Finally! I had some time to play with the car after doing some needed repairs and working on the house/yard this summer. This is a good mod; it is the first one I have tested to my satisfaction since installing and calibrating my MPGuino fuel economy gauge.
The theory of this modification involves two things, changing the shape of the rear of the car, and changing the size of the rear of the car. Both contribute to dragging around a smaller wake.
The air flowing over/beside the car needs a clean, crisp "separation" edge, so a fin extending past the rear of the car will help this. Many new cars have integrated these little fins into their design to improve economy. A common and easily seen example is the back of the Dodge Caliber, but smaller ones are all over, including on the top of new truck tailgates.
Reducing the size of the wake also yields benefits, so tapering at the rear of the vehicle (at the correct angle) Typically this angle is around 30 degrees below the horizontal. From the explanation by Phil Knox, the tail on your vehicle should point to a place on the ground that is 1.78 times longer than your car's height where you want the taper to begin.
My inspiration comes from 4 sources:
Darin's Firefly (Metro)
Ok, on with the photos!!!
I chose to use galvanized 1/2" metal conduit for the frame. Cheap, strong, and easy to bend with a hammer. This shows how it's connected to the top of the car's hatch. A sheet metal screw through the top now holds it in place permanently:
The conduit extends 1 meter from the top, is hammered flat and bent, then attaches to the bottom of the hatch:
A cross-bar is held in place by bolts on the hatch's underside and two small pieces of conduit go down the edges of the window:
This shows the completed frame. Next step is to figure out where to use coroplast and where to use plexiglass, to cut down on the cost:This looks like it might obscure the rear visibility with this much covered with opaque coroplast, but here's the view from the inside:Not too bad. Well, the wife didn't approve. Something about "no more campaign signs on the car" and "I'm not going to ride in it anymore" Something I'm sure many other ecomodders hear all too often! Well, the solution is to spend ridiculous amounts of money on all plexiglass:Note the placement of my two shiny new Ecomodder decals courtesy of Dan (Intrigued). They are way more visible than the static clings (they really are in the photos!)And all the utility of the hatch is preserved. I just have to bend my knees to open it; the tip reaches my shoulder when I lift the latch. Just have to remember it's on there when shutting it - it almost gave me a concussion the first time! :)
Drove it around for a couple days with no problems. Many more looks than I used to get though, so I definitely need to post my fuel economy up on the car to explain all the junk on the car. I even attracted the attention of two of my neighbors who are interested in getting better gas mileage and they're going to check out the EM forum now!
Now what you've all been waiting for: Fuel economy results!
I did A-B-B-A testing (After Kammback, Before Kammback) I left the frame on and just took off the plastic. My test route for this was highway only; 20 miles round trip with an overpass-turn around in the middle. I used pulse-and-glide on each hill, but set rules: I had to reach 60 mph before each coast, and I did not coast below 55 mph. Raw numbers:
A: 55.2 mpg
B: 51.2 mpg
B: 50.2 mpg
A: 54.8 mpg
Average of B runs: 50.7 mpg
Average of A runs: 55.0 mpg
I'm comfortable saying it is worth 4 mpg highway - a pretty good improvement.
Lifetime Fuel Economy: 39.49 mpg
View my fuel log here: http://ecomodder.com/forum/em-fuel-log.php?vehicleid=717