Rear Diffuser [Merged 4-9]

[red_devil]

What is the difference? Is my drawing a inaccurate misrepresentation of how air flows? To me, I think that's is how it should work but I'm in the dark b/c I never took a physics class and like I said...I'm not an aerodynamisis. Please feel free to explain how it works b/c I'm just throwing up ideas that I don't see anyone else trying. You had said in an earlier post that the main purpose of a rear diffuser is to create downforce. How would it create downforce if most of the air is flow on the underside? Wouldn't this create an up pressure/lift?

try here
http://www.ef.org.vt.edu/issue archive/2005_3_September/DesignConsiderations.htm

http://www.symscape.com/blog/secrets_of_diffusers

and some other F1 sites

Having a flat underside with a diffuser is the best way to create downforce with minimal drag. It increases the air movment underneath the car while the restricted slower moving air is directed over the top of the car. The diffuser just helps direct the air more smoothly. Just think of it as a reversed wing.

 

[No Mo STi]

Yea...i guess I never thought of it that way. So what your saying is, by smoothing out the air on the underside of the car it makes it easier for the downforce to work on the top side of the car?

 

[Boo5t_Addict]

"increases the velocity of the air below the car, so that the more slowly moving air above the car will push the car into the ground." - from your first link.

Quicker moving air on top of an airplane's wing, and slower moving air underneath = lift. You want the air underneath the car to accelerate (venturi effect) and be able to move quicker than the turbulent air traveling over the top and around the car. That's how you create downforce. So the key is to not slow the air moving under the car.

Having a flat underside with a diffuser is the best way to create downforce with minimal drag. It slows the air movment underneath the car while the faster moving air is directed over the top of the car

That's actually what an airplane wing does, to create lift. I think you know how it works, but you just accidentally worded the opposite.

Just think of it as a reversed wing.

Exactly. You're totally correct, but you may want to edit what you said about a flat underside vs the top so others aren't confused.

 

[No Mo STi]

Thanks for trying to give me credit for knowing it just wording it backwards, but I didn't know. I get bored at work and try to think up new little tricks that most people won't notice. I'm trying to get into road racing within the next few years and I'm just planning out how to make the car more aerodynamically sound. Thanks for the help. So back to the original topic...the bumper vents/holes. I know there isn't that much drag produced by the stock bumper, but I would be willing to be that it is in the neighborhood of 50lbs. I know I'm not going 8's but, you don't think they would shave off maybe a tenth?

 

[Boo5t_Addict]

F1 is the greatest form of racing! Most physically demanding, too. Those guys train 4 hours a day, every day to stay in such good shape. Back to topic though. In my own opinion, I don't think it will shave a tenth off your time. Cutting 100 lbs from your car as a rule of thumb will. What are your 60' times? I didn't look at your profile, so forgive me if it's already listed. Dsm's are actually more aerodynamically sound than you would think. There are much easier ways to shave more time off (tune for more boost, headwork, etc.), and your 16g definitely has more in it than 13's. Some have gone high 11's with it, althoguh I'm sure this was with race gas and a bunch of weight loss. When you start runnin single digits, then I would worry more about aerodynamics, but until then it's really not worth the effort. If you do decide to do this though, it'd be cool to see a before and after. Such as, what per cent throttle you're pushing to maintain a certain speed on a certain road with the same conditions (air temp, wind speed, etc.). Me, I wouldn't bother at this point though. Sorry for rambling, I tend to take a while to explain myself.

