Subaru Outback Kammback Experiment
 

Finally got enough coro lieing around to try this. I'm disappointed in the quality of pictures that turned out, my lens was smudged and I didnt realize it. As such, I only have one fairly lame picture of it, but you should get the idea.

http://lesbaru.com/stuff/kammback.jpg

Once again I apologize for my usual dark pictures, I pre-lightened this one up so people with dim monitors can see it.

It was just coro and duct tape, self supporting and suprisingly solid. Followed the lines of the car as best I could, although there was a crease on the top where the side met the roof. It rounded itself out more as it got further back though.

And as for the results....

Tested with scangauge, cruise set at 55 mph on a roughly flat, roughly straight 1 mile course (its hard to find anything like that in WV) down and back with no traffic. (Benefit of testing at night) Cruise was cancelled with the brake and then resumed. Weather was fairly calm, it was freezing though, winter gas. Blah blah. Sucky averages as a result.

Pretty much how I've tested everything else.

With The Kammback:
A 30.8
B 32.9
A 31.6
B 33.4
A 31.4
B 32.6
A 32.1
B 32.4

Average 32.15

Without the Kammback:

A 32.1
B 33.6
A 31.1
B 29.7
A 32.8
B 31.3
A 32.6
B 29.9

Average 31.63

So, just barely over half an MPG at 55 mph. Were this a free mod, it would stay, but a permanent solution would be costly, so its down on my list now. I encourage everyone to do their own testing and my post is just here as a reference for others. Blah blah.

As far as driveability, I didnt lose any rear view. I lost sight of the horizon behind me, but there were no blind spots created, which was actually pretty surprising to me. It also pretty much eliminated the need for the rear wiper. ;)

take the roof rack off and try it again? maybe the angle is too steep?

i'm surprised duct-tape held together so well.

I dont know, in person it didnt look steep enough. I'd say it was around 10 degrees. But what do I know?

As for the rack, thats staying. To take it off would involve disassembling the entire inside of the car and I'm still left with holes to fill somehow. AND I do use that thing at least twice a month now. Its fairly aerodynamic so without the crossbars it should be alright.

The angle should be no more than 15* from the steepest slope that it accompanies.

I'm honestly thinking that most of your airflow disruption will be coming from the relative height of your suspension, given that it's a Subaru and it's AWD. They're up there a bit.

I'm thinking an airdam will be more beneficial to you at first.

Interesting to see, even with the roof rack - would be a bigger gain for those of us with no racks I bet.
I agree, from the photos it looks steeper than 15 degrees, but a straight on side picture would settle the debate.
Matt

hm...maybe i should make a subee kammback just for old time's sake.

Can't see the pictures :(
lesbaru.com is not responding :(

i don't think this thread is breathing anymore.

I cant edit the original, so... FIXED

http://lesbaru.net/stuff/kammback.jpg

If I wasn't working 80 hours a week lately I would make another one of these.

very cool. thinking since i got my garage setup i'm going to do some tricky stuff automobilically, speaking. i might have to try something like this. I'd really like to make an aero trailer, though.

what do you do for 80 hrs a week?

Commission based Sales. (:

oooooooooooooooooooooooooooooh aaaaaaaaaahhh

Decided to do a quickie on my day off with some of the same coroplast I used originally. Missed the modding days.

Anywho, I think the angle is better on it this time, will let someone smarter than me be the judge. Can you heat up coroplast to make compound bends any easier? I'm ok drilling into the hatch for a permanent connection and making the shape good, but mine always wants to just get a crease. I know there have been bondo/fiberglass ones, but coroplast is cheap and if I could just get that bend on it would look/perform fine with very little time involved.

awesome! niiiiice

compound curves
 

I wouldn't try this in the kitchen oven,but if you constructed (threw together) some sort of insulated box,with some heat lamps inside;you could experiment with a scrap of the plastic.
The trick would be having a wooden mold to blow,or vacuum the plastic onto or into to create the shape.

You could get away with a more aggressive angle. Would this lower drag even more? We don't know. You should try enclosing the sides though. You could bring the sides all the way down to the bumper, creating a clean cutoff point for air. We don't know how much time the designers spent in the wind tunnel working on those corners, either, so testing is important.

Anyway. When making my lawn edging air dam I held the plastic close to the woodburning stove in the barn for a few minutes and it softened enough to bend better. You might try a hair dryer on its hottest setting.

How about a 1800 watt heat gun instead?

Got to try this out just a little so far. I do need to bring the sides down more as at 65 mph it is kind of flapping from the wind, not a lot, only around 2 inches up and down, but enough for me to reinforce it for sure.

heat gun
 

I don't even know if coroplast can be thermo-formed.It was originally extruded through a die set as melted pellets whereas ABS is a sheet compound and designed for thermo-forming,
If you're after compound curves I think you'll need tooling and full heating of the material.
Thermo-formed material has 'memory' and after formed and cooled will return to its pre-formed shape if re-heated.
You'll have only one chance to get it right.
Papier mache is a very low-tech way to achieve extremely complex compound forms and can be 'glassed' to protect it from the elements.

I'm going to try and do this tomorrow or Tuesday ish. I've got some materials left over from my grill block.