That's what I had in mind too.
I like Charles Bs' idea of pinching the wake inward with the roof rack extentions.
I wonder if adding a 'wing' like I illustrated above the roof extention ( lower spoiler ) would create a higher pressure to the airflow - again pinching the wake and forcing the wake to stay attached to the teardrop shape farther from the back of the car.

It would sort of a 'biplane wing' set up.


It would be easy to test - just strap a sheet of something to the top of the roof rack.

Did you draw this ? ( Nice ! )

I'm still trying to figure out why I saw my best tank on my Escort wagon with two bikes strapped to the back...

I figure the Audi A2 gives us some good clues about how to proceed though...
https://en.wikipedia.org/wiki/Image:Audi_A2_L_Silber.jpg
Very low Cd.

I figure you can either use either a rear window air deflector type spoiler to bring air down the back, or use a surface extension to entrain flow. I've also been considering the possibility of a deflector extending back abover the windshield by 3 inches.... yes it gains CSA, but by making the forebody "fatter" at the top, it might cause the air to take a path at an angle further back, meaning that it attaches to the rear surface longer, and reduces base drag. Anyone with a wagon with a tilt up sunroof, might like to experiment as to whether overall aerodynamics seem improved with it up or closed.

On my '88 Voyager, with rather square tailgate edges, I am planning to try attaching quarter rounds cut from pool noodles around the hatch, hoping the curved surface will entrain more air behind the vehicle.

Recently saw a Chevrolet Aveo that had a negative incidence rear spoiler/deflector, that looked promising. Not sure what I'm doing for the back of the Escort when it's back together again, think it might just need a deflector to keep the air nailed down further back. May try tumble strips/turbulators around the back.

Be interesting to carve humps out of styrofoam and stick them on top see if they make a difference.

Last summer I did some very informal testing with my Passat wagon with the sunroof open and closed. It is not a flip up design as you are talking about here but has a small piece at the front that snaps up to deflect air up slightly. I thought I was crazy but I consistantly was getting slightly better mileage with the sunroof open. Perhaps your theory explains it. When the weather gets nicer I will try a more serious A-B-A test!

Phil


Roadwarrior said:

"I figure you can either use either a rear window air deflector type spoiler to bring air down the back, or use a surface extension to entrain flow. I've also been considering the possibility of a deflector extending back abover the windshield by 3 inches.... yes it gains CSA, but by making the forebody "fatter" at the top, it might cause the air to take a path at an angle further back, meaning that it attaches to the rear surface longer, and reduces base drag. Anyone with a wagon with a tilt up sunroof, might like to experiment as to whether overall aerodynamics seem improved with it up or closed. "

Patience is a virtue I don't seem to have....... So when my VG's arrived today, I went right out and installed them......So much for the two month average oh well. we'll see if it all helps then.

last week I took a trip out of town this photo is upon returning.
https://www.fuelly.com/attachments/fo...6decae3a9e.jpg

I see you used your windshield washers a lot on the trip. :)

LOL that is after not using them for the whole winter I needed them and the resivoir was almost empty. DARN BUGS!!!!