Rear aero project - Civic notchback coupe

[brucepick]

Originally Posted by ChazInMT

Sorry, but I can't help thinking that the air needs to follow down the back end on something to get the best return. That pressure building back in my mind is helping to push the car along. When you do it in steps, it just is not the same.

On my 04 Civic, it worked out to where the back end of the "Kamm Over Trunk" was tall enough for me to put in a 8" window....were I to get off my lazy butt and build it. This would have allowed me to put a small rear window on that would have allowed me a view straight back.

Yes, that's pretty much what my plan is. I finally got convinced of the logic of a complete skin from roof line to final rear separation point.

The Lexan will start where the current off-the-roof gray one ends, will follow the top edges of the added side panels all the way to the back/bottom of those panels. I'm not planning a rear vertical panel, unless I learn of a good aero reason to close up that chamber.

[ChazInMT]

Yeah, sounds good. I read more carefully your post after I posted...sort of a post post thorough what did I just post regarding his post.....anyway, I apologize. You asked a specific question and I rehashed crap I been spewing for months.

I'd say you'd be better off curving the tops of the sides and curving the clear stuff "Front to Back", and, having the lexan be straight "side to side". It will be a compromise since getting it to "form fit" at the top of the rear glass will require the sides to be a little too high and the center too low, but if I were gonna choose, I'd go that route. I was thinking of having to get medieval on a 2 x 6 to make the complex top of glass transition. I was going to look for a chunk of lumber from the local purveyor of housing supplies for a 2 x 6 that was already cupped pretty good, then just start whittling away at it till it formed an airtight fit on the top of the glass. I have all the wood working tools (power hand planer, jig saw, & belt sander) to make that easy..er, but thought that’s what was going to be a key part for me. I'd think you could do the same and just sculpt the wood in such a way as to ease the transition from crowned to flat across the top there. Crazy talk?....maybe, maybe just crazy enuf to werk!

Any way, 2 more ¢ from me.

[Sven7]

If you're doing wood you could make a template out of cardboard to get the curve, then cut the wood and use foam body tape underneath to cushion it, keep it on and make up for any imperfections in saw work. I know on long curves it can get a little wobbly!

Also there was a Toyota Sienna boat tail thread awhile back that said it's best to enclose the rear. It might not be worth trying to find the thread though, since the opening you'll have will not be very big.

[COcyclist]

Originally Posted by brucepick

And, here's a shot of the view out the rear view mirror from the driver's position. The thin curved line across the middle is the spoiler center panel. You can see the vertical support, behind the center brake light housing. This nearly uninterrupted view is one of my design targets. It's a large spoiler, and you STILL can see out the back. Not bad, if I say so myself. The blurred blob at lower left is my hand holding my little Kodak.

Thanks for sharing the evolution of your Kammback. I like your view out the back and have been considering building a removable Kamm tail for my hatchback that would allow a view out the upper 1/3 of the rear window. I am a little concerned that the step-down to the Kamm may not allow the air to reattach even if I have a level section till it meets the template. I will be watching your progress with interest.

[brucepick]

COcyclist,

Thanks.

I'm also concerned about smoothness of the transition to the roof extension. Have shaved the leading edge of that panel to a knife edge hoping to keep attached flow, or at least not mess it up terribly. Duct tape to hold it down to the roof.

NO project is complete without duct tape!! Check out the dark green 3M "No Residue" type at Home Depot and other places. Holds great, no crud afterwards. Kinda pricey though.

The flat panel that appears as a line was pretty clever but coastdown testing showed only slight gains if any. Not sure if I put the test notes in here but...

I'm still going back and forth on this project.

Tonight worked on a roof extension panel that goes rear and down over part of the rear glass. At least for this weekend's Green Grand Prix, I hope to run it with just the roof extension and shortened side panels to match the extension length.

Found you can extend the roof line pretty far back, and down, before you intrude on the rear view. Use that info bit there! Also, use the defrost grid area as an indicator of what you NEED to see. That is, the top 8-10 inches of glass have no defrost wires. Your roof extension can block your view of that upper glass because you don't really need it. That's the theory I'm going on now, anyway.

[brucepick]

I did these test runs back on Dec. 3, 2011.
The results weren't very encouraging which is one reason I didn't post them earlier, sorry.

The results seem to show that the best version was the three panel one. The "better" design that would have been a full Kamm back all the way back, did not do as well.

See pic of four panel version below.

• Three panel version was the two side panels plus the "dropped" rearmost center panel, approximately parallel to the trunk lid.
• Four panel version had a roof extension panel coming off the roof extending over the rear glass (see pic below).
• Full cover had one more panel that completed the roof extension all the way back to meet the dropped panel's rear edge. So had it been done cleanly, that would have been a nice full Kamm back to a point even with the rear bumper.

Spoiler----- -----------Start--End--MPH
Version------Time------MPH--MPH--drop
Full cover---02:39 PM--65.4--48.0--17.4
Full cover---02:46 PM--67.6--49.2--18.4
Full cover---03:00 PM--68.6--50.5--18.1
Full cover---03:08 PM--65.6--48.3--17.3

Four panel--03:33 PM--67.3--49.3--18.0
Four panel--03:38 PM--69.0--51.0--18.0
Four panel--03:46 PM--64.7--48.8--15.9
Four panel--04:00 PM--68.7--51.0--17.7

Three panel-04:20 PM--66.8--51.8--15.0
Three panel-04:34 PM--67.0--51.4--15.6
Three panel-04:42 PM--67.1--50.8--16.3

No spoiler---05:20 PM--67.6--48.3--19.3
No spoiler---05:25 PM--67.1--49.0--18.1
No spoiler---05:32 PM--66.2--49.0--17.2
No spoiler---05:40 AM--68.6--50.0--18.6

The four panel and full cover versions should have been better than the 3-panel one. I finally deduced I hadn't built it very well, with the roof extension panel lifting up at the forward corners and just not sitting right. In April 2012 I built an improved version of the roof extension that worked well, I'll post some detail on that later.

Four panel version: