Installed Coro mini-spoiler & Wake Boards

[Xringer]

I've been wanting to try something on the back hatch of my CRV, but I didn't want to do anything that would look outrageous to my wife.
Dang, I should have used the yellow or red.. The blue just doesn't work..

I started off with 39"x12" on the top and worked my way down the glass. The two side sections are 22"x9".






Didn't create any blind spots and it's all on the lift-glass, so it pops right up cleanly for loading over the tailgate.

There is just a slight boat-tail pattern and I'm really wondering it this will have a positive effect on the MPG or will it be a wash..

What do you guys think?
(And no comments about the color, please)

[cfg83]

Xringer -

I like it. It is similar to PC mock-ups I made for my car. However, on MetroMPG's thread I learned that my original "flattish" slope was not optimal. That thread led me to these drawings :


(Compromise: kamm-back stops at end of bumper)

It looks OK from the side, but it becomes a reduced-visibility problem in the back and the third brake light is covered (therefore requiring clear top section) :



But maybe I'm mixing apples and oranges ...

Question: Are boat-tails and kamm-backs *both* legitimate solutions to aero design problems? Which one gives you a better solution?

CarloSW2

[Xringer]

Question: Are boat-tails and kamm-backs *both* legitimate solutions to aero design problems? Which one gives you a better solution?

I think that some may consider the $$cost vs gain as a factor in the "better solution"..
The Wake board and Frame extension ideas have been wind-tunnel tested,
because they were VERY cheap to install on 16 wheelers.
(Of course, an expensive full boat-tail is going to be way better).

My aim is to try out that cheap, quick and easy stuff to see what might work..

It's a crap shoot. But, if something quick-n-easy actually works, that would save a lot of people a lot of work and maybe some gas money..

One more step in trying to save the planet.. LOL!

[Sean T.]

I would think it would be more effective if the sides of the top piece were cut on a slight angle, to remove the "bowing" effect of the top panel.

With the bow, it kinda just looks like you extended the rear straight out. You can really see it in pic 3.

[Xringer]

Originally Posted by Sean T.

I would think it would be more effective if the sides of the top piece were cut on a slight angle, to remove the "bowing" effect of the top panel.

With the bow, it kinda just looks like you extended the rear straight out. You can really see it in pic 3.

I'm not sure what effect the bowing will have on the top panel, but if
you look at the last pic, you can see there is already a bit of a bow in the roof of the car.
The top panel is following the contour of the roof.

The main reason I cut the top of the side panels at angle to pull down the ends of the top panel, is to conform with the shape of the car a bit and try to get a little boat-tail effect in the top. The pulling stress also stiffens the coroplas to help keep it from flapping in the wind.

[COcyclist]

Good work! I've been thinking of doing something similar on the back of my Golf. Testing is the only way to know if you will get any benefit. I have a Scanguage but I prefer coastdown testing. It seems quicker and more accurate to me. I have noticed that if the wind picks up it really throws things out of whack.

Let us know if you get some good A,B,A test results with your Kamm back.

I know you want some taper to the top and sides but it's hard to see from the photos. Also, since your sides are set in from the high-mounted taillights the total angle should be measured from the outside edge of the car to the end of the side panels.

Happy Modding!

[aerohead]

Originally Posted by Xringer

Question: Are boat-tails and kamm-backs *both* legitimate solutions to aero design problems? Which one gives you a better solution?

I think that some may consider the $$cost vs gain as a factor in the "better solution"..
The Wake board and Frame extension ideas have been wind-tunnel tested,
because they were VERY cheap to install on 16 wheelers.
(Of course, an expensive full boat-tail is going to be way better).

My aim is to try out that cheap, quick and easy stuff to see what might work..

It's a crap shoot. But, if something quick-n-easy actually works, that would save a lot of people a lot of work and maybe some gas money..

One more step in trying to save the planet.. LOL!

Xringer,the boattail is merely an extension of a Kamm roofline which has attached flow.--------------------------- If you want a sense of what it will do,take a look at cfg83's Saturn,under MetroMPG's aero template/teardrop.Where the roofline begins to fall away,is the beginning of the Kamm roofline.From that point,to the end of the teardrop is the potential for drag reduction.If you divide that length into 10 equal segments,each segment is good for a 10% reduction in wake area,a 10% drag reduction,and a 5% increase in mpg (HWY).--------------------------- As you can see on the Saturn,the roofline allows for some drag reduction by the end of the roof and sides.Your CRV doesn't have as much taper,so your wake is basically as big as your frontal area.------------------------ The longer you make the tail,the greater the drag reduction,as you convert air velocity back to static pressure.That's really all there is to aerodynamics!-------------------- So basically,you can compare the length of your extension and new wake area,to the CRV's original wake.For every 10% reduction you pick up 5% better mpg.---------------------------------- Theoretically,if you had a lightweight tadpole trailer,with gap-fillers between the CRV and the trailer,which extended out the full length of the teardrop,you'd have a 100% drag reduction,and pick up 50% better mpg on the highway.So say your pulling 30-mpg on a good day,you'd be looking at 45-mpg at a steady 55-mph.

