The hybrids have "butt flaps". They are slight extensions of the left, right, top, and bumper to lessen the drag from the wake vorticies. Only the Tahoe and Yukon have them. And yes they may reduce total drag but they leave lots of "snirt" (snow dirt) on the back windows. lots of miles on those northern Ontario log roads for me.

If you park the hybrid right next to the non-hybrid you can see the differences.

 

Thanks Jet 1 for the web site. Marty I will park by a standard Tahoe to see the differance. Is the "snow dirt" a lot less on the standard Tahoe? The vortex generators I was thinking of would be made from 2024T3 aluminum "L" extrusion .063" 1/2' x 1/2", with a slight dihedral on the front face, positioned at the back slightly forward so you would not see them. Each vertex generator would also be placed on a slight angle approx 12 degress. This is just a though right now and I will complete a temporary installation for test an see what happens to the milage as well. If this works and eliminates at least 1/2 the "snow dirt" from the logging roads I would be happy, and if you cannot see them it would be much better.

 

that site is interesting but their concept is ugly. I was thinking along the same lines as you, KISS principle. I was also thinking about boundary layer interference as well.

http://www.fastskinz.com/index.html

 

Jet 1 you come up with some interesting sites. I will see how my experiment works out and share it here. I know we have a wiper but on snow covered roads my rear window is always covered and I like to know what's at my six.

 

Ken and Jet: I'm not getting a pretty (mental) picture here. One has the tail off the F22 and the other more like Chitty Chitty Bang Bang.

And yes my experience with my Suburban and these hybrids on the same roads shows less snirt on the Suburban. A very good friend of mine who teaches fluid dynamics and thermo at alocal university runs a 1/4 scale wind tunnel. He has been playing around with SUV wake flow reducer devices for almost a decade.

He has some solutions but they are about 3 feet long. UGLY The hybrid slight extensions look a little like his only much shorter.

 

I just aplly aviation aerodynamics to automotive searches....there is lots of stuff on the internet!

 

But aren't you guys talking about two totally different things here? If you reduce the amount of SNIRT on the back window, you have to redirect boundary layer air to the back window to do it...read drag. That is why there is not a 3 inch wide 5 foot long deflector on top of new vehicles...they put a wiper back there because it is more efficient than keeping the snirt off in the first place. My old suburban didn't get dirt back there because of the inefficient deflector. My 97 Honda Odessey didn't have a deflector by that date and the window would cover in snirt in one trip but it had a wiper. Short of changing the rear design of the rear of the vehicle, like Marty's friend with the wind tunnel is doing, to keep the flow from separating, I think you will find that it is why there is a wiper. Also, I just wonder...if the flow was smooth and accelerated, it would be a lower pressure. It is just my opinion but when a wing stalls, it is because the flow separates and the air is disturbed and the low pressure is disrupted. I would think that the pressure on the back of the window is higher or neutral than that pressure across the roof where the air flow is accelerated. Marty, can you get your friend to check the air pressure difference over the roof and at the back compared to in front of the vehicle? The back of a Prius is completely dirty too but not the aerodynamic back window glass.

 

Mel: the build up is thick and nasty. It bugs me but I live with it. I was thinking of an old 70s Vista Cruiser roof rack with integrated deflector.

 

I have also noticed the rear window collects a lot of dust, even when the car is parked. Specially when it get windy.
as if It is a dust trap, since it is a low pressure area surrounded by the Butt flaps Marty mentioned.

 

I have been sitting here in a country town watching vehicles go by both old and new. The newer vehicles all have a wing to let the air flow separate smooth without drag...and their glass is dirty. All the old vans suburbans older tahoes are clean. Do you think it could be that reduced drag let's the dust build? Marty! We had a vista cruiser! For those that don't know, that was an oldsmobile cutlass stationwagon...and the glass stayed clean with that deflector!