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E36 M3 (1992-1999) {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999


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Old Sat, Apr-03-2010, 08:51:01 AM   #1
Spahrticus
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Default Admiring BMW's aerodynamics (question inside)

So just how aerodynamic did BMW make the E36 M3? I don't know if any of you have noticed, I'm guessing a good few have, but when it rains, and you're on the freeway, the water that was back there forms in a very specific spot, right where the little dip in the high rise spoiler is. Those with out the spoiler, I feel sorry for you .

Anyway, I thought this might just be what cars in general do, but when I went out and drove my mom's C32 AMG, or my girlfriends Corolla, in the same conditions, both didn't do it. Now I don't post much here, but when I want a REAL discussion with knowledgeable people, I post here.

So what I'm asking is, how much (or how little) does the high rise spoiler, the one that came from the factory, help in rear aerodynamic down force.

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Old Sat, Apr-03-2010, 09:03:28 AM   #2
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Put rain-x on your spoiler, increases downforce by 10 lbs and you won't get that water spot
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Old Sat, Apr-03-2010, 09:11:55 AM   #3
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Put rain-x on your spoiler, increases downforce by 10 lbs and you won't get that water spot
this is what I get for being serious I guess......
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Old Sat, Apr-03-2010, 09:14:56 AM   #4
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In for high rise spoiler stats
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Old Sat, Apr-03-2010, 09:30:08 AM   #5
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unless its an LTW high rise wing, its just decoration.
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Old Sat, Apr-03-2010, 11:27:08 AM   #6
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Really the effects of even a high-rise spoiler are pretty minimal. To get real usable downforce that will actually increase grip on the road, you need a spoiler that is exposed to laminar airflow. The high arch (even the LTW high rise) spoilers are exposed to separated to turbulent airflow at best, which means the flow of air going to the spoiler is not a smooth flow, instead it has little vortices and uneven pressure gradients.

Now, the effect of the water moving in a nice, smooth pattern is actually a very different issue. Since it is located on the surface of the car, all of the water on the car is most likely within the boundary layer of airflow. This is (usually) a very thin layer of laminar air that envelopes any object. The flow of air right on the surface is much slower than the air several inches off the surface, due to the viscosity of air. The effect of the water gathering on the centerline of the rear window like that is a product of the boxiness of the car. Without getting into 3d aerodynamics basically there is a pressure gradient across the span of the rear window, and fluid always tries to flow to the lowest pressure region it can get to.

So as you can see, the issue of the water making nice patterns on the car and the effectiveness of the spoiler are two extremely different scenarios. The spoiler, I'm afraid, does very little to provide downforce. BMW knowingly does this, since adding downforce to a street car would decrease your fuel economy, which would likely make the car less competitive in the market.
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Old Sat, Apr-03-2010, 12:06:12 PM   #7
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not that its BMW but mt ACS roof spoiler, makes the rain for a V on my back window. The tops of the V being the top corners of the back window and the bottom being center base of the window.
When driving after the car is wet, the outside of the V is dry, water pushed away and the inside of the V is untouched, seemingly. It didnt do this before i had the spoiler...

As far as spoilers go, we must remember that sometimes theyre not for downforce, but for cleaning up airflow and reducing drag.
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Old Sat, Apr-03-2010, 12:33:25 PM   #8
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Originally Posted by snikwad View Post
not that its BMW but mt ACS roof spoiler, makes the rain for a V on my back window. The tops of the V being the top corners of the back window and the bottom being center base of the window.
When driving after the car is wet, the outside of the V is dry, water pushed away and the inside of the V is untouched, seemingly. It didnt do this before i had the spoiler...
Again, I'm not going to get into 3D flow because I'm too lazy but the V that formed on your back window is due to the low pressure region created by the roof spoiler combined with the rotational flow coming off the side of the car.

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As far as spoilers go, we must remember that sometimes theyre not for downforce, but for cleaning up airflow and reducing drag.
Uh...no. The very presence of a spoiler is going to increase parasitic drag, and if it makes any downforce at all it will create induced drag. What you're talking about are vortex generators, shown below. Their job is to delay airflow separation, which would create more drag. They do this basically by adding energy to the boundary layer flow.

A spoiler by its very nature does not clean up airflow; rather, it makes it worse.
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Old Sat, Apr-03-2010, 09:51:02 PM   #9
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A spoiler by its very nature does not clean up airflow; rather, it makes it worse.
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it is called a spoiler because it spoils the airflow causing it to get "dirty" (creating a wake behind it) and increasing drag

PS, the E30 M3's rear glass is more aerodynamic (teardrop shaped like the Corvette whose C5 ad C6 generations are very pronounced) than the E36's .....plus on the E30 M3, the rear glass area is structural
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Old Sat, Apr-03-2010, 12:38:01 PM   #10
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i was just adding some info to the whole thing man, thought id mention what i noticed my roof spoiler doing.
As for the other part, if some spoilers dont clean up airflow, how do you explain the CSL trunk spoiler on the E46 CSL? surely the CSL trunk is too low and out of the path of direct airflow to effectively add downforce.
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Discussing Admiring BMW's aerodynamics (question inside) in the E36 M3 (1992-1999) Forum - {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999 at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)