decklid spoilers

[Jean]

I hope OP doesn't go 120mph to test this added downforce lol.....

[eric (^__^)]

the back end of the old alfa giulia supers is a good example. The whole tail was recessed, and I think this design supersedes a trunk spoiler.

I think the IS spoiler just creates the same effect, getting the airflow away from the tail end, like probably a dozen other people have said itt

[BeirBrennerE30]

here is a basic overview of some auto-aero stuff

http://superhachi.com/theory/downforce/

and nomad was right. When air is flowing off of the rear edge of the roof it seperates because of the sharp turn. Having this air hit the trunk and flow over it before spilling ove rthe rear trunk edge reduces drag (vortex generators on the rear roof edge reduce this seperation). Ths lip on the trunk effectivly makes the trunk a little longer. also like nomad said... you would like the air to not tubble off the edge and head tward the rear bumper. this creates a lot of drag and is something the e30 is terrible at. ever notice the amount of road grime that accumulates around the rear licence plate area on the e30. Thats a massive low pressure area that is sucking the car backwards.

[Ray Smoodiver]

here is a basic overview of some auto-aero stuff

http://superhachi.com/theory/downforce/

and nomad was right. When air is flowing off of the rear edge of the roof it seperates because of the sharp turn. Having this air hit the trunk and flow over it before spilling ove rthe rear trunk edge reduces drag (vortex generators on the rear roof edge reduce this seperation). Ths lip on the trunk effectivly makes the trunk a little longer. also like nomad said... you would like the air to not tubble off the edge and head tward the rear bumper. this creates a lot of drag and is something the e30 is terrible at. ever notice the amount of road grime that accumulates around the rear licence plate area on the e30. Thats a massive low pressure area that is sucking the car backwards.

The vortex generators on the later model Evo's were insane, coupled with that massive wing made the car pretty slippery for something shaped much like a box.

[NOMAD]

and nomad was right.

Yay4me, I just had to quote it.


As for the wing comment asking if it was the accumulation of air under the wing that lifted it up, that's actually not the cause of lift. It's all about high and low pressure. High pressure is air being condensed and pressed together, (moving towards a compressed air tank idea) low pressure is air being separated out, (moving towards a vacuum).

Wing:



The air impacting the front of the wing builds air pressure. The air moving over the bottom of the wing travels the same speed as the aircraft so it maintains essentially the same pressure as surrounding air. The kicker is when the air comes over the top of the wing and has to slide down the backside. This is a low pressure area so the air moves faster down the slope. The low pressure on top and the regular pressure on the bottom is the critical differentiation. Say 14psi is stoich (same info the turbo and dyno guys love) then the bottom of the wing is 14psi pushing on it, but the top only has 10psi due to lower pressure on the top. That equals 4psi of lift over each inch of the wing. So the lift does come from the bottom of the wing, but not from greater pressure, but from lesser pressure on the top.

Increased speed = greater difference in pressure = greater lift.

Pretty cool huh?

[deutschman]

so do the roof spoilers do anything? the ones that sit right above your rear window? like the reiger roof spoiler on this page.

http://bdweb6847j.bluedomino.com/e30.htm

[Hallen]

Yay4me, I just had to quote it.


As for the wing comment asking if it was the accumulation of air under the wing that lifted it up, that's actually not the cause of lift. It's all about high and low pressure. High pressure is air being condensed and pressed together, (moving towards a compressed air tank idea) low pressure is air being separated out, (moving towards a vacuum).

Wing:



The air impacting the front of the wing builds air pressure. The air moving over the bottom of the wing travels the same speed as the aircraft so it maintains essentially the same pressure as surrounding air. The kicker is when the air comes over the top of the wing and has to slide down the backside. This is a low pressure area so the air moves faster down the slope. The low pressure on top and the regular pressure on the bottom is the critical differentiation. Say 14psi is stoich (same info the turbo and dyno guys love) then the bottom of the wing is 14psi pushing on it, but the top only has 10psi due to lower pressure on the top. That equals 4psi of lift over each inch of the wing. So the lift does come from the bottom of the wing, but not from greater pressure, but from lesser pressure on the top.

Increased speed = greater difference in pressure = greater lift.

Pretty cool huh?

OK, all of that is basically right, but the explanation in there about how it works is pretty much wrong.

The air going over the curved section of the wing moves faster due to Bernoulli's principle. It is the same volume of gas (or liquid) that must move through a constricted space (like using your thumb to spray water out of a hose). It does not go faster because it is going down hill.


