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ar151666  
#1 Posted : Tuesday, February 10, 2015 7:51:24 AM(UTC)
ar151666

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Some of you may know what this is right off the bat but others may not. I'm talking about the wing or spoiler at the top of the rear windshield on a few cars, most notably the 06 Subaru STI and the BMW E9 Batmobile. I'm wondering if these wings provide a reduction in drag on a car with a steep rear windsheild. We all know that a steep windsheild creates a low pressure area at the base of the glass in front of the trunk increasing drag. Does this little piece help the airflow down the back of the window creating a smaller wake?

I've done some research on these before and everything points to them being used to direct air to the large rear wing in order to increase its effectiveness. Any other benefits that anyone can see?

If you aren't sure what I'm talking about...

E9 CSL Batmobile

STI

Roof vane airflow
Would rather drive a slow car fast, than a fast car slow.
BoxedFox  
#2 Posted : Tuesday, February 10, 2015 1:49:21 PM(UTC)
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If cutting down drag was your primary goal, a more effective solution would be to build an extension off the back of the roof like you see on the new Fit:

UserPostedImage

It's a little tough to see in that overly yellow PR photo but the extension points out and down, which significantly cuts into the low pressure "wake" behind the car. It also helps keep the flow from separating at the roof, which is always a good thing.

I can't really see much benefit to those roof-edge arches other than to redirect the airflow towards a rear wing mounted further back on the body. It's a cheap fix for repointing the airflow without having to redesign the profile of the rear windshield. If the windshield profile isn't too far off what you need, you can get away with using vortex generators, which tends to be a more efficient solution to the problem. The only problem is they're much harder to tune.
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smartbomb  
#3 Posted : Tuesday, February 10, 2015 4:56:32 PM(UTC)
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We were trying one of those on our FD car to see if we could divert more air into our rear mounted heat exchangers. We could not tell if it helped our not.
ar151666  
#4 Posted : Wednesday, February 11, 2015 9:24:46 AM(UTC)
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Okay, so compared to the vane spoiler where all its doing is redirecting some of the airflow, vortex generators would "energize" the airflow helping create a cleaner separation and less of a wake, correct? I think I understand the difference between the two and what they would be used for, though both are pretty difficult or time consuming to test.

I almost wouldn't expect the small spoiler on the fit to do much of anything on a car like, say, an Evo with a steeply raked windsheild and then the trunk. On the fit it makes sense though.

I've seen things like this on SUV's before too

SUV air deflector

They claim it grabs air traveling along the top of the vehicle and forces it down the back window to help disperse water that would normally collect there. I assume that this has little to no effect on overall drag, but it seems very similar to the ideas behind the vane spoiler on the subarus.

Edited by user Wednesday, February 11, 2015 9:25:40 AM(UTC)  | Reason: Not specified

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BoxedFox  
#5 Posted : Wednesday, February 11, 2015 12:33:34 PM(UTC)
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From what I understand, the vortex generators on the roof of the Evo XIII MR didn't really energize the flow like you would expect from the wingtips of a jet aircraft. The idea there was to disrupt the flow as it came over the roof, because turbulent flow tends to stay attached to surfaces for longer before separating. By making the flow turbulent, they could force it to follow the contour of the rear windscreen for longer, which lined things up for the rear wing.

And yeah, if anything I would fully expect the little roof spoiler on the Fit to make things worse on an Evo. You'd get no airflow over the rear wing and it'd still have a draggy tail. Probably not as bad as the air deflector you're showing us on the Jeep though. That's way too far forward to be beneficial from a drag reduction standpoint.

If anything it'd probably create more drag and minute amounts of lift because of how far it's sticking up. It's not an efficient way to keep the water away from the rear windscreen, but if the rest of the car is a giant brick I suppose it doesn't matter that much.

Sidebar - If I needed a low drag solution for an Evo, I think I'd copy what Ralliart did on the Lancer WRC in 04-06:

UserPostedImage
UserPostedImage

It looks like a hideous twin element rear wing that's put too close to the rear window, but it's actually an ingenious way of getting the low drag, low-lift benefits of a hatchback body with the downforce of a sedan body while in yaw.

When the car is travelling in a straight line, the air flows straight off the end of the roof onto the top element on the arch. The air behaves as if it's going over the top of a big hatchback like the Honda Fit. They put a reasonably aggressive profile on it so it produces some downforce, but I do believe the main objective was to make the car fast in a straight line and keep it flat over jumps.

The lower element acts like a proper rear wing when the car is rotating (like when it's sliding around a long sweeper). Any air that comes around the C pillar towards the back of the car would be directed by the tall outer supports and flow past the lower element, which directly translates to downforce.

In both cases, the turbulent wake behind the car is reduced by the fact that it's an open area over the trunk rather than a big closed box at the tail end of the car. Mitsubishi apparently spent an obscene amount of money perfecting this design in a wind tunnel, and I love the final product.
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jcspiegel  
#6 Posted : Thursday, June 25, 2015 2:39:38 AM(UTC)
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Those rear air deflectors, like you see on that Jeep, were originally touted as a device to help keep the rear windows clean. Pretty common on station wagons from the 70's
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