Half height pickup bed cap?
 

Has anyone thought of doing an aerodynamic bed cap that starts halfway down the rear window instead of at the roof?

The benefit would be that if it starts below the sightline of the mirror, the driver's rearward view is not obstructed.
On the other hand, the driver's view downward and to the sides would still be blocked, which matters more the higher the vehicle is.

Would airflow reattach after the short step down from the roof?
How much better would it be than a flat tonneau?

http://img207.imageshack.us/img207/6...ewhalfline.jpg

Purely speculation here, but i watched the rain patterns on my brothers tonneau cover and there was quite a low pressure zone immediately behind the cab, which then vortexed near the center of the bed. Given this, I would say that the half cap would likely be an improvement as it would fill in the low space.

Oh, and you could build this out of cardboard and scrap wood easily if you wanted to test it.

Flow would not reattach. No benefit over a tonneau.

I agree that flow would not reattach. Even a quite shallow ledge that wouldn't preserve much visibility anyway would prevent this.

The bottom line is it depends on the truck. How the air back fills behind the cab varies across makes and models due to a plethora of differences - some negligible (style), some substantial (shape). On my xCab Tundra 4WD a hard tonneau was an improvement with highway 22 mpg. With open bed 20-21 mpg.

In general, I doubt it can be worse than open bed, and don't see how it could be worse than flat tonneau either. But my opinion is moot. Ask your truck. Testing a cheap prototype was an excellent suggestion!

Cheers
KB

it will be the same as a tonneu cover.
I used this method about 8 yrs ago on a Mazda B2200 longbed (hard to find hard tonneu covers in that size... very very hard.... think impossible) :D
I had to haul several things that wouldnt fit under the tonneu, but I couldnt leave out in open air (SWFL gets rain at the most inopportune moments....)
since some of the things weighed in around 100-150lbs, it made since they needed to be at the cab side, hence the initial design.

anyways, I used a piece of plywood, that I attached canvas on either side, that was push buttoned to the bed.
it came out to be almost identical to what you have posted.
I saw no difference in mileage at all over and above the soft tonneu I was using previous.

Possibly could be as good as a full aero cap if it was like Chrysler's PNGV rear greenhouse.

Isn't aerohead running a set up like this on his pick up truck? I think he said it makes no noticeable difference over the full height one.

neighbor Bob
 

My neighbor Bob did it to his Chevy 1/2-ton and for good effect.Doped fabric over EMT frame.Very light.Bob has gone to the big airfield in the sky now so I can't ask him to remember and share the results.I just remember his grin from the 1st tankful.

do you know if he did this from an open bed ?
if so, then Im sure he was happy as just going with a solid tonneu gave me an increase of about 1/4 per tank.
I ran 5 separate A-B-A tests with my ranger when I got my tonneu

1. w/o: drove from my home in Naples to Miami airport, and return. 3/4 tank of gas just to get me there.

2. with tonneu: less than a full tank of gas there and back.

3. w/o (A-B trip only): drove to Ft Lauderdale to pick up a scooter, about 37 miles shorter than driving to miami AP, used 1/2 a tank, just about exactly.
(cant use the B-A part of this trip due to adding the weight and height of the scooter in the bed)

4. with: to almost the same exact area as #3 (maybe 3 miles away) to look at a car, entire trip took me a little less than 3/4 tank

5. with: back to Miami AP, but had some suspension work, much taller tires (60mph read as 55 on odometer) and a full tuneup done prior to this trip, closer to 3/4 than empty for the entire trip.

all these drives had very similar conditions (as anyone in SFL knows, our weather is pretty consistent regardless of the date) :)