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Old 05-22-2010, 01:29 AM   #1
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Default Rear bumper aero or venting

Im curious to see what people have done to the rear bumper, to cut down or reduce the "parachute" effect that it gives.

Everyone has seen the classic civic with the huge holes drilled in it to do the same thing, and i know function>form, but im curious to how else you can do this, and make it look good, for those that dont use the car for track only
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Old 05-22-2010, 01:43 AM   #2
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Old 05-22-2010, 08:22 AM   #3
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I've thought about it. I think I'll just take the bumper off.
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Old 05-22-2010, 08:33 PM   #4
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Old 05-22-2010, 10:20 PM   #5
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This is a dumb thread, no offense to the OP. Oscar answered your questions as to what most of us do, Double 086 gave you another extremely attractive method. /thread.

edit: And by /thread I mean if anyone else posts anything beyond an amazing zero cost rear bumper solution that isn't cutting the bumper or taking it off AND adds power (or praises my amazingness), I'm locking the thread.
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Old 05-22-2010, 11:19 PM   #6
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Curly you are a god and i wish i could carry your children......


i wouldnt mind more info on the elise diffuser......im sure its gonna cost a good penny though...


EDIT! yeh just saw the price on some aftermarket diffusers for elises, **** that price. where can i get one cheaper?

Last edited by WonTon; 05-22-2010 at 11:31 PM.
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Old 05-22-2010, 11:36 PM   #7
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Quote:
Originally Posted by WonTon View Post
Curly you are a god and i wish i could carry your children......
ew.

https://www.miataturbo.net/showthrea...iffuser&page=2
https://www.miataturbo.net/showthrea...iffuser&page=2

I should rename this thread "how to not search".
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Old 05-23-2010, 07:38 PM   #8
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Quote:
Originally Posted by Dlaitini View Post
Im curious to see what people have done to the rear bumper, to cut down or reduce the "parachute" effect that it gives.
I'd like to see data showing there actually is a parachute effect. I left the bumper off the original OGK for weight savings and to let the wheel wheels vent. I think the air mass moving under the rear subframe is slow moving, low pressure and turbulent enough that any form drag from the overhanging bumper skin is minimal and probably more imaginary than actual.
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Old 05-24-2010, 04:22 AM   #9
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whos got a wind tunnel?
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Old 05-24-2010, 11:58 AM   #10
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Quote:
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whos got a wind tunnel?
If someone really wants to test this you do not need a wind tunnel. Just a straight flat uncrowded road. Do a coast-down test preferably with a GPS unit logging velocity. Do two coast down runs back-to-back after an aero change, plot the results in excel, and you can see if it reduces or increases drag. The faster the starting velocity, the more clear the difference will be.
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Old 05-24-2010, 04:27 PM   #11
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A 100-60 MPH data set would probably do pretty well there, good idea.
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Old 05-24-2010, 06:28 PM   #12
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Real world data is best. Take the bumper off and track it. Get before and after times. If there is no time benefit on the track, then it's not a problem.
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Old 05-24-2010, 06:59 PM   #13
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Tire wear/temp/pressure, track temp, driver fatigue, engine temp, ambient air temp, traffic, fuel load and other things will have an effect on lap time. Not so much on a coast down test.
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Old 05-24-2010, 11:40 PM   #14
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Quote:
Originally Posted by Sparetire View Post
Tire wear/temp/pressure, track temp, driver fatigue, engine temp, ambient air temp, traffic, fuel load and other things will have an effect on lap time. Not so much on a coast down test.
Yup, coast downs are pretty repeatable. Recommend two directions to help further isolate wind.
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Old 05-25-2010, 02:57 PM   #15
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So who's going to do it?

I'm still back and fourth on hacking up the rear bumper, but if it shows a decent gain, I'm in.
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Old 05-25-2010, 03:14 PM   #16
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I have been thinking about doing this test with the LS1 car but it is offline right now and will not be back online for at least month or so. Plus I am working on the turbo car right now and have some higher priority projects going. So, if you want a quick answer don't wait for me

Whoever does it, do multiple runs in each config, +1 on Emilio's suggestion to run it in both directions for wind correction. And if you cannot do the comparison runs on the same day, back-to-back, at least try to make sure the air temp and other such variables are the same.

Ideal test would be 2+ runs in stock form, 2+ runs with bumper mod (or removed) and 2+ runs back in stock form, all within an hour of each other or less. If you already have a rear bumper cut out, a piece of stiff plastic or cardboard taped to the bumper (on the inside, well secured) would probably be good enough as long as it does not deform at speed.

Run the car up to say 100+ MPH (closed road course, professional driver, etc. etc.), let it out of gear and coast in neutral, going dead straight, no control inputs, engine revs, whatever. Just go straight, car in neutral, idling. Try to be consistent with the whole procedure. Look at the coast down time from say 95 MPH to 45 MPH. Accelerate to a speed higher than your top speed time bracketing start point (before going to neutral coast-down) so the transients (throttle off, neutral shift) can settle out. You could pick this point after the fact if you have logged data. When plotted in excel for speed versus time it should look like an exponentially decaying curve, like a discharging capacitor.

If you do not have a logging GPS then a stopwatch will do if you are very, very careful with marking the times. A passenger working the stopwatch while looking at a GPS unit for speed would be best though the typical 1 Hz refresh rate could be problematic.

Use a smooth road course so the car is not riding up and down a lot relative to the road surface which would skew the data since the airflow under the car will vary accordingly. A test and tune night at a drag strip could work great if they have a LONG coast down area before the turnoff otherwise you will have to use the brakes.

Last edited by ZX-Tex; 05-25-2010 at 03:28 PM.
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Old 05-30-2010, 03:34 PM   #17
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See the picture in my sig. Bumper is cut fairly high, the rear bumper crash support behind it is obviously removed and custom brackets were made to hold the bumper on (since it normally attaches to the crash support) and Lotus Elise rear diffuser. All the brackets I made for the diffuser are adjustable. I think the angle as-is a a bit high in terms of what is optimal. It currently sits at the same angle as on the Lotus, but I intend to lower the angle soon.
-Ryan
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Old 05-30-2010, 05:59 PM   #18
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I wonder if a coast down from 90 to 60 would be enough to show the difference? The bulk of the efects of drag are going to be above 60 or so, after that they tend to be overshadowed by rolling frictionon cars like these AFAIK.

Plus its not so hard to find a safe place to do that.

Other than that, everything 'ZX just said.
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Old 06-03-2010, 04:37 AM   #19
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Those Elise splitters look beautiful on a Miata.
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Old 06-13-2010, 02:10 AM   #20
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In case I'm not the only one who likes having a hitch on my Miata- The Hard Dog hitch hangs down 3 inches under the bumper and I cannot live without a hitch, soooooooo.....
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