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):

http://ecomodder.com/forum/attachmen...1&d=1521387812

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

http://ecomodder.com/forum/attachmen...1&d=1521387812

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! :thumbup:

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.

http://image.superstreetonline.com/f..._rear_view.jpg

http://image.superstreetonline.com/f...rett_civic.jpg

These are built to be low-drag first, not low-lift, so they're generally a pretty good example to follow.

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.

http://ecomodder.com/forum/member-fr...studebaker.jpg

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:

http://ecomodder.com/forum/member-fr...n-tubik-21.jpg

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.

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.

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?

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?

"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
https://scontent.cdninstagram.com/t5...Mjg0NA%3D%3D.2

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:

https://i.imgur.com/8KE6oJx.png


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:

https://i.imgur.com/afCVJ9j.png

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

https://i.imgur.com/2YI1xUJ.jpg

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.

:eek:

Why didn't I think of this. :rolleyes:

Makes sense, although I kinda thought the air settled back down over the rear roofline and then created drag after the rear.

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.

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?

http://ecomodder.com/forum/member-fr...1-100-0866.jpg

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

http://ecomodder.com/forum/member-fr...ni-desktop.jpg

You can scallop the back edge to be up-to-date.

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.

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.

Here's a thought: It's not as good as a kamm, but you could try grabbing the trunk lid spoiler used on the Civic Hybrid of similar year. I believe Honda made a few changes to them worth 0.03-0.04, the lid spoiler being one of them. You shouldn't have a hard time finding one in a junkyard.

https://i.ytimg.com/vi/tVePTwn2JHg/maxresdefault.jpg

...$50? Wow, from my research I calculated over $200 just for parts alone. I can see how a recycler would be a good resource though.

Maybe I could remove the rear window, rear hatch, etc. and use a kammback to double the storage space in my car...don't laugh at me for this lol.

7th gen HCH is 0.28 and the standard sedan is 0.36 (I think).

I thought spoilers in general added drag?


How about this as a mini-kammback? Or would this add drag? Fit 2001-2005 Honda Civic Em 4-Door OE Rear Roof Window Visor Spoiler Wing 1Pc
Or these vortex generators? Fit For Roof Shark Fins Spoiler Wing Kit 9Pc Vortex Generator Evo-Style PP
Or would they not be worth it?

Cost plus? :)

The $40 part is similar to what kach22i has on his pickup IIRC. The 9 dollar part would require careful placement. In a wind tunnel.

If you tuft test the rear window, you'll find there's a bubble of flow detachment right in the center, at the base. I added 6 vortex generators a few inches down from the top of the rear window, angled at ~15 degrees to the direction of flow, and it kept attached flow across the entire window. Effect? Yes. Measurable effect? No.

http://ecomodder.com/forum/member-vm...810-182146.jpg

http://ecomodder.com/forum/member-vm...810-182215.jpg

Without a wind tunnel to test in, you're probably more likely to do harm than good with something like a spoiler, but they can help in some cases - I'm sure Honda did wind tunnel testing. Ditto with anything else you bolt or tape on which sticks out in the wind. Wings always hurt because by design they generate downforce at the expense of drag.

Pretty much the only easy, guaranteed DIY modifications are those which cover aerodynamically dirty areas with smooth panels, e.g. wheel spats, radiator blocks and belly pans.

Since all the Photobucket links are a mess, here is a shot of it from a few months ago.

random pics that YOU have taken....... - Page 605 - Pelican Parts Technical BBS
http://forums.pelicanparts.com/uploa...1520442287.jpg

I put the American flag decal on it because it cracked recently - many years of service before it happened.

The smoked acrylic originally came off the side of the building of an architect I was helping out years ago, it had just been sitting there for years - I cleaned out his yard.

Thanks, but I was thinking of the roof-top part.

The way that works is the gap or air-slot between the roof and the wing. The air gets accelerated thereby lowering the pressure, the higher pressure air above it then drops (in theory).

I think one of the after-market roof wings (spoilers/visors) could be modified to have such a slot creating an air-fence down the back window.

https://www.ebay.com/itm/JDM-2000-CI...-/290780346895

Looks like they run +$100, maybe a home-brew creation will be more cost effective in retrospect, especially if you mess it up the cut.

Another approach is a bi-wing?

Harder to pull off by just guessing your way though it though.

http://ecomodder.com/forum/showthrea...-26678-10.html
http://kep.cdn.index.hu/1/0/437/4379...69e3c4c_wm.jpg

Ford spent like a gazillion dollars to perfect that one I assume.

