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Old 11-16-2008, 11:34 AM   #1 (permalink)
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Rear spoiler designs

After being on here so long, I can't help but analyze every vehicle I see pass me on the highway every day.

One thing I have noticed is the genral shape of lip & roof spoilers on many modern vehicles. I have noticed that they do not often follow the flow of the roof line but rather shoot straight out parrallel to the ground or even have and upward curve to them.

Can someone give me the "for dummies" explanation as to why? I would think that these spoilers would be shaped to continue air flow as efficiently as possible. This of course excludes size, because I also feel that many hatch back suv's would benefit from larger roof spoilers to reduce drag.

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Old 11-17-2008, 05:25 PM   #2 (permalink)
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I think that, in many cases, the reason is "because it looks cool and people will buy it for that reason"...

Not sure if there is really a valid aerodynamic reason for it.

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Old 11-19-2008, 11:53 AM   #3 (permalink)
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To create downforce! I passed a turbo VW beetle yesterday, they have the automated spoiler which rises at highway speeds. Same with audi TT and several other smooth backed sporty cars. Must suck for aero....

Even minivans and the like need downforce to keep the rear end planted.
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Old 11-19-2008, 12:12 PM   #4 (permalink)
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Deezler, it's a detail but I'm pretty sure the correct answer would be to reduce lift. I don't think the beetle or TT produce much, if any, downforce at speed. I remember the first gen TT didn't have the spoiler and there was a high rate of high speed accidents due to lift.
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Old 11-19-2008, 12:18 PM   #5 (permalink)
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doh, you are correct.

I think maybe the Subaru WRX is the only car I've seen with a wing big enough to create actual down force.
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Old 11-19-2008, 12:25 PM   #6 (permalink)
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it depends. i agree that a lot of what we call spoiler are just for looks and a few of these are also there to generate downforce, but there's an increasing ammount of lip soilers wich i believe are really there to help aero as well. if the angle of the roof or windshield goes beyond a certain angle, the flow will separate but if there's a spoiler that extends far enough, even when its parallel to the road or even pointing upwards this might catch the separation bubble and create an overall aerodynamic profile.

a good example of this is the audi A2

the back of the roof and rear window is a curved one piece glass element with a spoiler attached to it.

the round shape is a result of the construction method of the windshield and part of the overall round shape of the car, but it's very bad for aero, the spoiler prevents the air from tripping over but captures the small separation bubble and has the air continue at a 16° angle. (the red line) with a 0.26 Cd i think this is one of the cars where a spoiler actually helps aero

the 16 degree angle is something that seems to be present in a lot of low drag cars as this comparison of rear profiles shows


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