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Discussion Starter #1
Are they functional or just eye candy? Do they reduce wind noise, keep rear window clear of rain, snow, improve aerodynamics, etc.???
 

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It really depends on the car body. If the back window drops away very quickly (rapidly increasing pressure gradient), as in the 3.0 CS, Bavaria, E12, E28, etc. body, then it may be possible to gain some drag reduction with a "properly engineered" roof spoiler like seen on the 3.0 CSL factory spoiler kit. Notice the unique design of this spoiler in that it can trap and direct the boundary layer flow around the transition. In this case the spoiler can help the boundary layer to remain attached longer around such a sharp transition and therefore reduce pressure drag behind the car (i.e. reduce the size of the counter-rotating vortices that form off this transition).

If you are referring to the sort of "sun visor" type of top-of-the-window thing on an E39, then my take is that would be worthless and may actually be detrimental to the well designed roof/window transition region of the body. Going to a larger trunk spoiler (not a wing) would be a potentially much better choice than the roof visor. ;)

Chuck
 

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it's just for show. i used to like it and wanted it. but now i don't. glad i didn't buy.

-joe
 

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Doesn't Black M5's uhh... Black M5 have a roof spoiler? He's driven the E39 faster than anyone has, and he still lives to talk about it, so it can't possibly be doing any harm.... :1: :1: :M5launch:
 

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If anyone is interested I have an AC Schnitzer version that I am selling. It is already painted LeMans Blue but I have never installed it and still have the complete installation materials.

I changed my mind on putting it on.
 

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Discussion Starter #11
mricorp said:
If anyone is interested I have an AC Schnitzer version that I am selling...
In the FOR SALE forum someone was looking for a Hamann roof spoiler. Maybe shoot them a PM.
 

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I'm interested shoot me a pm.

mricorp said:
If anyone is interested I have an AC Schnitzer version that I am selling. It is already painted LeMans Blue but I have never installed it and still have the complete installation materials.

I changed my mind on putting it on.
 

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Chuck is right, there is some merit to them. Control the boundry layer better and have more control over the shed vortice, and you can reduce lift and hence drag on the end of the car. Will a small one on the back of the window do it? Maybe maybe not, and I doubt they did much wind tunnel testing on them. Call it race on Sunday sell on Monday, as it has a purpose, but not really on the street.
I'm still waiting for WRC style wings with vertical elements to show up on street cars... I cant wait to tell someone that those are there to control spanwise flow when the car is sideways. hiha
:cheers:
 

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BmwNut said:
Chuck is right, there is some merit to them.
Whoa! I was talking about the 3.0 CSL type of roof spoiler that creates a channel that traps the boundary layer along with a good chunk of freestream airflow as the one having some merit. These little E39 eyebrow things are highly suspect to me...I would have to see some surface pressure data to believe they aren't detrimental. ;)
 

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Guess there was a small communication error there Chuck. I know that they have a purpose, but in the next sentance I pointed out that it isnt really true for street applications. Yes, a race prepped 3.0 CSL is worlds different than a hunk of plastic tacked to a window of an E39. Sorry for the miscommunication there.
:cheers:
 

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Bren did a wonderful job there guys...

The ACS parts are actually aero tested and hence do actually perform as expected. The rear window spoiler on the m5 actually does shed high velocity lift off the rear end of the m5....drive a car that has one and one without and you will notice it very easily. BMWUSA (Josh) has driven mine and has commented on how well the car feels planted with the aerobits and suspension in place.

The total effect of the wing is not really to disrupt airflow directly but more indirectly. As with the E39 the curvature of the passanger compartment creates lift itself (curvature of something passing through the air contributes to both Bernulli's Prin and the theory of Coanda) and as such the wing really does nothing more than extends the length of the rear passenger compartment thus ensuring that the air extendes itself and hopefully resists normal temptation to follow the rear glass down (creating lift). By extending the length of travel, a roof spoiler actually does pushes the air away from the back of the car and with a rear deck spoiler, large amounts of lift can be decreased.

Man, i wish i had some airflow diagrams to help this discussion out.

Pretty much it comes down to this...if it comes from a high end tuner (ACS, RD Sport...and such) it more than likely has had some aero testing done and will perform (if only marginally) as it is expected.
 

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For being balls deep in a midget that was a great explaination Rob...LOL.
I am really impressed that ACS actually took the time to wind tunnel test their product. In the age where actual aerodynamic bits are emulated and thrown on any car under the sun, taking the time to put out a superior product gets a big thumbs up from me. Three element wings on a FWD Honda are usually the norm...
:cheers:
 

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Regarding aerodynamics the roof spoiler would increase drag. It may clean up the boundary layer on the roof top and actually delay detachment of the flow a bit. However, it will reduce the base fill behind the rear window and rear body near the license plate. You will have a much lower pressure area behind the car resisting forward motion. The slope of the rear roof area and the angle of the rear window are designed to encourage a somewhat controlled filling of the dead water region behind the car including behind the rear window. The roof spoiler will create an increase in drag without doubt. I have read about vehicle aero. for a long time and studied it at high speed on the public highways at night. The SAE has numerous publications on the evolution of vehicle aero. that are quite good. Roof spoiler = bad aero.
 

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Discussion Starter #20
HBRAMSTEDT said:
Regarding aerodynamics the roof spoiler would increase drag. It may clean up the boundary layer on the roof top and actually delay detachment of the flow a bit. However, it will reduce the base fill behind the rear window and rear body near the license plate. You will have a much lower pressure area behind the car resisting forward motion. The slope of the rear roof area and the angle of the rear window are designed to encourage a somewhat controlled filling of the dead water region behind the car including behind the rear window. The roof spoiler will create an increase in drag without doubt. I have read about vehicle aero. for a long time and studied it at high speed on the public highways at night. The SAE has numerous publications on the evolution of vehicle aero. that are quite good. Roof spoiler = bad aero.
Wow - thanks for the detailed analysis! :thumbsup:
 
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