Designing a partial kammback on a sedan

[wyatt]

Originally Posted by AeroModder

On the kammback: I'm wondering about meeting in the middle. In it's current form, it looks like vorticies will be made that will tumble over the trunk. I'm not sure if this is benefitial in helping to fill a void or not.
I'm considering adding to the sides to make the ends vertical to eliminate any vorticies formed by the angled edge, and trimming off the top plane to meet the new edge. I would only be losing a few inches of the kamm, so it wouldn't be too much.

I would just add to the sides, but go all the way to the end. We all know you will get max MPG gain with max length, but if visibility is an issue, I understand. I used thin acrylic that I had to make my windows, they are just duct taped into place and do a good enough job, but be sure to tape all your edges to prevent cracks from causing pieces from falling out if it cracks. If you have access to Plexiglas or something similar (that is thicker and more crack resistant than the super thin acrylic I used) you will be better off. Use it to finish out your sides, and you could extend the whole back if you wanted.

[AeroModder]

Finally got a dry day to redesign the kamm. I flattened out the sides and trimmed 2 inches off of the overhang, but I needed to clip some off of the side to allow the trunk to open.

I also redid the supports and got them more evenly spaced, and gave it a nice gradual curve over the rear glass.











EDIT: Either today was a good day, or the change really helped. My usual coastdown check went farther than it ever has today. There was a bit of a breeze out (crosswind), and the temp was around 60. I finally hit 35 MPH at the second 35 speed limit sign, when I usually come to that speed at the city limit sign between the two 35 limit signs.

EDIT 2: Another coastdown test today was close to the last one. A semi truck slowed down my coast a bit, though.

[Arragonis]

I'm intrigued, what are those parts on the rear lights for ?

[AeroModder]

Those were to extend the inward angle of the rear end and give a cleaner edge for flow separation. I ended up removing them, and just recently revisited them with an altered design and a better attachment method.

I'm starting to think of making a full permanent kammback with a hinged top for trunk access and some clear plastic panel for the rear window. I'll probably get another sheet of coroplast in red and some brackets and braces after I sketch up the plans. The proper angle will come up about 4 or 5 inches above the trunklid. I might even extend it out to the end of the bumper to have a completely rectangular end.

[wyatt]

Originally Posted by AeroModder

Those were to extend the inward angle of the rear end and give a cleaner edge for flow separation. I ended up removing them, and just recently revisited them with an altered design and a better attachment method.

I'm starting to think of making a full permanent kammback with a hinged top for trunk access and some clear plastic panel for the rear window. I'll probably get another sheet of coroplast in red and some brackets and braces after I sketch up the plans. The proper angle will come up about 4 or 5 inches above the trunklid. I might even extend it out to the end of the bumper to have a completely rectangular end.

I drew these, they may give you a heads up on sketching... The first one is in Google Sketchup, and the scale is approximate. The second one is very close to actual. These were for my car, but both mine and yours are sedans, so you should be able to draw lots of parallels. Mine was 4' long (because I had a sheet of 4'x8' coroplast). I think the overall width was around 6' for mine. It could have used some more trimming, but I was extremely happy with the results.I had considered doing the square sides down to the bumper, but really didn't want to obstruct the tail lights at all.

[AeroModder]

That looks quite good. Were you able to get a number on approximate mileage gains on that? My only mileage calculator is distance/fillups.

[wyatt]

I did coast down testing, and saw a 12.5% reduction in drag, which is about what I had expected considering the reduction in trailing area. That translated into about a 6% increase in FE at highway speeds, or 3mpg for me. If you are getting closer to 30mpg initially, and you do a 4' extension like I did, you should see about a 1.5 to 2 mpg increase at highway speeds. My MPGuino has been out of service for a long time, so the MPG increase was comparing two 1800 mile trips with similar driving conditions, speeds, etc.

[AeroModder]

Alright. I'll call up the place I got my 4x8 sheets last time and see if he has some in red. The rebuild won't be until next month, though. I also no longer have my camera, so I can't take pictures of the build until I can afford a replacement.

[Weather Spotter]

Originally Posted by AeroModder

Alright. I'll call up the place I got my 4x8 sheets last time and see if he has some in red. The rebuild won't be until next month, though. I also no longer have my camera, so I can't take pictures of the build until I can afford a replacement.

I would see if a neighbor kid can take a few with his iphone and email them to you http://forum.ecomodder.com/images/smilies/biggrin.gif who knows, he might ask what you are doing and you could convert him into an ecomodder.

[battlewagon]

I'm also making my own.