Downforce is exactly the same as lift. It is just that downforce is a racing term for lift that is directed towards the ground. Most wings on cars are made like airplane wings... they are just oriented upside down.

The most important fact of most spoilers on race cars is to break up the natural upwards lift generated by the shape of the car body. Look at most cars, they are basically shaped like a wing. The spoiler reduces the lift effect and therefore does reduce the vorticies that get created behind a car and so it also reduces drag.

Anyway, I don't have any direct data on the IS spoiler, but the suspicion is that it does reduce upwards lift, which is bad, and also reduces induced drag because of that. Those two benefits are greater than the increased drag that the spoiler produces, so to speak.

[Hallen]

so do the roof spoilers do anything? the ones that sit right above your rear window? like the reiger roof spoiler on this page.
http://bdweb6847j.bluedomino.com/e30.htm

Yeah, they do something. I am not sure what they do. I suspect they will direct more dirt towards your rear window and generally increase overall drag. They might reduce some upwards lift too. But I am not an aerodynamicist, so I don't know for sure. Some cars do come with a small lip on the rear of the hood just before the window, so on those cars, I am sure they do something.

[mikejungle]

check out beirbrenner's link, it explains roof spoilers: http://superhachi.com/theory/downforce/

[NOMAD]

OK, all of that is basically right, but the explanation in there about how it works is pretty much wrong.

The air going over the curved section of the wing moves faster due to Bernoulli's principle. It is the same volume of gas (or liquid) that must move through a constricted space (like using your thumb to spray water out of a hose). It does not go faster because it is going down hill.

Correct. A good reminder to me that semantics is important. I was trying to make it simple by using common terminology to point out what part of the wing I was referring to. No, "down hill" has nothing to do with it. The "downward" slope or "angle of the rear" does create the open area for the high pressure gas at the lead edge to expand into the void of low pressure.

I was not clear in my explanation. Good to remember. Yep, it's all about Bernoulli and some of Newton.

http://science.howstuffworks.com/airplane8.htm

[Ral]

OK, all of that is basically right, but the explanation in there about how it works is pretty much wrong.

The air going over the curved section of the wing moves faster due to Bernoulli's principle. It is the same volume of gas (or liquid) that must move through a constricted space (like using your thumb to spray water out of a hose). It does not go faster because it is going down hill.


Downforce is exactly the same as lift. It is just that downforce is a racing term for lift that is directed towards the ground. Most wings on cars are made like airplane wings... they are just oriented upside down.

The most important fact of most spoilers on race cars is to break up the natural upwards lift generated by the shape of the car body. Look at most cars, they are basically shaped like a wing. The spoiler reduces the lift effect and therefore does reduce the vorticies that get created behind a car and so it also reduces drag.

Anyway, I don't have any direct data on the IS spoiler, but the suspicion is that it does reduce upwards lift, which is bad, and also reduces induced drag because of that. Those two benefits are greater than the increased drag that the spoiler produces, so to speak.

not to split hairs again, but spoilers will not increase fuel efficiency or top-end speed (though they will create downforce). There is a high-pressure zone created above the trunk and spoiler which creates downforce. However, since the high-pressure zone is above and ahead of the spoiler, it will push the spoiler down and backwards. Also, someone was correct in stating that there's a "tube" of air that forms behind the trunk (it's the reason pickups will throw trash into the cab if you leave the back sliding window down) but by increasing the pressure ahead of the spoiler, it exacerbates the difference between that high pressure above the spoiler and the low-pressure air behind the trunklid and under the car. This, combined with the much sharper corner that the air must turn through, increases drag.

One last thing that helps visualize all this: Bernoulli's principle does not produce lift, it is a byproduct that is created by the flow of compressible (read: molecules are "springy" and can stretch and compress between each other as if connected by springs) and viscous (meaning: sticks to solid objects and itself) over a solid body. As the body moves through the air, the air sticks to it, but still flows around it. So, air flowing over the roof sticks to the rear window, and gets essentially "thrown" downward. It's this downward motion of the air that gives aircraft lift. (air goes down, airplane goes up). If the air can't stick to the body as it curves downward (really sharp corner, or air just can't hold on that hard), then it breaks away and forms an eddy current. So, in this instance, a spoiler makes a really, really sharp edge for the air to stick to and turn downwards, so it breaks off and spins back toward the bumper, or meets the air spinning up from the road under the car.

source: aeronautical engineering degree at the United States Naval Academy for credibility

/nerd rant.. excuse me while I go rip out some chest hair and drink whiskey at 10 am