 

[No Mo STi]

F1 is the greatest form of racing! Most physically demanding, too. Those guys train 4 hours a day, every day to stay in such good shape. Back to topic though. In my own opinion, I don't think it will shave a tenth off your time. Cutting 100 lbs from your car as a rule of thumb will. What are your 60' times? I didn't look at your profile, so forgive me if it's already listed. Dsm's are actually more aerodynamically sound than you would think. There are much easier ways to shave more time off (tune for more boost, headwork, etc.), and your 16g definitely has more in it than 13's. Some have gone high 11's with it, althoguh I'm sure this was with race gas and a bunch of weight loss. When you start runnin single digits, then I would worry more about aerodynamics, but until then it's really not worth the effort. If you do decide to do this though, it'd be cool to see a before and after. Such as, what per cent throttle you're pushing to maintain a certain speed on a certain road with the same conditions (air temp, wind speed, etc.). Me, I wouldn't bother at this point though. Sorry for rambling, I tend to take a while to explain myself.

I understand what your saying. 60's are around 1.7-1.8. I've made one 12 sec pass but lost the slip. Either why it's irrelevant. i was taking to the person that hold the small 16 record the other day when I was at Buschur's and he was telling me that the back half of his car is basically a tube car . I wanna say he said he ran something like a 11.2 and the car weighed something like 2600-2700 lbs. I was half asleep when I was talking to him so its hard for me to remember. Gotta love working midnights.

 

[Black_Bullet]

A couple yrs ago on my old gst I had removed front and rear bumpre guards for weight reduction.
( I DONT recommend this if you drive on the street daily.)
But at first it didnt occur to me that removing the metal rear bumper would cause more air to pocket in the new empty space in the back therefore I had cut two squares out to allow the air to flow out... I cant say i noticed a negligable difference even though the car did seem to accelerate nicer, but that mightve just been the weight reduction...

I know several civics who've done this.
Also I was browsing random threads and pics and dsm sponser driver, Mavisky has cut holes out on the back of his talon for the same effect your going for... Pm him with your ideas.

I thought about making a rear diffuser before out of sheet metal but scraped the idea.
Wasnt worth the effort imo, and felt like the rear diffuser would actually lift the car from under neathe if not done correctly. Idk.

 

[No Mo STi]

I thought the same thing until I read one of the links that were provided by one of our fine members. I'll give him a shout and see what he has to say about the bumper holes. I saw it on a few n/t civics at nopi a few years ago and I thought it looked cool, but I had a brand new STi that I didn't feel the need to cut up. Thanks for all the advices everyone.

 

[Defiant]

Please feel free to explain how it works b/c I'm just throwing up ideas that I don't see anyone else trying.

You're onto your answer, there.
No one is trying it because it's a pointless exercise below a hundred miles an hour. F1 cars are so extensively aero-engineered that they can barely herd themselves around corners below sixty, and if a car in front of them louses up their crucial airflow, their handling becomes more like an International Harvester Farmall.
The best thing you can do for a street car's aerodynamics is to divert air around it at the front with a deep chin spook. Keep in mind you do not want to undertray a street car, because your radiator's exhausted air is leaving out from under the car.
Both generations of DSMs have amongst the lowest Cx of any cars ever produced.

 

[Black_Bullet]

Heres maviskys car i managed to dig up this from another thread..
( Mavisky hope you dont mind. )

You must be logged in to view this image or video.

Nothing wrong with being innovative.

 

[Black_Bullet]

Does anyone think the stock wing that comes on 1g talons and eclipses are aerodynamicaly worth- ful? Lasers dont come with the cool liftback wings ...

 

[jtmcinder]

Both generations of DSMs have amongst the lowest Cx of any cars ever produced.

I believe that you are confusing Cx and CxA. There are tons of cars with a lower Cx, including the 3G design. The beauty of DSMs is that they have a below-average Cx combined with a below-average frontal area (A). So, I agree that our cars have low drag, but it's not coming from the Cx alone.

- Jtoby

 

[Black_Bullet]

Thats interesting to know...
Does anyone think the stock wing that comes on 1g talons and eclipses are aerodynamicaly worth- ful? Lasers dont come with the cool liftback wings ...

What about my question reguarding the stock 1g talon/eclipse wing...
I guess to rephrase that post,
Would I see less aerodynamic prowlness in the lighter wingless laser vs the 1g eclipse/ talons
or is the wing negligable...?

