Partial Boattail/Kammback on a sedan?

[19bonestock88]

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

[kach22i]

Originally Posted by 19bonestock88

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...

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

[freebeard]

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

[samwichse]

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.

[19bonestock88]

Originally Posted by kach22i

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

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

[kach22i]

Originally Posted by freebeard

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

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?

[19bonestock88]

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

[aerohead]

Originally Posted by freebeard

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)

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

[kach22i]

Originally Posted by 19bonestock88

I accidentally tore the thing off backing up at around 20mph ......

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.

[aerohead]

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.