roof bubble on most cars
 

I have an idea that might help mostly on cars that have almost flat roof.
here is the idea: take a sheet of lexan or sheet metal and make a bubble top start at the top of the windshield go over the roof and stop at the back window. however in the migddle of the roof have it 4?6? 10? inches off the top of the factory roof. this would extend the angle of the windshield up onto the "new" roof and help the air flow better over that part of the roof?

or has this been tryed and I need to learn more'?

I had that idea too http://ecomodder.com/forum/showthrea...-suv-8918.html

I wonder if the aero benefits would outweigh the increased frontal area. Interesting.

My thinking in this is as follows: the increased apparent frontal area is of no consequence, because when the wind leaves the upper line of the vehicle, it detaches, and the space below it may as well also be frontal area.

Really, someone should give it a shot and tuft it.

I am sure there is a "sweet spot" (angle) that is just right. Id think some vehicles like my jeep wrangler might even need a different angle of attack between the back and the front.
Speaking of which. Mycurrent Jeep has a hardtop. but 15 years ago I had a old CJ with a sof top and when you got up to 50 the front of the top got sucked up and half way back it was flat and of course the back was getting pushed down.
I wonder if that is the way that would work best.

Plus Robert Cumberford could wear his top hat in it!

While researching drag reduction for busses and trucks I came accross the following drawing in Piechna's book:


It was used to illustrate the effects of rounding leading edges and using air deflectors, but the author mentioned that the reduction of drag by using a "bubble" in the turbulent area would be outweighed by the penalty of increasing frontal area. A much better way to get the desired shape would be to change existing angles without increasing FA, though this is not always an option.

Of course, what is true for box-shaped busses may not be so for smaller (different Reynolds number) cars with better aero. A-B-A testing?

one of the aero experts, probably Phil Knox, quoted a surprisingly small radius required to maintain attached flow. I think it was 10 cm = 4 inches or something. If true, almost any car made in the last decade or more already has attached flow on the roof.

update: searching but can't find it yet.

Radius doesn't equate to angle, though. If the angle between windshield and roof is only 45 degrees, the air being forced up the windshield at high speed may detach at the roof line, if the transition radios is less than 4". The bubble idea would create an optimal radius from the angle, training the flow back onto the roof line. With proper tapering, the bubble can also lead into the rear departure, lessening the required length for a boat trail or kamm shape to be effective, by increasing the angle at which they can be mounted relative to horizon.

Good points.