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Old 07-02-2014, 06:05 PM   #1 (permalink)
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Old square sedan kammback

I have read a lot of sedan kammback builds, most of them using newer, more rounded cars. So I need your help on this one.

I have mocked up this cardboard kammback.


However, I think that it could turn out to have a negative result because the air 'overshoots' the trunk.

Without kammback there is lots of turbulence in the middle of the rear screen, but some reattached air on the spoiler.

The kammback can't be much longer, because then the trunk will hit it.

What do you think?
- Would the car benefit from the kammback?
- Should I leave it alone?
- Or should I make the spoiler on the trunk longer, so flow can reattach?

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Old 07-02-2014, 09:05 PM   #2 (permalink)
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From my gleamed knowledge I think your trunk is to low to hope to attach the flow to. Unless your spoiler was 18" high (forget it) so you should concentrate on a kammback

Could a longer Kammback go on the outside of the trunk seam ?
If so maybe the trunk could be left operational.
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Old 07-03-2014, 11:23 AM   #3 (permalink)
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Have you confirmed you have re-attached flow on the trunk lid?

Myself, I'd look for a way to make a Kammback that is attached to the trunk or trailing edge of the roof, but can open up to preserve trunk access.

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Old 07-03-2014, 01:42 PM   #4 (permalink)
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Thanks for the answers! I have done some tuft-testing, but before I start on the kammback I should do it again (better).



It looks like there is some reattached flow on the rubber spoiler. The back window and middle of the trunk are all turbulence. I'm I seeing this right?

I can't (will not) drill holes in the roof to secure hinges, but I just realised I might be able to use the raingutter as a mounting point. I have a spare trunk that I can hack up so I might go for the full (to the bumper) kammback and make it hinge via the raingutters.
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Old 07-03-2014, 02:51 PM   #5 (permalink)
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I would spend lots of time developing the kammback before fabricating one.

You need to simplify the installation and mounting method to a point that no one could improve on it, at least without great effort & Knowledge.

Off hand I think silicone mounted would be enough, if you gave it lots of surface area to adhere to , and designed it to be self supporting (hooked in rain gutter etc.). My Aftermarket rear top of the glass roof spoiler is mounted with silicone as per instructions.
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Old 07-03-2014, 03:03 PM   #6 (permalink)
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I did yarn testing of my VW. I put on some heavy wire masts on the back and tied a flag every 3". Once you see where the wind is not turbulent you can use that point for an "anti spoiler" What surprised me is how quickly air went from being fairly organized to a frenzy. It would only stay together a few inches past the edge of the body.


Here is an over the shoulder pic of it a few inches behind the back window.

I used a section of cattle fencing. to hold the tufts. I know a guy who uses a fiberglass wand (fishing pole) with a bit of yarn on it. He can hold it where he wants to test.
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Old 07-03-2014, 05:36 PM   #7 (permalink)
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You could attach the wand to you rear window wiper.

/me neither.
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Old 07-03-2014, 09:31 PM   #8 (permalink)
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attach tufts on the wiper


Or.. you could get some duct tape and wool ..
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Old 07-07-2014, 04:55 PM   #9 (permalink)
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Kamm-back

From the Porsche 914,you can see what Lotus did with their Europa,with boot height and length pushing the rear out near the 'Template' for Cd 0.29.
If your Kamm addition filled in this compromised area,you'd be poised for a nice drag reduction.


You can see how the 1st-gen Golf/Rabbit would have the lowest drag if it's roofline had been 'filled in.''

You can also see how Chrysler lowered their Charger's drag by filling in this area.

A quick cardboard mockup would give you your fuel savings numbers.
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Old 07-07-2014, 05:42 PM   #10 (permalink)
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Thank you Aerohead, I will get some pieces of cardboard and start testing (tuft-testing, coast-testing and fuel consumption testing). My neighbour has a Lotus Europe, really cool car! Incredibly small and light. Would be a perfect car to put in a small 3 cylinder engine.

But first I have to fix a dragging wheelbearing. Found out today that the front left is on its way out.

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