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Old 06-24-2022, 02:36 PM   #1 (permalink)
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Rear diffuser?

Has anyone on here made a rear diffuser for any of their vehicles?

Any tests done?

Since Iím doing a smooth under tray, Iím going to obviously fabricate a rear diffuser. Seems like itís all benefits and no downsides

So far my inspiration is making one thatís like on the vision eqxx by Mercedes

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Old 06-24-2022, 05:50 PM   #2 (permalink)
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Yes.

Yes.

I haven't done one myself, the VW Beetle body doesn't lend itself.

I have squirreled away some examples:

Start big and whittle down


If the outer fences are curved, you have a start on the rear wheel spats.



That was the closest I got to that car. The fences start at the axle line and flare to the outside face of the tire.
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Old 06-24-2022, 08:11 PM   #3 (permalink)
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Wonder why they donít use any strakes or fins. Was going to try and make mine look like that and connect to the rear wheel skirts
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Old 06-24-2022, 08:45 PM   #4 (permalink)
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Wonder why they donít use any strakes or fins. Was going to try and make mine look like that and connect to the rear wheel skirts
Hey Phase! Strakes are only beneficial when they prevent vortices from forming because of other elements. Otherwise they deflect air that doesn't need to be deflected, i.e. they cause drag. It's easier and better to just have a well shaped smooth surface and let the air determine it's own least energy path. This is especially true if you are making mods upstream, as the optimal strake angles depend on what's happening along the entire length of the car.

If you are putting on rear wheels skirts, it might be worth having a strake on each side, as an extension of the wheel well. Then have the outside skirt and the inside strake meet, making a boat tail for the wheel. That's what all the solar car winners / serious aero cars do. (They even go a step further and make a whole fairing for each wheel.)
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Old 06-25-2022, 05:18 AM   #5 (permalink)
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I always like the skegs on the 1961 Oldsmobile:


https://cdn-0.barnfinds.com/wp-conte...-3-630x390.jpg
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Old 06-25-2022, 09:44 AM   #6 (permalink)
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Im a 57 chevy fan myself.

Has anybody ever run very long tufts to see exactly what is going on back there in the turbulent section?
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Old 06-25-2022, 02:19 PM   #7 (permalink)
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Wonder why they donít use any strakes or fins. Was going to try and make mine look like that and connect to the rear wheel skirts
Like mentioned above, arbitrarily adding strakes can be harmful to aero. They should only be added where they're actually needed. And to add strakes without disrupting airflow, you'll need CFD and/or wind tunnel testing.
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Old 06-25-2022, 10:33 PM   #8 (permalink)
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Like mentioned above, arbitrarily adding strakes can be harmful to aero. They should only be added where they're actually needed. And to add strakes without disrupting airflow, you'll need CFD and/or wind tunnel testing.
i figured id just skip the strakes and go for the eqxx style diffuser
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Old 06-26-2022, 02:09 AM   #9 (permalink)
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Sounds like a plan. Also, you need to be mindful of the departure angle. If it's too aggressive, the air stream can detach and create a bunch of turbulence (i.e. drag). If you could do some tuft testing, that should help.
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Old 06-26-2022, 02:12 AM   #10 (permalink)
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Quote:
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Sounds like a plan. Also, you need to be mindful of the departure angle. If it's too aggressive, the air stream can detach and create a bunch of turbulence (i.e. drag). If you could do some tuft testing, that should help.
im going to keep it fairly flat and a mild angle like the eqxx also. pretty much just extending the existing underbody shape out another foot or so

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