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Old 03-18-2018, 11:48 AM   #1 (permalink)
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Question Partial Boattail/Kammback on a sedan?

I know there have been myriads of threads related to kammbacks and boattails for hatchbacks, but how about sedans? Could a partial boattail or kammback be implemented on a sedan as well, and would it have the same FE benefit? I'm considering doing a kammback on my Civic sedan and was wondering about the following templates (color code interpretations below the images):



The white indicates the kammback and the black indicates a possible partial boattail.



The blue and green indicate the pieces for the kammback, and the purple and red indicate the pieces for the lower part of the partial boattail.

Any thoughts on this? Location, possible FE gains, durability (coroplast w/ zipties, tape, and clamps)?

I know it would be better if the sides extended to the outside, but this would require taping to the exterior paint, and I'd rather not risk the car's paint job. I might have to relocate the license plate as well so it remains visible.

Any tips/suggestions welcome! Or if it's just a bad idea just let me know!

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Last edited by mpg_numbers_guy; 03-18-2018 at 05:05 PM..
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Old 03-18-2018, 12:59 PM   #2 (permalink)
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You don't need to worry about damaging paint with tape; I've still got tape residue from the Gorilla tape I used to attach a temporary tail last fall, but it will come right off with Goo Gone.

If you look at LSR Civics, they all have some sort of decklid extension with fences.





These are built to be low-drag first, not low-lift, so they're generally a pretty good example to follow.
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Old 03-18-2018, 03:55 PM   #3 (permalink)
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Your search term within Ecomodder could be 'hitch mount'. Most of the implementations of your proposal fail in the same way yours would. (sorry). If you envision the airflow over the aft of the vehicle you would see vortexes erupting from the point of separation triggered by things like longitudinal edges and trim. This creates a wake bubble that will completely enclose the construction you propose. The most you get is wake filling.

The way you could make that work (and it would) would be to remove the roof completely and extend it out to the sides with a taped or molded transition. The taillights and license must change to accommodate the airflow.



The reason Bonneville spoilers work it that they provide a surface for the turbulence streaming off the roofline to reattach. If you push the Kammback/boat tail concept to it's conclusion you get something like the Citroen Tubik:

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Old 03-18-2018, 05:04 PM   #4 (permalink)
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Quote:
Originally Posted by Vman455 View Post
You don't need to worry about damaging paint with tape; I've still got tape residue from the Gorilla tape I used to attach a temporary tail last fall, but it will come right off with Goo Gone.
Eh, I'm still kind of wary of taping up my car, even for ecomods. Don't want any risk of damaging it since I do plan on reselling it sometime down the road.

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Originally Posted by freebeard View Post
Most of the implementations of your proposal fail in the same way yours would. (sorry).
No need to apologize! I was half expecting it to be a bad idea anyway, just thought I'd throw it out there and see what y'all thought.

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If you envision the airflow over the aft of the vehicle you would see vortexes erupting from the point of separation triggered by things like longitudinal edges and trim. This creates a wake bubble that will completely enclose the construction you propose. The most you get is wake filling.

The way you could make that work (and it would) would be to remove the roof completely and extend it out to the sides with a taped or molded transition. The taillights and license must change to accommodate the airflow.

The reason Bonneville spoilers work it that they provide a surface for the turbulence streaming off the roofline to reattach. If you push the Kammback/boat tail concept to it's conclusion you get something like the Citroen Tubik:
So basically there really isn't a way to do a kammback or boat tail with a sedan like there is with a hatchback or a wagon without completely modifying the car?
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Old 03-18-2018, 06:06 PM   #5 (permalink)
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The closest I could find at New Car Prices and Used Car Book Values - NADAguides is a '98 in my ZIP area. Mid book is $3575 and the high-low spread is small. What value do you hope to preserve?

Quote:
So basically there really isn't a way to do a kammback or boat tail with a sedan like there is with a hatchback or a wagon without completely modifying the car?
"So, what you're saying is..."

For certain values of 'modifying the car' — YES. You're trying to modify the preformance envelope without modifying the engineering.

However if you allow removing the trunk lid, tail lights and back bumper and storing them in a clean, dry place then the story changes...

