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:





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

EG:

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.

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.

That's what I meant to say: coastdown test, coastdown test, coastdown test.

damn coastdown tests... :D

Coastdowns: They keep beer cans from making rings on the furniture, right?

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:

I thought that's what all those old America Online CD's that came in the mail were for.

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.


http://www.tercelreference.com/Downloads/paseo.jpg
http://www.tercelreference.com/Downloads/paseo2.jpg

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... :D

Sorry but I can't see it helping by adding another planar surface for the air to flow past, although I really doubt it hurts much. Something you might want to try would be to tape some chloroplast from near the bottom of the rear window molding across the top of the spoiler to effectively raise the height of the rear deck.

I suspect that Paseo spoiler is more likely to help than hurt for a few reasons:

1) It's going to promote cleaner flow separation than the trailing end of the trunk lid. That lid edge isn't as sharp as it could be, and minus the spoiler, there may be downward flow over the trunk lip toward lower pressure, which will increase drag & lift;

2) It's deep enough in the front-to-rear direction that it could permit better flow reattachment coming off the roof if it has separated. If you've got attached flow at the trailing edge, you minimize rear wake.

In a sense, it may be similar to this extreme rear deck treatment from Autospeed.com:

http://us1.webpublications.com.au/st...07773_11lo.jpg
Australian VL Walkinshaw Commodore

Now the goal of this piece of work was solely to increase downforce (which in this car's case also increased drag). But what makes it similar to the Paseo's is that if the trailing end of the vehicle is below separated flow, it's useless. By raising the height of that end surface to get reattached flow, you can take advantage of it - to either achieve cleaner separation, which we want, or to generate downforce.

Disclaimer: I'm no expert. I'm just spouting my interpretation of the Paseo based on things I've read.

Another very interesting article...thanks MetroMPG! :D

Not that it makes much difference to the author's points...but he uses the term "laminar" when he simply means "attached". There is very little laminar flow on any car, because any irregularity bigger than a couple of hundredths of an inch will trip the flow. The flow may still be attached, but it is now turbulent. Any body parting line (hood, ornaments, bug splatters, paint chips, trim, doors, etc.) will trip the flow. And there isn't a lot of drag difference between laminar and mildly turbulent flow, either, so long as the flow remains attached.

Other than that relatively minor point, good article!

I ran a business as a fiberglass body kit builder in the late 80's. I made molds from this mockup. Unfortunately not too many Sentra owners were buying, so I had to close up shop.

Kidding.

I actually set out searching for a photo I'd seen before of a wood wing on the back of an 80's Caprice. Didn't find it - but this one works. :D

spoilers
 

I feel like most contemporary spoilers are simply for styling."The Fast and The Furious"inoculated the public mind with the notion that the spoilers portrayed in the movie were the hot-ticket for any performance-minded street-racer.Those spoilers would begin to produce measurable downforce at 120-mph,below that velocity,they're virtually useless.Since they are intended for downforce,by default,they increase drag.

In the 1970s,when rear spoilers began to show up,they were a paliative for poorly designed cars.Modern day cars would have no need for a spoiler,as,simply raising the height of the trunklid accomplishes the same thing, while adding useful trunkspace.

If your curious,a rule of thumb would be to measure the angle of incline from the back edge of the roof,to the rear top edge of the trunklid,as measured from the horizon.If that angle is greater than about 12-13-degrees,you'll have flow separation and high drag.If the angle is 12-13-degrees,flow will be re-attached at the trunklid,with a captured vortex spinning within that defined space,a smaller wake,and lower drag.

For a notchback car,the patented Ford Bi-wing spoiler,as used for the Sierra concept car/ Merkur XR4Ti of the 1980s offers the best drag reduction for anything cooked up that I know about.

http://forum.ecomodder.com/attachmen...1&d=1202848856

An angled aluminum plate as created by CAR and DRIVER for their Crisis-Fighter Pinto project for 1974,made a very productive rear spoiler,projecting up at no more than a 30-degree angle to a line which intersects an imaginary 12-13-degree line measured down and back from the roof.P.S.,the "tunnel" style,hollow type spoilers need to be "deep",projecting as close to the rear windshield as possible.