 

[jtmcinder]

Cx is the dimensionless measure of aero (un)smoothness. This is independent of the size of the car, so it's not very useful. A 1/24 scale model of a DSM will have the same Cx as a full-size DSM, but the model suffers a lot less drag.

CxA is the above multiplied by frontal area. This is what matters.

In other words, there are two main ways to decrease drag: make the car smoother (Cx) or make it smaller (A).

- Jtoby

ps. please don't jump all over me for talking about drag when I haven't specified the velocity and/or the altitude, etc; I think most people will get the point I'm making without these

 

[Black_Bullet]

For example why drag cars have no front bumper opening, anything that can pocket air or disrupt flow I suppose will hurt the Cx ...
So wouldnt a sti hood scoop for instance hurt the Cx pretty bad because all the air that enters gets trapped in the engine bay or is forced to go down? ( Not trying to talk about stis again, just closest example. And I know the scoop is for the top mount since its a rally based car ) but Ive considered hood venting to release heat out the engine bay and i guess a vent that doesnt allow air in is nessecary to keep good Cx. And the wing question I asked was never answered so im going to answer my own question and just assume it doesnt help aerodynamics as far as air passing smoothly around the car, and that it only helps downforce to the rear slightly after x amount of speed.

 

[jtmcinder]

All cars are compromises.

The STi has a boxer engine, which keeps the weight low. Partly because of this, the best place for the intercooler, they decided, was on top of the engine. Problem is, the air moves very fast across the hood, making it a low-pressure area (i.e., extraction vents move air out of the engine bay, not in). So they needed to use dynamic pressure, not static pressure, to get any air to the IC.

Would it be better from a Cx point of view to not have the scoop (which seems to get bigger every couple of years)? Yes, of course. But you have to cool the IC. And...

All cars are compromises.

- Jtoby

ps. all wings cause drag; if you need more downforce, you might accept the higher Cx, but the wing is going to hurt

pps. have I mentioned the fact that all cars are compromises?

 

[Black_Bullet]

Thanks. The only aero mod i think id ever do to my car is make/ or buy a front lip/ chin splitter type ordeal to keep less air from passing under the car.... Good to know im not missing out on anything in not having a rear wing. I suppose lowering the car another half an inch may help due to lowering the cars center of gravity. Its only lowered 1 inch on H&R springs with kyb gr2s, Once I get coilovers and koni yellows 1.5 inch drop will be in order.

 

[No Mo STi]

All cars are compromises.

The STi has a boxer engine, which keeps the weight low. Partly because of this, the best place for the intercooler, they decided, was on top of the engine. Problem is, the air moves very fast across the hood, making it a low-pressure area (i.e., extraction vents move air out of the engine bay, not in). So they needed to use dynamic pressure, not static pressure, to get any air to the IC.

Would it be better from a Cx point of view to not have the scoop (which seems to get bigger every couple of years)? Yes, of course. But you have to cool the IC. And...

All cars are compromises.

- Jtoby

ps. all wings cause drag; if you need more downforce, you might accept the higher Cx, but the wing is going to hurt

pps. have I mentioned the fact that all cars are compromises?

The trade off on the STi is that the scoop is helping produce more horsepower. And actually the hood scoop went back down to a WRX size when they switched to the new body style. I remember people saying they were getting pulled over by cops saying it was illegal. I wouldn't put a scoop/cowl in a DSM hood just because there isn't a whole lot of need to force air into the engine compartment. BTW...why isn't there a whole section devoted to aero...?

 

[Defiant]

Does anyone think the stock wing that comes on 1g talons and eclipses are aerodynamicaly worth- ful? Lasers dont come with the cool liftback wings ...

They're far enough back in the burbling air to not really matter. But they look racy. Kind of like the door wings on the last Talons. And the boomerang wing? rolleyes, rolleyes, rolleyes.

