Window vent visors

[CarlP]

I have an 05 Suzuki Forenza that will be my daily driver now. It has vent visors on the windows. Has anyone done any testing to see if they make a measureable difference in fuel economy? Is there any other advantage to them I'm missing?

[Daox]

Rip em off!

I think the only advantage to using them is opening up the windows while its raining/wet.

[wyatt]

I know on my car, and most other cars, that there is separation of air around the a-pillar. They may not be hurting anything. If you do take them off, you should see if you can measure a difference.

[PaleMelanesian]

On the other hand, the effect might be like this: Aerotesting - Sunroof open vs. closed. - CleanMPG Forums

Tape some strings to your side windows, front and back, and see if / where you have attached and separated airflow.

[MetroMPG]

They increase frontal area, so there is likely an adverse effect, even if the flow normally separates/spins into a vortex at the A pillar.

I'd bet they also increase Cd, too.

If you also have them on the rear windows, they're probably also disturbing flow at the outside roof edge / C pillar.

Tuft testing will show you the extent of the disturbed flow. But I'm with Daox: I'd take 'em off!

Will it show up at the fuel pump? I'm guessing the difference is probably smaller than the normal variation you see from tank to tank (but it doesn't mean they're not hurting).

[wyatt]

Originally Posted by PaleMelanesian

On the other hand, the effect might be like this: Aerotesting - Sunroof open vs. closed. - CleanMPG Forums

Tape some strings to your side windows, front and back, and see if / where you have attached and separated airflow.

Interesting... I will second the tuft testing idea. We could hypothesize all day about whether or not they make a difference, but the tuft testing should tell you outright.
One thing you could try is only removing it on one side of the car, tufting both sides and seeing the difference in real time.

[MetroMPG]

I think we already reasonably know they have to have a negative impact: they increase A at a point where the increase can be of no aero benefit (smooth sides - assuming window shut).

[wyatt]

Originally Posted by MetroMPG

They increase frontal area, so there is likely an adverse effect, even if the flow normally separates/spins into a vortex at the A pillar.

I'd bet they also increase Cd, too.

If you also have them on the rear windows, they're probably also disturbing flow at the outside roof edge / C pillar.

Tuft testing will show you the extent of the disturbed flow. But I'm with Daox: I'd take 'em off!

Will it show up at the fuel pump? I'm guessing the difference is probably smaller than the normal variation you see from tank to tank (but it doesn't mean they're not hurting).

I would guess the rear ones (if present) would act more as a parachute based on the picture... so those should come off. The front ones are debatable... it will probably come down to how much use you get out of having them.

[Daox]

I'd have to disagree, I don't think its debatable.

#1) They increase frontal area which inherantly increases drag.

#2) There is no way they don't cause separation. Its an edge hanging out from the car a good inch. Without it, there is at least has a chance for the flow to stay attached.

[sepp]

ive been wondering about this to, but apparently they are only advertised as weather deflectors. and never for any aerodynamic gain, wich they certainly would do if there was any.

but i noticed that all modern trucks have these wind guides to the sides of their cabs, to help keep the airstream attached.



why not place such guides on all the edges of our cars where the airsteam detaches? like above the nose, above the windshield, around the A-beams... would that do any good?