Designing a partial kammback on a sedan - Page 3 - Fuel Economy, Hypermiling, EcoModding News and Forum

AeroModder · 08-17-2010, 05:04 PM

I did some measurements and found that a 4' kammback would reach just past the edge of the trunk and sit 4" high off the trunklid. I'd be sealing off the trunk more or less, but I could remove and replace it if I absolutely needed the trunkspace.

wyatt · 08-17-2010, 05:11 PM

I maintained trunk access by using the fold down rear seats. I don't know how well that would work for you, but it worked really well for me (since I never once NEEDED all the trunk space, all the space in the rear seat, and all the space in the passenger's seat area). It actually really wasn't that hard to pack/unpack the trunk through the rear seat either.

AeroModder · 08-17-2010, 06:20 PM

Unforunately, Tempos don't have rear seat trunk access.

What thickness of acrylic sheet did you use? Home Depot has a 30"x36" sheet of .096" acrylic listed as shatterproof for $15 I was going to pick up.

wyatt · 08-17-2010, 06:47 PM

The acrylic I got was from Hobby Lobby for $3 with a half off coupon. It was super thin and definitely not shatterproof! I would guess it was maybe 1/32 or thinner, and a similar size.
It may be possible to do a good enough job attaching your Kammback to your car (and make it stiff enough) to have the ability to cut it and hinge it allowing access to the trunk. I knew mine was temporary and I still had trunk access, so never went forward with this route.
If you were going to do the full kammback, and want it to last, you could look into getting a bike carrier and mounting it on your trunk lid to attach the kammback to... or maybe get a second trunk lid to do a "permanent attachment" to (so you don't put holes in your original)... something better than duct tape.
Something else to keep in mind, dust collects on your rear window with a kammback in place, so either you would want to seal off the interior of the kammback completely, or have some way of cleaning the glass. Sealing it off completely would be difficult, I used duct tape to seal the edges, and around the glass, but after a couple months, there was water leaking in when it would rain (just in a couple places, but I could never track down where the weak points were...). Having the ability to see out is pretty nice.

AeroModder · 08-17-2010, 07:01 PM

I was going to seal it off completely. Did you use 4 mil coroplast or something a little thicker? I was thinking of getting 6 mil so it doesn't bow in the middle and using some aluminum strips to stiffen it. I've already drilled holes for the side skirts, but for the kamm I'll use 3M double-sided moulding tape. That stuff holds incredibly well.

wyatt · 08-17-2010, 07:07 PM

I used 4 mil (since it was what the campaign signs were made out of). I had a 2x6 at the end of my trunk lid, and added additional supports inside to give it a bow in the middle. This prevented the sagging, and added a surprising amount of stiffness to the coroplast. Make sure you have a good fit, and that it is attached sufficiently well. You don't want to have something sticking up and catch the wind at 60mph, I just don't see it ending well, no matter how strong your adhesive is!

Weather Spotter · 08-17-2010, 07:09 PM

I made my Kammback out of 4mm as well. It holds up just fine.

wyatt · 08-17-2010, 07:12 PM

Just not as stiff as I would have liked... I would have liked to get the 10 mil... but free is free!

AeroModder · 08-17-2010, 07:46 PM

Oh, you did have supports in there. I couldn't see them in the pictures. The black coroplast I used probably absorbed too much heat and started sagging because of that.

Speaking of sticking out and catching air, I actually had one of my undertray panels come loose at 50 mph and fly off. I bolted it back on with some wider head screws and washers, and added a zip tie to the front and made double sure it doesn't catch air under it.

AeroModder · 09-08-2010, 12:27 PM

Okay, I've got some basic plans. I'm going to Prius my rear end.

I'll have the angled Kamm extend out to the end of the trunklid, and a flat spoiler extending to meet the side extensions, with a couple pieces of coroplast to have the sides meet the spoiler, and have the rear edge all on one plane. Then I'll take a sheet of clear acrylic and cover the entire rear end so it doesn't end up acting like a suction cup, but a clean, flat surface.