Hope it helps,Phil.

MetroMPG -

I like these spoilers the most, the "flush-kamm-back" looking ones :

Chevrolet Cobalt SS
http://www.worldcarfans.com/2060810....eak-new-ground
http://images.worldcarfans.com/artic...05.Mini18L.jpg

Ford Hydrogen Fusion 999
http://www.autobloggreen.com/photos/...on-999/306097/
http://www.blogsmithmedia.com/www.au...7/img_7330.jpg

Ford Hydrogen Fusion 999
http://www.autobloggreen.com/photos/...on-999/306098/
http://www.blogsmithmedia.com/www.au...7/img_7332.jpg

Ford Hydrogen Fusion 999 in grey primer
http://www.autobloggreen.com/photos/...esting/313341/
http://www.blogsmithmedia.com/www.au...7/img_8011.jpg

They are for salt-flat speed records. I don't *think* they are for downforce. Aesthetically, I think they are "as cool" as the raised spoilers. I'll bet they are less practical in parking lots because they extend the length of the car and are therefore vulnerable to damage.

Hmmmm. In the "grey primer" image, there is a parachute tucked in the back. Maybe they're main purpose is to act as parachute shields?!?!?!?!?

CarloSW2

I wonder... The Cobalt one has struts where the parachute would be.

is the reason some wagons have a spoiler is to extent the roof-line to make it slope more downward before the air separates from the car, kinda like on a caliber?
http://www.dodge.com/shared/2008/cal..._phtgal_03.jpg

I just wanted to hear what you guys think of the stock spoiler on my 96 saturn. This is not my car, but i have the exact same spoiler. Looks fairly pointless to me.


http://www.getmejoe.com/1996SaturnSL2/LeftRear2.jpg

http://www.getmejoe.com/1996SaturnSL2/Rear.jpg

Exactly. Speaking only of roof extensions on hatchbacks/wagons: if the extension slopes slightly downward, it's a safe bet that it's an aerodynamic benefit. If it slopes upward relative to the roofline, it's Fast & Furious.

igo: that's a tough call. The width of it suggests to me it's making things worse, not better.... BUT the height and depth look reasonable.

The best way to tell for sure would be to tuft test the trunk with it removed, to see if there's any flow attached at the back of the deck lid.

Like this... Video: tuft testing (rear) 1993 Ford Mustang notchback

What angle do you get if you follow Phil's advice and draw a line from the trailing edge of the roof to the trailing edge of the trunk (a) without, and (b) with the spoiler?

DPV: a clear illustration of roof spoiler differences... (NOTE that trunk spoilers can be angled upwards and reduce drag... just talking about roof spoilers here...)

If you look at, for example, Volvo's own C30 "Efficiency" model, among several aero enhancements, they reduced the rear spoiler angle from the regular model, essentially turning it into a roofline extension (extending the slightly downward slope):

http://bioage.typepad.com/photos/unc...efficiency.png

(source)

vs. original model with higher cD:

http://z.about.com/d/cars/1/7/h/T/08...30_profile.jpg

venturi??
 

is your rear window more sloped than your other cars
or it there a possibility that the distance of the rear spoiler to the trunk lid is carefully determined like the boundry layer (usually the lenght of both wings combined) as it provides a cushion of air that has allowed many ocean flying, out of fuel, airplanes to "coast" for many miles and make a safe terra firma landing.
perhaps in your case it creates a venturi, creating a low pressure area in front of the spoiler thus allowing air to speed up as it flows down the rear window?

accuracy of coast down tests
 

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!

Your usual street car spoiler should have an impact of roughly +-0.01 on the CD depending on how it's designed. The error in a coast down test is bigger than that so there's no way you'll really know.

Spoilers work by increasing the pressure between the back of the roof and the spoiler itself, so there is less of a low pressure zone at the bottom of the winshield. If the slope from the back of the roof to the back edge of the trunk is more than 15 degrees, you can benefit from a properly designed spoiler as long as the slope from the back of the roof to the top of the spoiler is around 12.5 degrees, or as close to it as possible. Also, if the air has someplace to go below the spoiler the aerodynamic purpose is somewhat defeated.

