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Old 01-09-2008, 07:23 PM   #1 (permalink)
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How to decide whether to remove rear spoiler to decrease drag, improve mileage

This question has come up a number of times: "Does a rear spoiler lower my mileage?"

I'm wondering if we can come up with a few guidelines about whether or not to advise rear spoiler / wing deletes, considering that ....
  • some OEM spoilers reduce both drag & lift:
  • others are designed to reduce lift at the expense of increased drag;
  • and still others increase drag solely for the happiness of their owners and the joy they bring their viewing audience:





Also consider that while some OEM & aftermarket wings & spoilers improve high speed stability, the type of person who is going to remove a spoiler to increase efficiency probably doesn't drive at speeds where reducing rear lift is an issue.

Generally speaking, I'd say that spoilers which extend the decklid, or sit flush on top of it at a shallow angle should be left on. These are the style typically added to high efficiency vehicles to promote clean flow separation, or to enable flow reattachment at the end of a 3 box shape:




The Renault Logan Eco2 sported a deck lip extension and other mods to increase FE.






Honda Civic Hybrid has a rear spoiler not found on the non-hybrid version.



Is it reasonable to say that spoilers (wings) that stand off the rear deck should be deleted for better mileage?

EG:




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Old 01-09-2008, 07:27 PM   #2 (permalink)
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Or do we need to say that every model is different, and the only surefire way to know is tuft test, tuft test, tuft test.
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Old 01-09-2008, 09:07 PM   #3 (permalink)
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I would think a basic coast down test of some sort would work. Find a flat spot, get the car up to 80mph and let it coast in N measure the time from 75mph to 50mph. Do it enough to get a good average and consistent numbers then remove the spoiler and do the same run again.

Some variation of this would probably be easy for most people to test. I don't think saying always remove it is better though. Remember when I did a coast down test with my rear spoiler a while ago and it was actually better with the spoiler on It was a tiny difference but it was repeatable so it had to actually be helping.
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Old 01-09-2008, 09:16 PM   #4 (permalink)
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That's what I meant to say: coastdown test, coastdown test, coastdown test.
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Old 01-09-2008, 11:09 PM   #5 (permalink)
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damn coastdown tests...
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Old 01-09-2008, 11:13 PM   #6 (permalink)
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Coastdowns: They keep beer cans from making rings on the furniture, right?
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Old 01-15-2008, 11:04 AM   #7 (permalink)
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From a relatively uncommon thread about mods at cleanmpg, a member there removed the "stand-off" style rear spoiler from his 1999 Honda Accord after determining to his satisfaction that it increased fuel consumption slightly:




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Old 01-15-2008, 11:07 AM   #8 (permalink)
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Quote:
Originally Posted by Silveredwings View Post
Coastdowns: They keep beer cans from making rings on the furniture, right?
I thought that's what all those old America Online CD's that came in the mail were for.
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Old 01-15-2008, 11:25 AM   #9 (permalink)
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This is a great question, and one I've been asking myself lately WRT the Paseo. Now, the Paseo's spoiler doesn't look like much. There is barely a gap between the trunk and the bottom of the spoiler (less than an inch). This leads me to believe it may help with aero. Also, I have noticed that on a rainy day the back windshield stays completely dry at highway speeds. I believe this is a good thing? None of my other cars stay as dry as the Paseo in rain. So, is it possible that the spoiler helps this (since it is downstream of the airflow)? I think I speak for most here in saying I don't want to rip off the spoiler (a big PITA from what I've heard) just to see that it does in fact help aero.



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Old 01-15-2008, 12:09 PM   #10 (permalink)
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MetroMPG, I just gotta ask...where did you get that photo of the blue Nissan with the plywood skirts and rear wing? Is there a "story" that goes with that photo?

I was ROTFLMAO...

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