Spoiler over rear window
 

I've seen two kinds of these spoilers, mainly on Mitsubishis and Hondas, do they help or hurt aerodynamics?
This is one http://istargazer.ca/blog/wm-pic13.jpg

Here's the other http://desmond.imageshack.us/Himg46/...jpg&res=medium

It looks like they would would in conjuction with a regular spoiler, like on the bottom picture, but I know aerodynamics can be tricky so I'm not sure.

I think it depends on the design of the car, I seriously considered doing something like this to my 04 Civic Sedan, but seeing some “Shape Studies” that show a large radius roof to rear glass transition area returning very good Cd results got me to wondering if Honda hadn’t very carefully “tuned” the shape of everything on the rear portion of my car to minimize weird pressure gradients that would cause vortices to form. This it would seem to be not too difficult to do, and would account for a fairly good Cd value I get for a fairly large compact car.

IN other words, doing something like this could very well mess up something the designers of your car crafted very carefully. Each case would of course be its own study.

The first one that you posted is a Subaru-style vortex generator, so it is not, technically, a spoiler.

http://images.motortrend.com/roadtes...oiler_view.jpg

I don't have much confidence in the designers over at Chrysler 17 years ago :rolleyes:
Even at low speeds the vortex at my rear window is quite noticeable. So I guess once I got time and materials I'll see what I can make in regards to that kind of spoiler/VG, and see if it helps.

The first one on the white car redirects the boundary air current down so it stays attached to the rear window.

The second one on the gray car provides a clean edge for the departing air, and launches it slightly down and out. I should note that there is no hope of the trunk wing on the gray doing much of anything.

I think they both create uplift, but in my opinion most drag saving devices and forms do, just as most down-force created devices and forms cause more drag. If someone can prove me wrong with those statements, I welcome the learning experience.

Every shape has some drag whether lift is positive, negative, or zero. Shape and angle of attack are the main features that affect drag. The overall shape of a car is generally a flat bottom and a curved profile so there is lift unless something is done to counteract it. Typically downforce is generated by air dams, foils, or wings and will cause drag. Wings cause drag whether generating lift or drag. There's a good bit more to it, some reading on vehicle aerodymanics will help.

I can't speak to whether the vortex generator itself creates uplift or downforce, but it is designed to maximize the effectiveness of a rear airfoil, which creates a MASSIVE amount of downforce. That's why those cars are able to jump ravines despite a ~60/40 F/R weight distribution.

I think you'll find the downforce isn't very "MASSIVE". Nose-heavy doesn't mean the front will fall faster than the rear. Watch the "Dukes of Hazzard" for examples of that, the 69 charger isn't balanced or aero.

-soD

You're right. Only hundreds of pounds of downforce at speed. And the General Lee had better weight distribution by 5-10% F/R. And there's no telling whether they used ballast for those jumping shots.

I would not call the top foil on the white car a "vortex generator".

I would call the tiny shark fins "vortex generators", and you did not post a photo of those.

I agree that if a garnish/device redirects air somehow to a second device intended to increase drag and down-force, that this second garnish/device will operate more effectively. I have this same exact situation with my roof wing and rear spoiler, see roof wing in my signature below.