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Old 05-10-2009, 07:31 PM   #1 (permalink)
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rear diffuser build thread

Hey guys.

I am planning to build a rear diffuser for my car this summer. I want it to end up looking something like what you see inside the red box here:



Before you have anything to say about how effective a diffuser will actually be, I am planning to supplement it with underbelly trays, and if that still isnt good enough for you.. well then shhh.

The race car guys seem to think that much more than 7 degrees is too much, for the slower commuters, it seems like 10 degrees is more suitable for freeway speeds.

They are usually talking about cross sectional stuff, in my case the top surface of the diffuser itself is going to be level with the ground and the fins will provide the diffusing action. Hopefully you can bear with my microsoft paint skills and help me out with this. I mocked up an exaggerated top view drawing of the idea I have, but I don't know what these angles should be.



What do you think?

*edit, I noticed I did not specify how long this thing is going to be, I have available the space directly behind the rear control arms, to the edge of the rear bumper. Basically almost the length of the trunk.

Thanks
John


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Old 05-10-2009, 10:36 PM   #2 (permalink)
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So this rear diffuser will be parallel to the ground, with fins extending downwards for the "diffusing" effect you are going for? What's the purpose of this panel? Drag reduction?

To me it seems the point of these things on cars is to bring the air back upwards to minimize the wake behind the vehicle. Thus the >7 or >10 degree angles you referenced. I think the fins themselves are really for stability under cross winds, turbulence, etc. If you want to energize the boundary layer of the air flow as it leaves the panel at the rear of the car, why not just place a row of delta-wing style vortex generators? If you build the entire panel flat and then extend fins down into otherwise clean(er) airflow, and at an angle to the axis of the car (why?), you are only going to create drag.

I would suggest doing whatever it takes to put some vertical angle into the panel, from the lowest otherwise mostly smooth portion of the underbody behind the rear suspension / fuel tank, up into the lower edge of the rear bumper. Then fins parallel to the car. my 2 cents.
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Old 05-10-2009, 10:51 PM   #3 (permalink)
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Have a look this should clear it up

http://en.wikipedia.org/wiki/Diffuser_(automotive))
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Old 05-11-2009, 12:06 AM   #4 (permalink)
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Yeah basically the purpose of a diffuser is to decrease the pressure of the high pressure air running underneath your car to give it a smoother transition to the pocket of low pressure air directly behind your car. Similar idea to the vortex generators, they decrease air pressure to better contour to the back windshield or around a sharp corner or something.

Diffusers don't have to curve upwards but it probably helps a lot, that way they are diffusing air cross sectionally like I said before, not only left and right but up and downways also.

Basically, if you take that higher pressure air rushing under your car, and drop its pressure before you dump it out the back of the car, you will end up with less low pressure air behind your car, in other words, less drag.

Theoretically it works beautifully and I think they look frickin sweet too. But I am anxious to see if it makes a big difference or not.

Thanks guys.
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Old 05-11-2009, 08:33 AM   #5 (permalink)
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Ah ha. So my technical concept of the diffuser purpose was off just a bit. But I still stand by my recommendations.

The picture you posted here and each example on the wiki page all show straight fins, directly parallel to the main axis of the vehicle. They also ALL come up at an angle from the under-body to the rear of the car. This vertical angle is what provides the de-pressurization of the air moving under the vehicle.

So you are hoping to lower the pressure of this air by allowing it to expand horizontally. But if you conceptualize the air flow coming toward this panel, you are actually forcing some of it to turn when it hits the inner fins and is re-directed outwards. While it may leave the diffuser panel at a lower mean pressure, you could actually raise the pressure locally where the air is forced to turn.
And if you add fins extending further down that any other piece under the car, you are effectively adding frontal area, especially when they are cocked at an angle like that.
Is there no room under a civic rear end for some vertical angle? I have even contemplated cutting up my rear bumper somewhat to gain more angle and build one of these myself.

In any case, yes, they do look frickin sweet. What materials / construction methods are thinking of employing?
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Old 05-11-2009, 01:49 PM   #6 (permalink)
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Well, yes and no.

Not all diffusers always curve upwards towards the back of the car, and also the fins are often times angled relative to each other in one way or another. They are not always straight.

The fins will add frontal area to the car, but only the leading tip of them. The side of the fins that you would see from the front of the car are not working as drag, because the "channel" that the air is traveling through, while one side is pushing the air horizontally, the other side is "pulling" it in the same direction, and the overall distance between the sides of the channel is increasing as the air travels rearward, while the pressure of the air is dropping.

Also keep in mind that the drawing I made is kind of an extreme case and they wont actually be angled that much, I dont think.

Like I said, theoretically this thing should work great but I guess we will have to see if it is really worth anything for our applications. The pictures you saw of the yellow corvette, the c6r, and similar applications work to reduce drag of the car so much that it makes the wing on top of the car more effective for producing downforce. Of course this is while they are cornering at over 100 mph, but still. Its rad.

Thanks
John
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Old 05-11-2009, 04:48 PM   #7 (permalink)
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I was under the impression that diffusers created extra downforce (Formula 1) which is bad for FE. Any Thoughts?
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Old 05-11-2009, 08:01 PM   #8 (permalink)
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Didn't know what your fabrication techniques you're gonna use, but here's a FG or CF fab. rendering. Took your rear wing roots as base inspiration. Narrower entry with a widening exit & flaring angles. Dunno how you want to finish rear tire wings?

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Old 05-11-2009, 08:20 PM   #9 (permalink)
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wow that looks cool, did you find that or did you pshop it?

I was actually going to go a little cheaper than carbon fiber... haha. I was just going get some abs plastic sheets cut and use one sheet as the belly pan piece and chemically "weld" the fins on like a t joint style. So just five pieces together and screw it into the bottom of the trunk. I think it will end up being about $120 to make if I buy thicker plastic and get it custom cut at TAP plastic. I don't have a table saw....

So nobody can say what those angles should be?

And I doubt that a diffuser alone can generate any downforce, but they can make a spoiler or wing more effective at creating downforce, like I said earlier. Its kind of like saying that vortex generators can create lift... I don't think so. And just as a disclaimer, I might talk like an authority on the subject, but I could be wrong so correct me if I am.

Thanks!!
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Old 05-11-2009, 08:30 PM   #10 (permalink)
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Go about 20 seconds in and you will see the diffuser "working"


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