However, this is the similar direction I want to go in with my next experiment.

http://forums.pelicanparts.com/off-t...taken-606.html
http://forums.pelicanparts.com/uploa...1520790897.jpg

Anything that plumps up the rear deck to near aero-template profile should work to some degree in my opinion.

This angle below (red car) is close to that of the Civic in question, maybe it isn't all that bad?

http://ecomodder.com/forum/showthrea...-26678-13.html
http://image.superstreetonline.com/f...ght_window.jpg

Kamm/template options
 

If you like,go to the car blueprint website and check out the 3-view ,4-view images of:
*Cd 0.23 1993 Volvo ECC sedan
*Cd 0.23 2018 Audi A4 sedan
*Cd 0.22 2013 M-B CLA 180,Euro.spec.Blue.
*Cd 0.22 2017 BMW 5-Series G30 sedan
These are all non-Kamm,but industry-leading type low-drag sedan shapes,and if you'll study all the different views of their bodies,you might see some features you could use with the Honda.

So you couldn't measure any benefit other than the airflow remaining attacked? Even non-FE related gains such as increased coasting distance?

Maybe I'm missing a point that's obvious to you. If the idea of a boattail/kammback on the end of a sedan won't work because the airflow goes over it and doesn't reattach, wouldn't it do the same thing with a spoiler, regardless of it having an aero shape or not?

How about something like this that I posted earlier?

One would have to basically rebuild the Honda for those...

same FE effect?
 

The Kamm-back car is based upon a 1/2-streamline body of revolution.These bodies cannot produce separated flow due to their perfect sectional density.Drag coefficient is simply a function of body elongation.
The notchback car is a mutilation of the ideal form and technically,is 'challenged',as far as drag goes,and will not (cannot) react to boat-tailing as will a K-form.
It is true,that up to a point,the notchback can have little penalty as far as drag goes,compared to the same car with a K-form greenhouse,but after that,you'd need the K-form to get the drag lower.
It may have been MetroMPG who modified a Honda similar to your's with a K-form addition,only to see no real drag reduction after all his effort.
You might want to select a Cd target,then inspect vehicles(especially Honda's) which already meet that target,and see if your car might benefit from some of their features.
Research by Audi,on their Audi-100 III demonstrated 'saturation' points for specific mods,whereas some part would respond positively to a certain alteration,after which,no additional shape change could produce additional benefit.This is exactly what 'optimization' is all about.
There are production Honda's of Cd 0.27,and Honda concepts of Cd 0.20.Honda solar racers as low as Cd 0.10.

I did vortex generators on the Malibu that I had and water tested the back glass before and after... I copied the formula from the Evo, and had them about 4” above the glass... beforehand, there was flow separation in the center of the glass, that showed itself in water(dew in the morning) taking forever to run off if at all on my commute... after I added the vortex generators all the water run off as soon as I got up to 50mph, but I couldn’t detect any net improvement in drag... it took the same(or within a window of error) throttle/load to hold 55mph on flat ground as it did beforehand... now I never did a true coastdown test to produce more accurate data than just engine load at cruise, but eventually I removed them(after two tanks I think) both because I didn’t think they were helping and because my then wife was starting to refuse to ride in the car...

The visor isn’t a half bad idea(might use on my build), and they can be found for $30-50 for the Civic... a shame I can’t get one made specifically for my car, I’d have to cut up one for a Corolla or something... like mentioned earlier, cut a slot near the leading edge(about 2” wide(est.) and almost as long as the whole visor) to allow an air fence and I’d really think of using thin Plexiglass to extend the trailing edge to about where the trunk lid comes up to when open... if you follow the template shape, it *should* help

Which I proposed at Permalink #5?

visor
 

Mercedes-Benz used a combination backlight-header deflector,combined with a rear wing on their 190 Evo.I don't know if the two created a drag reduction,or just dealt with lift issues on the Autobahn.

It’s safe to assume that the giant wing was to reduce lift/add downforce but the rear window deflector I’m not sure about... maybe to increase the effectiveness of the wing? That’s a lot of why Mitsubishi put the vortex generators on the Evo...

That item in the "this" link is what I call an air flow "disruptor", more commonly called a spoiler because it spoils the air flow. This is in the Gurney Flap or Wickerbill family of aerodynamic devices, it creates drag because of the swirling vortex of air behind it, but that same swirling mass of air also pulls the air back down faster than it would without it.