[Xringer]

Originally Posted by aerohead

Xringer,the boattail is merely an extension of a Kamm roofline which has attached flow.--------------------------- If you want a sense of what it will do,take a look at cfg83's Saturn,under MetroMPG's aero template/teardrop.Where the roofline begins to fall away,is the beginning of the Kamm roofline.From that point,to the end of the teardrop is the potential for drag reduction.If you divide that length into 10 equal segments,each segment is good for a 10% reduction in wake area,a 10% drag reduction,and a 5% increase in mpg (HWY).--------------------------- As you can see on the Saturn,the roofline allows for some drag reduction by the end of the roof and sides.Your CRV doesn't have as much taper,so your wake is basically as big as your frontal area.------------------------ The longer you make the tail,the greater the drag reduction,as you convert air velocity back to static pressure.That's really all there is to aerodynamics!-------------------- So basically,you can compare the length of your extension and new wake area,to the CRV's original wake.For every 10% reduction you pick up 5% better mpg.---------------------------------- Theoretically,if you had a lightweight tadpole trailer,with gap-fillers between the CRV and the trailer,which extended out the full length of the teardrop,you'd have a 100% drag reduction,and pick up 50% better mpg on the highway.So say your pulling 30-mpg on a good day,you'd be looking at 45-mpg at a steady 55-mph.

You are so right about the lack of taper on the CRV. It's way too boxy.
That's the main reason I found the improvements being used on trucks so interesting.
If this stuff doesn't contribute anything to the MPG or causes drag, I should be able to see the effect on the MPG after a while.

I'm thinking about some shorty rear wheel skirts next.. Maybe the over-all effects will add up to a couple extra MPG some day.. I'm looking for 35 MPG at 55 MPH..

I'm also thinking seriously about adding an EV motor. A mild-hybrid conversion.

[cfg83]

aerohead -

Originally Posted by aerohead

Xringer,the boattail is merely an extension of a Kamm roofline which has attached flow.--------------------------- If you want a sense of what it will do,take a look at cfg83's Saturn,under MetroMPG's aero template/teardrop.Where the roofline begins to fall away,is the beginning of the Kamm roofline.From that point,to the end of the teardrop is the potential for drag reduction.If you divide that length into 10 equal segments,each segment is good for a 10% reduction in wake area,a 10% drag reduction,and a 5% increase in mpg (HWY).--------------------------- As you can see on the Saturn,the roofline allows for some drag reduction by the end of the roof and sides.Your CRV doesn't have as much taper,so your wake is basically as big as your frontal area.------------------------ The longer you make the tail,the greater the drag reduction,as you convert air velocity back to static pressure.That's really all there is to aerodynamics!-------------------- So basically,you can compare the length of your extension and new wake area,to the CRV's original wake.For every 10% reduction you pick up 5% better mpg.---------------------------------- Theoretically,if you had a lightweight tadpole trailer,with gap-fillers between the CRV and the trailer,which extended out the full length of the teardrop,you'd have a 100% drag reduction,and pick up 50% better mpg on the highway.So say your pulling 30-mpg on a good day,you'd be looking at 45-mpg at a steady 55-mph.

I was working on a version of the "tear-drop line" for Xringer's CRV, but you can see I may be off a bit on how I aligned the tear :



However, the alignment may not be too important because the angle all along the "trail" of the tear (comet?) is almost the same. The hard-to-see trailing red line is about 10.5 degrees below 0 degrees horizontal.

Xringer -

You are so right about the lack of taper on the CRV. It's way too boxy.
That's the main reason I found the improvements being used on trucks so interesting.
If this stuff doesn't contribute anything to the MPG or causes drag, I should be able to see the effect on the MPG after a while.

I agree with your logic on using trucks as your inspiration. To me, the truck solutions make sense for Scion XBs, Jeeps, and "boxy-shaped" SUVs.

CarloSW2

[Xringer]

10.5 degrees.. Thanks for doing that work. I just went out and checked the angles.
Above the drivers head, the roof slopes down at 4.5 degs. But move towards the back seat, and the roof becomes flat. It stays flat (zero slope) all the way back to the rear hatch.

The good news is, the hatch lid has some down slope.
I checked my coro angle on each side (at the pulled-down edges) and they are almost dead on.
10.5 deg on the left and 11 deg on the right. (sloppy tape job)..

The big rise in the center of the top panel needs work, since it's only 4.5 deg.
I'll have to see what I can do to flatten it out some.