Kammback for the Nut Wagon?
 

I'm thinking about doing it! I wanna break 60 MPG! The hatch is terribly rusted (old pic) and I have a better one, so why not experiment? I don't want to cover the taillights, so I want the Kammback to be above them. Anything goes above the taillights, what should I build? Should I keep the corners, basically mimicking the cars shape like the snow did, or transition to a rounded shape at the end? How long should it be? I don't want to go too far past the bumper. How much angle can I get away with on the top and sides?

http://img.photobucket.com/albums/09...gonprofile.jpg
http://img.photobucket.com/albums/09.../snowkamm1.jpg
http://img.photobucket.com/albums/09.../IMG_6220a.jpg

I have been told that round would be better than angles, so I did go with round shapes with mine.

One I made is 50cm long from top and top angle is around 10 degrees, side angle is around 15 degrees, but I guess 10 would be better there too.

snow form
 

Build it like the snow.Go all the way to the bumper and cut in fenestrations for the taillights to shine through on the sides.3rd brake light too.

my theory is 15 degrees is the goal on the sides, and 20 is the goal on the top.

pretty cool pic of your car with the snow. build it like that.

Yeah, just use some acrylic sheet on the sides. Try drmiller's angle and tuft test it to make sure it isn't losing flow attachment. If you can, transition to rounded at the end. It'll help with side winds.

What he said!

Do kind of like I did with my SedanKamm, start out mimicing the car, and then go to a "chamfered" back end, which mimics round. My top angle started out at about 10deg and went a bit steeper (maybe 15deg?) about half way down (it had a post stiffening it in the middle). The sides can come in at the same angles IF you have good attached flow on the sides like you do on the roof. Otherwise take it a bit more easy. Test for attached flow with a tuft test. They are easy to do if you have windows, which I would suggest so you can see when driving. Build in and around all tail lights, as Phil said.

Thanks everybody! I'm still thinking about what to build it out of. And it needs to open with the hatch, but that shouldn't be a problem. I have a collection of LED 3rd brake lights I can use, so I don't have to worry about covering the factory one. The bottom and back will most likely be left open. Is that going to be a disadvantage?

The more enclosed and lengthened you make it, the more benefit you'll see. If you can make it go around the sides and bottom, do it. If you can make it extend two feet back from the hatch, do it. It's very much what you put in is what you'll get out.

Other mods like wheel covers and skirts and belly pans will compound the benefit.

Disadvantage of the bottom being left open will only be seen if you have good flow under the vehicle to begin with. Do you have a full belly pan?
I would build directly off the hatch if you have a spare one. Building it this way will let the hatch and kammback open together.
Materials can be a wide range of things... How long do you want it to last?
1) I built my SedanKamm out of a used campaign sign that was made of coroplast. It was very appropriate for the build, and will last a long time even if it's exposed to constant rain/sun/ etc. Coroplast is easy to work with, and easy to come by, just call your local sign shop and ask for prices of both new coroplast, and used coroplast. New it should be about $20 for a 4'x8' sheet, used it is sometimes free. If you get used that has been printed with vinyl, you can take your time and peel the vinyl off (tip, let it sit in the sun for a while and heat the adhesive). If it's campaign season near you, look for guys with big 4'x8' signs, and offer to help take down, and dispose of, their signs after the election. If you find ones that have full color printing (a person's face), it's printed on a large piece of vinyl that is easy to peel off. Mono-color, or even multi-color signs are typically colored vinyl that is cut and applied, and takes much longer to remove.
2) Lots of the big signs are actually Alumalite, which is thick coroplast that is sheeted on both sides with a fairly thick aluminum. This stuff is really rigid. We worked with some, and a guy offered to bend it on his 30 ton sheet metal break, but after seeing it, he decided it was a bad idea! Sharp edges can cut right through heavy leather gloves... ask me how I know. Be careful if using this stuff!
3) There is another sign making material that is aluminum over a foam core. This we did bend on the big 30 ton press, but the operator refused to do more than a few degrees at a time, and would only bend it in the 4' direction. 3 grown men could not make the stuff bend over a 55 gallon drum.
All three of these options come in multiple colors, which you will get to choose if buying new, but have to put up with what you find if getting it used. I don't think I have seen brown for matching your car, but I have seen black, which is pretty stealthy.
4) For windows I used thin acrylic sheet (Plexiglas is a brand name of acrylic, Lexan is a brand name of polycarbonate). I got mine at Hobby Lobby for under $5 by using a 40% off coupon (which they have about every other week). I got the biggest one, and sized my windows to make a single sheet make all the windows (and a grill block for my wife's car). The thin acrylic worked well as windows on the SedanKamm, but worked very poorly as a grill block (rocks go straight through it). Coroplast, on the other hand, makes an excellent grill block, but that's another story.