How to decide whether to remove rear spoiler to decrease drag, improve mileage

[aerohead]

Originally Posted by tripp323

Ok. You say " the lift can not be enough to offset the added drag, by definition" and you are the expert. But then how does a plane take off? Obviously you can generate enought lift using a wing at higher speeds. Your point about loss of traction is well taken. But put that aside for a sec - say we are on the Bonneville Salt flats and I want to lighten my prototype high-mileage vehicle - at some speed couldn't we use lift like a plane about to take off? I may have trouble controlling this - but if I control lift so that I still have enough traction to keep moving forward? Sorry to challenge your paradigm - hope you will keep an open mind for this curious newbie question.

At Bonneville,weight is your friend.If you lose traction at speed and go sideways,there is a very good chance you'll be airborne.If you land on your side and barrel-roll,that will be the last drive you'll ever take.The centrifugal forces will tear your organs loose and they'll end up in your feet and mouth.It's a horrible way to go! If your fortunate,you'll get your parachutes out in time to keep you straight down course 'til you grind to a halt.--- Forget about rolling resistance reduction by unloading them with a lift device.We don't want to see you at the morgue.--- Get the best LRR tires you can find and live with them.Emphasis on "live."

[ebouwman]

Originally Posted by brucepick

<snip>

Of course it takes energy to lift part of the car's weight off the ground, which ultimately comes from your engine and your fuel. From what I've read here before, that penalty occurs as drag. All other things being equal, a car with more lift will have a higher drag coefficient. Nuttin' comes from nuttin', and NSTAAFL: no such thing as a free lunch.

EXACTLY!!!!

And what you said about mass and stuff is bang on, i'm a physics type guy, i haven't pursed it yet in post sec, but loved it in high school, and flying and aerodynamics are another love of mine, so when i kinda read some of this i kinda have a hard time seeing how somebody thinks this will work.

The best way to increase efficiency through aerodynamics in the sense of lift vs downforce, is to remain as neutral as possible.

In flying there are 2 types of drag, induced drag, (created by the production of lift) and parasite (created by things that don't produce lift)

You can trim little bits here and there from induced by creating winglets and such. But really not that much, and you'll certainly get almost no gains fiddling with it on you're car, so you're best off to eliminate as much as possible by making things lift/downforce neutral.

Parasite drag is really what you should focus on, and is what may be eliminated by having a wing that isn't for downforce, but for guiding air, just like the ones shown in some of the earlier posts.

So the way i see it is, rip off wings that are obviously used for downforce and just leave the ones that look like the may be used to guide air.

Lets start it off

This is my car


And there is a MASSIVE upturn so it's just adding downforce and drag.

Oh and i just did a quick calculation of fuel mileage last week, and with that wing, and thrashing it, and driving quick and doing all the wrong things, still managed to get roughly 35 mph. And it's an automatic.

[daedalus921]

Hey Daox! I remember you from Tercel Online. To see what the air's doing on your rear window, cut a bunch of pieces of yarn about 2" long. Then tape them all over the window. The "tails" will act like little mini windsocks to show you the direction of the air flow. Sport Compact Car magazine did that on a notch-back 240sx. The article was to show how vortex generators decrease the size of the low pressure zone behind the rear window. They used AirTab vg's.

As far as I know, the optimal length to width/height ratio is 7:1. Therefore whatever you can do to lengthen the rear deck (or roof on a hatchback) may very likely lower the drag coefficient.

[Istas]

Actually, ebouwman, I think that wing on your car might be doing some good. It's hard to tell without a profile pic, and the only reliable way to know is by experimentation, but if that wing is catching the stream of air coming off the roof of the car, it will be helping to separate it cleanly from the car, rather than dragging it into the car's wake. It's less than the 30-degree angle aerohead quoted. I'd try some A-B-A-B testing, if you can, before conclusively claiming one way or the other.

[tanjulio]

So the best stock spoilers for a 'hatchback' are from Honda??

what about a 1988-1991 Honda hatchback??
are these good?

[MetroMPG]

Someone PM'd me wondering if this type of spoiler is effectively a "deck lid extension":



Answer: no. It's a roof extension. (Obviously you know that.) Some might call it a mini, mini Kamm back.

Like a deck lid extension, most would agree it's beneficial for reducing drag, vs. not having one, but for different reasons.

A deck lid extension helps by giving detached flow off the rear of the roof/rear window the opportunity to reattach; also a deck lid extension may promote cleaner flow separation from the rear deck.

A roof line extension - provided it continues plan & profile taper - reduces drag by reducing the size (cross sectional area) of the trailing wake where it separates from the back of the car. It may also reduce drag by reducing rear lift (vs. not having one), depending on the shape of the underlying glass/bodywork.

[3-Wheeler]

Originally Posted by 3lr

coast down tests, with your engine disengaged, provide very little resistance to an errand swift "wiff" or wake
from a jetstream on an otherwise totally still day.
especially on an lightweight vehicle, esentially flushing
your efforts.
for all aero improvement testing of my coasting racers i have found an very steep spot in the forrest surrounded by
dense pines, thus shielded from all extracurricular ariflow.
i you have no shielding of any kind perform your tests
during complete windless nights in the early AM, but not too late as i have noticed slight wind suddenly arising around 4 am in various paces around the globe!

air is always "thicker" at night time making test conditions
much more discerning!

And that's why it's better to coast down test in the winter...

Makes it harder to swap parts however!!

Jim.

[aerohead]

Originally Posted by 3-Wheeler

And that's why it's better to coast down test in the winter...

Makes it harder to swap parts however!!

Jim.

But it's absolutely crucial to run back-to-back,with and without the mods before atmospheric conditions "creep."
And remember,all your data must be normalized to standard conditions.
The only variable with your testing should be aerodynamic.

[k.civic.f4i]

thanks metro

[meelis11]

Hey,
what do you think Mitsubishi Eclipse GS (1 Generation, 1992).
It looks like this

I took image from here: 1994 Mitsubishi Eclipse Pic

Is that spoiler just for downforce?
How much I can benefit if I remove it?

Maybe somewhere is info about EPA fuel mileage - same 2.0 DOCH 110kW nonturbo engine, with/without spoiler.

I dont want to rip it off, fill holes, repaint half of the car just to find out that this change does nothing to improfe FE