extend roof spoiler on N Beetle

[ecomodded]

I put a top of rear glass roof spoiler on my N. Beetle in anticipation of giving the air passing over the roof a point to exit my car, rather then going half way down the back glass causing lift and increasing drag. I was thinking if i continued the roof spoiler down over the sides 6 inches would stop more air from being sucked along the body to its departure point which is not defined and has no trailing edge. I want to give it a clear departure point across the roof pillars on my car.
Any thoughts or advice on my plan to extend the roof spoiler would be appreciated and helpful, I do not want to make matters worse if this idea is more harmful then good.

[ecomodded]

No one chimed in to suggest other wise so i will assume it is borderline and nobody can or will take a chance with a assumption. Personally I can visualize how a smoke wand smoke trail would look, to some degree that is. It seems to be predictable.
I can fashion the spoiler ends out polyurethane (PUR and PU) then prime and a can of color match paint.
It will be contoured to minimize its height/ profile in the wind as the glass spoiler is.

[focus1.6uk]

It could be worth extending the spoiler using cardboard and seeing if there are any mpg gains or tuft testing it, if possible, to see if the airflow improves.

[kach22i]

I would not do as you propose, at that intersection you need to let it flow by doing nothing, or attempt to manage the converging forces there.

Automobile 2 - Odds and Ends pictures by kach22i - Photobucket

[ecomodded]

Thanks for your input Kach, and bringing that to my attention. I see your point, I also noticed one more flaw with my add on, the sides of the Beetle actual do have a defined trailing edge.. and my extension would only deny the airflow its already good flow path off the side of the car. Where it does meet with some roof edge flow and as you stated and the cars wake in the rain confirms that a large vortex spirals off the cars top two corners.

My new thought is to make a smaller extension that will attempt to keep the roof edge airflow in line with the roofs airflow. A small change in the right direction I now deduce..
A video of the car cruising on the hwy in heavy rain would determine the/any changes.

[razordave]

Sorry to dig up a zombie thread, but kach, what do you think of my fences?