Bolt the outer corners of the box cavity to the sockets for the tail light housings and run flying buttresses to the roof, like a Ford GT40. And what XKCD/37 would call a big *ss-diffuser:


https://scontent.cdninstagram.com/t5...Mjg0NA%3D%3D.2
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Old 03-19-2018, 01:52 PM   #6 (permalink)
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Check out the forum's aerodynamic streamlining template, at the top of the page under "Tools". I plugged your car in and came up with this:




While not strictly 100% true, anything below that black line is in the car's "wind shadow" and isn't going to help maintain laminar airflow.

Marked in red is the shape of a kamm that would help lower CD:



EDIT: For comparison, here is how my Insight follows the template:

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Old 03-19-2018, 08:36 PM   #7 (permalink)
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Quote:
Originally Posted by freebeard View Post
The closest I could find at New Car Prices and Used Car Book Values - NADAguides is a '98 in my ZIP area. Mid book is $3575 and the high-low spread is small. What value do you hope to preserve?
Based on similar cars' selling prices it would still fetch close to $1K when I'd consider selling it. If I do. It all depends when 2015 or newer Honda Fits come into the sub 5K price range.

And then again later on I may decide my car looks too ugly and want to restore it to stock.

Quote:
Originally Posted by freebeard View Post
"So, what you're saying is..."

For certain values of 'modifying the car' YES. You're trying to modify the preformance envelope without modifying the engineering.

However if you allow removing the trunk lid, tail lights and back bumper and storing them in a clean, dry place then the story changes...

Bolt the outer corners of the box cavity to the sockets for the tail light housings and run flying buttresses to the roof, like a Ford GT40. And what XKCD/37 would call a big *ss-diffuser:


Quote:
Originally Posted by Ecky View Post
Check out the forum's aerodynamic streamlining template, at the top of the page under "Tools". I plugged your car in and came up with this:
Why didn't I think of this.

Quote:
Originally Posted by Ecky View Post
While not strictly 100% true, anything below that black line is in the car's "wind shadow" and isn't going to help maintain laminar airflow.
Makes sense, although I kinda thought the air settled back down over the rear roofline and then created drag after the rear.

Quote:
Originally Posted by Ecky View Post
Marked in red is the shape of a kamm that would help lower CD:
VERY helpful, and much appreciated!! Something I will definitely keep in mind if I can come up with a way to make it attach without drilling and yet be sturdy enough, and also be cost effective enough since coroplast would block the line of vision.
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Old 03-19-2018, 09:11 PM   #8 (permalink)
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Fit and finish. If it has obvious utility, to even a fraction of the public, and you can post performance information in the driver's side quarter window, why wouldn't the resale value go up over one otherwise equivalent?



This is Polymetal. It is most famous for being the cladding on the Grenfell Towers [inferno], but it is available in every town in America for signage. These samples were formed by hand with an 18" lever arm. It costs ~50% more than plywood, but is equivalent in stiffness to 5/8" plywood, and most importantly —it's prefinshed.

It's all down to using very clean rollers for that street/show level of finish. And that red area is definitional



You can scallop the back edge to be up-to-date.
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Old 03-20-2018, 09:07 PM   #9 (permalink)
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I would honestly do it if I could figure out a way to actually do it without spending 100s of $$ in parts. I'd want to do it in some kind of clear plastic or something so visibility out the rear wouldn't be impacted, and that in itself would already dramatically increase the cost over the coroplast I'll be using in my mods here soon. And then again I can't really think of a way to mount it securely without drilling, since I doubt clamping/zip-tying it to the hinges/edges of the trunk lid is anywhere near secure enough.

I really should take all these planned mods one bite at a time instead of trying to do everything at once lol.
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Old 03-21-2018, 02:30 AM   #10 (permalink)
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Somewhere near you vicinity there will be a recycler. At the very least Habitat for Humanity. In Bleugene Oregon it's BRING.

I've obtained a variety of materials there, Coroplast, double-wall greenhouse glazing, 1/8" ABS triangle cutoffs, sheet metal flashing, & etc. Your mileage may vary, but it would be less than full sheets for minimal cost. For a three-sided Kammback hood, I'd budget $50.

Sit in the driver seat and watch in your mirror while someone moves a shop rag or something around and note where the blind spots are. Then you can see what needs to be clear. ....or cut away. The transition onto any addition will be in the blind spot on the C-pillar.

It's good to have a plan. I try not to rush things too, eventual success counts.

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