So yes it would do something, something aggressive but it's main function is to create down-force on it's front face via build up of air pressure, and this also results in a lowering of air velocity at that point by the air hitting this blunt plane.

This as an analogy is a hammer, and I think you may want to use a knife. The right tool for the job as it were.

Have you seen some of the things forum user Sven posted?

http://ecomodder.com/forum/showthrea...car-20303.html
http://farm8.staticflickr.com/7013/6...84b68a7b_b.jpg

He also put together a PDF that shows the roof wing on my pick-up truck (still cannot find it - sorry), the intent on that design originally was to have a clean sharp edge for the roof air to release from. However by leaving an air gap it became some sort of weird Venturi channel generator and helped pull the air down on to the tonneau bed cover (a good thing).

Here is an idea, using clear acrylic make a Funny "U" looking similar to the below image.

http://ecomodder.com/forum/showthrea...n-22468-5.html
http://i188.photobucket.com/albums/z...k/100_0681.jpg

The F-U (standing for Funny-U) as we will now be calling it is only attached at the bottom two sides, the back/rear is open, and the front leading edge is beveled or rounded but not touching the roof - there is a air gap 1/4" - 1".

Just a concept.

Many people want to go with the tried and true for a known results. Others like sticking their head out a little and taking chances, the thrill of discovery and all that.

Make your choice.

If you are a "if it's not broken - then don't fix it" type of guy, then why are you posting here? :D

I don't know what's best for you.

Just have fun doing some thing.

Building a cardboard and duct tape mock-up is a good way to start.

Enough talk, just build something.

FYI: I think your car is currently in the Cd .30 range.

http://ecomodder.com/wiki/index.php/...t_of_Drag_List

EDIT:
It should come out a bit like this.

http://www.sarona.com/single_product.php?id=4973
http://www.sarona.com/media/images/4...ork_jersey.jpg

Or like this:
https://www.carid.com/aero-function/...f-spoiler.html
https://www.carid.com/ic/aero-functi...oncar-01_1.jpg

It really depends on the spoiler. I see aerodynamics as a black art.

https://i.stack.imgur.com/L5rdw.jpg


A spoiler won't reattach flow, but it can reduce turbulent wake if it's very carefully placed. It's not nearly as effective as a kamm or boat tail, which can keep laminar flow, but it does affect wake.

Here's an article you might enjoy:

https://tylerlinner1.kinja.com/a-pra...=1437153595522

A good example is a pickup truck bed. It's usually the case that drag is lower with the tailgate up, rather than down.

https://i.kinja-img.com/gawker-media...3904262440.jpg

https://i.kinja-img.com/gawker-media...4098079528.jpg
That's the PDF (link), thanks for posting it.

Hey, good post overall, hit on a lot of good points.

Makes me wonder why people react negatively to kammback shaped vehicles such as the CRX and Prius when they're so much more streamlined...after all it isn't that hard to produce an aesthetically pleasing fastback car.

The description of the ebay visor SAYS it helps with FE....but then again practically every exterior vehicle accessory claims to help with FE, even the ones that obviously don't.

cringes Yes, lol.

That is an AMAZING chart. Saved it for reference! :thumbup:

A Civic is broken if it can't easily beat 40 MPG on a tank, and my Civic is down to 36-37 even with hypermiling (due to short, cold engine trips). Therefore my Civic is broken and I need to fix it. :D

Just waiting to get my timing belt replaced before modding.

I know my Civic isn't a pickup truck, but just curious, is this because having the tailgate up creates an enclosed area that contains the airflow more?


Long post, I know..hopefully y'all can find all my responses to your posts in here. :o

S'okay. I come to Ecomodder because people pay more attention to what I say here than in real life, in general. Sometimes I even get thanked. Unheard of.

Yes, tailgate up is a weak version of the half-tonneau, which is close to equivalent of an aerocap.

I'd stopped using backlight in favor of window. I now feel justified in reverting. (It's all DayLight Opening)

SO I got bored at work and had a few min to kill, so found some scrap coroplast and some metal strapping material and tape and made a partial Kamm for my car... at the moment it’s adjustable in angle so I can tweak it as needed and if it doesn’t help it’s only held on with tape and the presence of the car’s original bodywork...

Pictures are crappy, but I literally did this inside five minutes lol

That's the perfect car to prove or disprove the principles I've discussed about a roof wing with vented and non-vented leading edge.

