tuft test it. If flow is not attached lift it 2 or 3 degrees and try again. It should work best somewhere between 15-20*
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Old 08-21-2012, 10:50 PM   #5 (permalink)
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That is actually exactly what I had in mind! Great minds think alike Im trying to figure out the best way to lay out the coroplast, and how many sections to do it in. More brainstorming time.
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Old 08-21-2012, 11:07 PM   #6 (permalink)
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hmmmm........

if you have squared sides, and things are not perfect, you will induce vortices which will "fix" the imperfections.

build the thing.

keep side and top tapers to 20 degrees or less, and bottom taper to 10 degrees or less.

i would not worry about the corners.
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Old 08-21-2012, 11:24 PM   #7 (permalink)
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Quote:
Originally Posted by 2000neon View Post
That is actually exactly what I had in mind! Great minds think alike Im trying to figure out the best way to lay out the coroplast, and how many sections to do it in. More brainstorming time.
Nice! I'd align the ribs longitudinally so it bends easily, but you probably knew that as well.

If you have a large 4x8 sheet it should pretty much cover the whole thing, right? Otherwise let the split be at the middle because the corners will want to peel themselves back off.

Edit- 1,111th post! Hah
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Old 08-22-2012, 08:50 AM   #8 (permalink)
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Quote:
Originally Posted by 2000neon View Post
I am really considering a much more dramatic, full kammback for my Neon. Allowing the sides to taper in as much as possible without interfering with opening the trunk. Also, I am considering making a hinge system on the top, to allow me to fold it back and use the trunk. My issue is with building rounded corners on the top-to-side transition.

I know that rounded corners are preferred aerodynamically speaking, ease of construction is another story. To the best of my understanding, this is because with just a straight, creased edge, the airflow "trips" over the edge, causing vortices which will increase drag and severely limit the drag reduction potential, am I correct?

So how drastic of a rounded corner does it need to be? I assume the more rounded the better, but is there a known minimum amount of curve that is required?

Finally, I know people can compare the amount of roundness by talking about the size of it, for example a 3/4" radius. So does that mean that if the curve was a whole circle, it would be 1.5" in diameter? Is that sufficient for avoiding air tripping over the edge? I am just trying to get my head around what kind of curve I need, to figure out the best way to build it.

Thanks for any input, you guys have already helped me out so much in building my Neon to where it is, and I appreciate it. This site is great!
Edge radii solve a problem. Ideally (perfect design) flow wouldn't need to mix between top and sides. In practice cross winds mean up/down wash can't be avoided. From Hucho





Using a nominal 6' body %5 would be 3.6" radius.
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Old 08-22-2012, 09:45 AM   #9 (permalink)
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Take a good look at the Prius, almost all folded corners. Is it ideal? Probably not, but it has been tested and it has low drag.
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Old 08-22-2012, 10:03 AM   #10 (permalink)
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Quote:
Originally Posted by drmiller100 View Post
hmmmm........

if you have squared sides, and things are not perfect, you will induce vortices which will "fix" the imperfections.

build the thing.

keep side and top tapers to 20 degrees or less, and bottom taper to 10 degrees or less.

i would not worry about the corners.
Mr Miller,

I think you need to reconsider this. Creating a vortex off a c-pillar is not what you want. You may be thinking cute little burbles of air that help the air "Re-attach". This would be an almost impossible thing to engineer with a team of scientists, a wind tunnel, mission statement, and $2 million NASA grant. So I doubt a guy in his driveway with duct tape and coroplast will nail it.

The Vortex creation that needs to be avoided are the long trailing tornados like you see off the wing tips of airplanes. These are very common. The energy required to create and maintain these is tremendous and it can only come from one place, the cars engine. We all know what that means.

The C-pillar is the most likely spot for these to form and they're caused when large pressure differences are within close proximity to each other, the air tries to move sideways from high to low pressure and in doing so, sets itself spinning.

So, in rough numbers terms, if Neon were to "Just Build it" yeah, anything is likely better than nothing, and he may realize 30% of the potential gain out of building a kamm extension just due to the fact that he's "Stuffing the Wake". But if he builds it properly, and radiuses the edges, he'll have a more optimized device that will see 70% of the potential gain. So 25MPG base, if he stuffs the wake, 26.5MPG, builds it better, 28MPG. Would he be happy with 26.5? Probably, but why build something half-baked when if you take the time to do it right, the result is so much better?

Also the 20° thing is not a very good rule of thumb. One has to consider what you're starting out with. If the slope at the top of rear glass is already at 10° or so, then yeah, 20° works, but if it's flat, 20° would be too drastic a change and would not be optimal. I really don't think you want more than a 10° change at any one point. This is why a blended curve works so well when adhereing to the template, it allows the air to transition as undisturbed as possible optimizing the result.

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