[UnicycleDan]

Hi,
I'm new to this site, but I've been considering aeromods to my SUV for a while and researching quite a bit. I drive a 2003 Enovy XL, 14 mpg city, 19 highway. I've changed my driving habits and best I've hit so far is 23mpg. I consider this a success, but of course now it's time to make it better. I know you're thinking: ditch the guzzler and get a car. Sorry, I'm in college, with limited (but thankfully existent) funds. Also, I've used the full capacity of the vehicle moving apartment stuff and people countless times, so I'm keeping it for now.

I've been recording my driving habits with each tank to see how they affect FE, all recorded for my vehicle, BigBlue, in the EcoModder Garage, with notes next to each tank entry. I plan to read through your 100+ Hypermiling tips and improve my driving further, but as an engineer, I want to build stuff, too, so I'm looking into ecomods. I'm kind of sad, though, to find hardly anything on kammbacks and other aeromods for SUVs, just a few for trucks, and one for a van.

To start off, a self-imposed limitation: I want all of my modifications to be non-invasive to the stock car, no holes drilled, etc. I attempted to attach a picture of my car...we'll see if it works.

I have a handful of minor aeromods planned: front grill partial block, gap and headlight recess filler pieces in front, front/side airdams or underbody panels, roof rack, mirror mods, etc. But, I'd like to start the designs for a more major mod: a Kammback attached to the hatchback. My thought for sturdiness and being non-invasive is to hook a bike rack in place and mount the kammback's frame to the bike rack as if it were a bike (only hopefully a bit more aerodynamic).

The attached picture/template shows a basic view of what I'd like to do. I live at an apartment with very limited tools, so the kammback will likely have flat panels (5 panels: 1 top, 2 on each side, since the side of the car is roughly vertical on the bottom half and changes to angle inward toward the roof on the top half-1 panel for top angle, one for bottom). I also plan to used a 90-degree wedge of roughly 4-in PVC for each of the top corners where the side panels meet the top panels so that I get large-radius corners on top. I plan to make the kammback 2.5-3 feet long for biggest aero effect, while trying to keep a reasonable-length car (already 17ft long to start).

Okay...lots of background...now for my first question. My attached picture shows roughly a 9-degree angle slope for my kammback (top and sides each 9-deg inward). I found pictures and comments on your threads and elsewhere online with "best angle" claims for flat-panel kammbacks ranging from 7-12 degrees. Also, Trailer Tail kammbacks for semis are roughly 10 degrees. I know tuft testing will probably give me the best answer, but is 9 degrees a reasonable place to start? During tuft testing, am I essentially looking for the steepest angle that will retain laminar flow?

Thanks.