I say this because of the dirt roads you apparently drive on.

You most likely have a good idea how much dirt you have on your rear window normally, this can be used as a crude measurement of attached airflow, yes?

Now you have your partial Kammback or coroplast, and can observe dirt attachment.

Then you can move back or cut out an air-slot at the leading edge to see if you are getting some Venturi effect.

The dirt has a less chance of settling on surfaces with attached air-flow, yes?

I guess other methods are tufts or rolling down a hill tests, or even your gas gauge.

Tuft testing kammback spoiler and rear wheel skirts in a vw lupo 3l
https://www.youtube.com/watch?v=Md_Zl9tEk08

What do you think about one or two rows of speed holes rather than a full slot, for structural purposes?

On the OG Civic Hybrid, that tiny spoiler on the rear of the trunk lid is part of the aerodynamics package that resulted in it having a 0.03 lower Cd than the non-hybrid model (0.28 vs 0.31 IIRC). Of course, there was more in that, like the more closed off upper grill, additional underbody plastic, and more aero (though not particularly???) wheels.

The cars currently in the shop for suspension work I don’t have the tools to accomplish... I don’t drive on dirt roads but my driveway is extremely muddy... so far as I’ve tested it I’m getting more drag now... angle might be a little off... when I get the car back I’ll adjust and try again

In my opinion, that is a dirty turbulent solution.

Some sort of edge strengthening or even a middle spar for support as done on some rear wings could work. And like those solutions some aero-fairing of the returns (back portion) should be done.

19bonestock88, how did you determine you are getting more drag now?

Seat of the pants feeling?

Roll down tests?

MPG?

It looked like a very good start to me.

Was it flexing at all?

Did you hear fluttering?

My assessment is based on both coastdown(on points where I usually EOC) and instant MPG measured at 55mph...

I completed the mod at work so I tested it on my commute home, and found that in places I usually coast in, I’m arrivig st the other end at a slightly slower speed... for instance there’s a section where I arrive st the top of a slight downhill at around 40mph and at the end, almost a mile later I usually arrive at 38-40mph depending on surface water and temperature... the night I tested the mod, I arrived at 34mph...

Additionally, I found that my instant MPG at 55mph(with severe misfire and being in open loop) dropped from ~31-32 to around 27-28... with the engine making so little power, the extra drag is plainly evident...

HOWEVER...

I made a couple mistakes during construction that I addressed...

1. I put the leading edge of the Kamm on the sealant seam at the top of the back glass, making a 8-10mm lip for air to jump

2. I used metal strapping material taped to the trailing edge(underside) to keep the trailing edge straight but as low as I had the sides pulled down it bulged up in the center and pushed the center up to a flatter angle than I wanted...

I accidentally tore the thing off backing up at around 20mph and when I put it back on I addressed the above flaws(but I didn’t have/lost the metal strip) so the center of the Kamm now saga, but only when the car is stopped... it lifts above around 35mph, to be flat across its width

backlight
 

'backlight' is the industry's term for the rear windshield.Whenever I can,I like to use these terms.:)

If you kept that leading edge open a little I bet it would have been okay.;)

I'm sure you know this but 20 mph in reverse is hard on the transmission.

The only time I see people doing this regularly is in Dearborn, Michigan where I grew up. The immigrant Arab population is very fond of going backwards for some reason, will go down an entire block and even corners backwards.

Must be a cultural thing, I never understood it.:confused:

a cheap prototype
 

So far,I've identified 165 vehicles,from 54 different manufacturers or universities,which used template-like bodies,or located a rear spoiler along the templates pathway.
Maybe there'd be a chance that the Kamm-back mod would show at the gas pump.
MetroMPG coined the term 'Cardboard And Ductape' (CAD) design,years ago,and I was thinking that you could fab up something using this technique,then attach it with 3M,Blue Painters Tape.
The cardboard and tape would hold the Kamm together enough for testing.The 3M tape would hold it securely to the Honda,without any chance of leaving a residue.
Just thinking out loud.
The Insight-II,Accord Crosstour,Clarity,and some of Honda's lowest drag concepts are all Kammbacks.
On my '84 CRX,I created flying buttress C-pillar extensions with an upper deflector,then rear wing,to 'enclose' a vortex within the template footprint,creating a rudimentary Kamm greenhouse,then added a foot of boat tail.It helped it get down to Cd 0.235.I can't imagine something like it not helping.Just a hunch.