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Old 11-07-2011, 07:15 PM   #1 (permalink)
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Help w Perfect Kammback/Boat Tail Optimization

I excel at the enthusiasm, innovation, and construction of mods but struggle a bit on the science and mathematics end of things. I'm hoping some of the more math/science gifted aero guys can help me get on the right track to optimize my kammback and get the best results possible.
A few people have said it looks too steep and that to find the optimum shape, I needed to use only the top half of the teardrop image and line the bottom edge with the groundplane. Doing this does indicate that my angle is too steep. I planned on remaking the sides anyway to extend them beyond the frame so I'm thinking when I do this, I will also take the opportunity to round the corners and raise the angle. Before I do anything though I want to make sure I understand things right and that my thoughts toward a full boat tail are also on the right track.
Here is a 3 part picture that illustrates well what I am thinking. Here is a very short explanation of the picture:
  1. The top half of the teardrop (A) with the bottom edge along the groundplane and the highest part of the drop the highest edge of the roof. (I believe this is where I want the kammback to line up)
  2. I added a bottom half teardrop (B) with the top edge PARALLEL to the groundplane and the tallest point lining up with the rear skirt/bellypan edge. My thinking here is that a future boat-tail extention would follow the darker part where A and B overlap.
  3. Here I added a third layer, this time of the entire teardrop (C) sized so the tallest point spans the difference between the top (roof) and bottom (bellypan) of the car. It looks slightly steeper than picture 2.

It looks like I know where to raise my kammback to, which boat tail is best (2 or 3?)
Also, as for narrowing the width of the kammback and boat tail: if I took an arial photo of the car would i simply taper of following the full teardrop such that it's tallest point spans the entire width of the car?

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Old 11-07-2011, 08:09 PM   #2 (permalink)
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I like that you did the underside, I have not see that done before.

Just go with "C", it's the shortest and you are splitting hairs, they are just too similar to my eye (and all under 20 degrees).

Are you going with a 10 degree incline on the sides?

How round are your corner edges going to be?
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Old 11-07-2011, 09:10 PM   #3 (permalink)
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You cannot slope the underside up as steeply as you slope the top and the sides. The reason is, the ground slows the air flow back in, and so you would create a low pressure wake. The underside has to be kept flatter so this doesn't happen.

Here's the template that includes the curve on the underside:

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Old 11-08-2011, 12:28 AM   #4 (permalink)
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If you have positioned the template so its peak matches the highest camber of the roofline then you do seem to have a moderately too steep angle on the kamm. If you raise the angle, you could also lengthen the kamm and not lose any more rear visibility. Nice numbers in your fuel log--and no EOC on that last one. Nice. What are building the Kamm with? How are you attaching it?
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Old 11-08-2011, 02:25 AM   #5 (permalink)
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Best way is to do tuft testing with different angles. I was able to make my kammback spoiler with much higher angle. Template however lead me to close the optimum. Now I have also problem with rear visibility with my 3 mm thick black abs spoiler but you can correct that with tinted polykarbonate kammback .
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Old 11-11-2011, 12:26 PM   #6 (permalink)
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Here is the current spoiler tuft testing video:
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Old 11-11-2011, 04:34 PM   #7 (permalink)
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Very nice testing video. Looks like you were getting about the same level of turbulence that I got on the Insight. I did notice some reverse flow on the tuft which was mounted at the rear edge. I think someone said this type of extension worked best if closed at the back.

The thing I find missing from all our efforts is any sense of the quantity of improvement. Is a bit of flutter at the ends, like we both had, a serious problem, or is there genuine and significant improvement so long as the tufts are not detached?

Nice approach, brave friend!
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Old 11-12-2011, 06:03 AM   #8 (permalink)
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I tested 3 angles and this last angle is 6.3 degrees more than the last. Angles were with 3 degrees steps:


Like that it is quick and easy to test various angles. When you tilt the spoiler more it is wise to cut the exess material off from the sides. I just measured the current angle it is about 11.6 degrees so about the max is that 17. I might have to do more tests even higher angles. I will also try to get better visibility to roof tufts.

Here is a video of the first setup:


I did ABA test on this first angle and got results that it saved about 4.15% on average to both directions. Better test need to be made.
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Old 11-17-2011, 07:41 PM   #9 (permalink)
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rebuilt the kamm back

I spent a good 5 hours rebuilding the kammback on Tuesday. I radiused the corners and raised the angle, while I was at it I also extended the length of the sides so they go all the way down to the tail-lights and then horizontally back, rather than angling up to the top.
I also wrapped the whole thing in green vinyl so it looks nice and pretty.
Now all of my visible mods match and look nice color-wise.
I'll post pics as soon as I can but I like it already. I was 2/3 into the tank already but now it's sitting in the parking lot with a range record on the tank, I'll get gas on the way home and hopefully post my best #s ever!
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Old 11-18-2011, 04:42 PM   #10 (permalink)
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Kammback 2.0 - optimized

Thanks everyone for helping me understand the ideal shape better. I printed a picture of the car with the ideal teardrop slope then cut it out with scissors. To optimize the Kamm I simply stood to the side of the car closed one eye, then held the cutout at arm's length, lining it up so it eclipsed my view of the car. I then simply adjusted the Kamm's angle until it matched that of the cutout.
I'll take better pictures from more angles to better illustrate the Kamm construction but for the purpose of this thread these pictures should be helpful:
Below you can see the new Kamm, then the half teardrop overlaying the car lined up along the groundplane so that the highest roofpoint is the highest of the template. Looks pretty spot on

Check out my fuel log for the latest tank which had the optimized kamm for only the last 100 miles or so. Set a personal best range record and FE record!

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