Spoilers can reduce drag AND give downforce
 

I don't understand making thinks look cool, unless it does cool, sorry.

A spoiler should function. This one does:

www.max-mpg.com

Reduces Cd from 0.38 to 0.30, and gives downforce

Ernie Rogers

Does it actually generate downforce, or just reduce lift? (Not just a question of semantics)

Does it actually generate downforce, or just reduce lift? (Not just a question of semantics)

F = ma
 

That's a sharp question, Metro,

The answer is to be found in Newton's laws. The total force acting on the car must be balanced by an equal reaction, seen as air accelerating opposite to the force.

My wing actually slopes downward slightly toward the rear, and is arranged to suppress flow separation near the top. After the car goes by, there is no air moving upward. So, the total force on the car must be backward and slightly upward (lift) Strictly speaking, the wing reduces lift over the rear of the Beetle.

Ernie Rogers

How was that determined?

I suspect coast-down testing. Ernie's a proponent and has posted a few how-to's in the past.

Not soley downforce...if it was only downforce that I wanted, there are much more aggresive designs I could have chosen. Like the hatch-mounted wings that were used on 'Group A' racing wagons in the 1980's. Besides, how much downforce do you think I'd get from a 1" high lip?

Well, I'm not gonna lie to ya'....yes, there is a part of me that wanted to improve the asthetics of the car which is why I made the extra investment in having it painted to match the body color. But, more importantly, I wanted to explore the possibilities of actual function of the rear lip on a two box after seeing them used in many other OE applications.
After all, what do you think the motivation is for auto mfgrs installing rear lip spoilers exclusively on thier high efficiency models? (eg: Honda Civic Hybrid, Volkswagen Jetta TDi)

Just out of curiosity, Ernie...why are the 'side pedestals' of your spoiler so large? Are there lateral stability benefits there?

might the spoiler work a bit like a turbulator? most of the time the airflow does not keep fully attached to a surface and a turbulent layer tends to build up between the surface and the laminar airflow. a turbulator is usually a zigzag ridge of a few mm high that trips the laminar flow back to the surface at a point where it would otherwise totally shed from the surface thus causing more dragg than the turbulator does

the spoiler might act somewhat similar to a vortex generator also, that extends above this turbulent area and draws in laminer flow.

i imagine that the spoiler might serve a bit like a dam that stops the slow turbulent layer from going over the edge and somehow messing things up there and serve as a sharp solid edge for the "clean" airflow the shed of the back into a more favorable path?

anyway it's just a theort that might not have much to do with actual science... i wish i knew more about aerodynamics.

lunar,

It could - but the spoiler is mounted at the trailing edge of the body... It's going to trip the flow and then there's nothing for the flow to stick to...

-----
I'm really curious of this whole "clean edge" separation thing... sometimes called "clean separation" - something tells me this isn't really possible. Logically, the flow above a flat spoiler will be higher (lower pressure) than the flow just behind the car/under the spoiler (higher pressure).

I'm starting to get into CFD modeling... maybe I should just make a simple scenario and see what happens (that won't be for a couple weeks though)

I think the majority of those 'spoilers' could be better described as 'extensions'. Most are meant to retain attached flow longer over already tapering body lines, or to move the flow separation point away from bodywork that would otherwise draw the flow inwards or downwards in such a way that it causes more lift/drag.

By the looks of the angles at the rear of your wagon, it already had clean separation on both the top & sides. Unfortunately, since there's little taper to the car's existing shape, there would be little to be gained from simply extending bodylines, such as with the roofline 'extensions' on the C30 Efficiency, Polo Bluemotion, or GMC Yukon Hybrid, for examples.

EDIT: The best bet on your car would be to mimic the partial boat tail setups that have been tested on tractor trailers. Or jwxr7's Metro.

I know it's lots of work, and probably not the look you're after.