As for Mav's tailholes, I'd like to see a tuft-test run on them. I'd expect you'd see them draw in air instead of exhausting any.

 

[Talesin]

Problem being, the bumper doesn't generate excess drag at speed. If anything, the diagrams on the first page should contradict the author's position... if it works that way, a high-pressure bubble would form inside the bumper, and encourage the higher velocity low-pressure air moving under the car to move more smoothly under the rear bumper and out the back. Kind of how trucks with bed lids get worse mileage than ones with open beds... a high-pressure area forms behind the cab, providing makeshift streamlining.. the bed lid destroys that. If anything, adding the Honda-kid holes to the rear bumper would only increase turbulence in the underbody airflow, and actually increase drag.

 

[TurboATM]

And without some real data, I'll continue to wonder if putting holes in the rear bumper is worse than leaving it stock. The key to a diffuser is allowing the high-speed air beneath the car to slow down smoothly, preferably as far back as possible. By allowing air to go through the bumper, the slowing could occur just behind the gas tank, instead, which is sub-optimal to say the least.

Go find a Lotus Elise and stare at it for a while. That diffuser was tweaked in a wind-tunnel for months, if not years.

- Jtoby

i have a carbon fiber diffuser for my friend 07 Exige S sitting in my room...the other night we put it on the rear of the talon for laughs...then taped it down to the sunroof upside down haha

 

[jtmcinder]

Kind of how trucks with bed lids get worse mileage than ones with open beds... a high-pressure area forms behind the cab, providing makeshift streamlining.. the bed lid destroys that.

This doesn't match what I've read. From what I can tell, a bed lid has either no effect or a slight benefit. The surprising and significant result is that, assuming an open bed, it is better to have the gate up than down (by a lot) and it's better to have a solid gate than one of those nets (by a moderate amount). This is where I've seen the discussions of the air "bubble" in the bed, not in discussions of lid vs no lid. It was one of the times that Tom and Ray were completely wrong, MIT be damned.

- Jtoby

 

[Black_Bullet]

This doesn't match what I've read. From what I can tell, a bed lid has either no effect or a slight benefit. The surprising and significant result is that, assuming an open bed, it is better to have the gate up than down (by a lot) and it's better to have a solid gate than one of those nets (by a moderate amount). This is where I've seen the discussions of the air "bubble" in the bed, not in discussions of lid vs no lid. It was one of the times that Tom and Ray were completely wrong, MIT be damned.

- Jtoby

And I always thought it was better to have a lid on the bed as to were it wasnt
pocketing air in the untopped bed with the gate up...

 

[roadrunner2176]

For those in doubt read this near the bottom of the page.
http://en.wikipedia.org/wiki/MythBusters_(season_3)#Tailgate_Up_vs._Tailgate_Down


And watch the whole video clip here. http://video.google.com/videoplay?docid=-2400605931519184533&q=tailgate

 

[Talesin]

The tailgate up/down thing is similar to the bed lid... the bed lid also does not allow that pressure system to form, and creates more turbulence/drag than the open bed (and associated 'vortex') would cause.

In any case, this is about people who cut holes in their rear bumper to 'let trapped air out'. The idea is similar to a truck with a mesh net tailgate. Instead of high pressure air forming a 'cushion' inside the bumper and encouraging lower-pressure/higher-velocity air to flow under the back bumper, those holes allow turbulent airflow to pass through, increasing drag from the air passing under the bumper as well as the air that now passes through the bumper. It changes from a streamlined high-pressure bubble to a plastic 'drogue chute'. It also disrupts the vacuum system behind the car, reducing stability and reliable handling at-speed as the turbulence has a chaotic effect on how the back end holds steady.
It might not be blatantly noticeable on a DD that only ever hits 65 on the freeway, but I personally don't underestimate the effect that kind of response can have. You don't want to have the rear end unexpectedly jerk to the side, doing 95 into a sweeping turn on the track.