In a way it works a bit like a tailgate on a truck.

Here's a good example of a working spoiler

stability
 

the wing on the back of the modern bug will help
streamlining and economy.
the herrod's helper on original bugs were used to increase
stability.

yes you are correct, air of a rear wing is dammed,
extra molecules - higher temperature.
just look at the sandbox on the back of a double-a
funny car.

i was inquiring if anyone thought the paseo's rear wing's
close proximity to the trunk may work in a way to
create a low pressure area thus reducing seperation
of the air once spilling off the roof's rearmost edge??

anyone, anyone

Correction
 

[QUOTE=3lr;9720]<> like the boundry layer (usually the lenght of both wings combined) as it provides a cushion of air that has allowed many ocean flying, out of fuel, airplanes to "coast" for many miles and make a safe terra firma landing.

The boundary layer is air attached to a surface. You are referring to ground effect, which reduces induced drag on aircraft wings. Even if an out-of-gas plane entered ground effect at top speed, it would soon coast below stall speed. However, low flying was used to usefully increase the range of flying boats. Bryan Allan also discovered that one should not fly lower than several wave-heights - over water, lower does not keep getting better.

Is the Cobalt front-wheel-drive or rear-wheel-drive?

The Paseo's wing is roughly a half inch above the trunk, possibly less.

At Bonneville one of the more popular stock bodied cars to race is the 50's Studebaker. Without mods to the body racers found above +/- 150mph the rearend became very light causing spins. The Studebaker racers pleaded with rules committee for some relief to the rules. Hence, the 50's Studebaker can be modified with two ducts running from underneath the body and exiting aft the rear window. The effect was to lower the pressure under the rear end and raise the pressure behind the rear window creating a higher downforce. Some classes do not allow a spoiler unless the spoiler is year specific OEM.

Regarding the "flush-kamm-back" Cobalt racecar: the kamm back or spoiler probably has two effects on the Cobalt one being downforce to increase traction/decrease wheel spin. Second the kamm effect is used to keep the center of pressure of the race car aft of the center of gravity.

Remember in the movie World's Fastest Indian Burt Munro did a demo with a pencil, the pencil became stable when the CG was moved forward of the CP. Burt wanted to add weight to the front end of his MC to shift the CG forward, thus make it more stable.:eek:

The Cobalt kamm spoiler (creating drag) has the same effect, it moves the center of pressure aft of the CG. Many if not all Bonneville streamliners use the Kamm effect to stabilize the car at high speed.

IMHO a spoiler on a eco-car is for looks and does nothing to decrease the CD.

:thumbup: Great looking Black Deuce Coupe. :thumbup:

I'm curious, why a "deuce coupe" which is usually associated with the 1932 Ford Coupe and the Beach Boys. http://static.howstuffworks.com/gif/...-hot-rod-2.jpg

I know that I have a good size spoiler but it's OEM. Since the eclipse already has a low drag coeffecent would this spoiler be better on or off?

http://memimage.cardomain.com/member...773_9_full.jpg

http://memimage.cardomain.com/member...73_22_full.jpg

It's too tall for drag reduction,and it's chord is too short for any appreciable downforce(which you wouldn't need anyway).I'd say its just for styling purposes.It probably generates a little drag,so removing is okay,however,drag may be so low,its a wash.It's your call.

BlackDeuceCoupe

Love the :thumbup: 5 window coupe w/ the Olds thru the valve cover spark plug wires.

Best friend has a 34 Plymouth 4dr. PE with a '57 Dodge Hemi.

I've got a '33 Ford chopped full fendered Tudor w/ 350/350. :turtle:

At $3.90/gal it is once a month driver.

99clipsgst

You might tuft test w/ and w/out the spoiler. Have someone photo the tests. I would place the tufts on the back of the rear decklid. I suspect the test will show no effect on the tufts providing you aren't running flat out.:eek:

Black Deuce Coupe,

Maybe a 32 Hiboy with a turbo 4 banger, 5 speed. Probably 2200-2300 lbs. HP dependent on boost.

Have you been to the Bonneville Nationals in August?

CB

The Bonneville Nationals is a PhD in streamlining.

Seeing the Nebulus Theorem, Turbinator, or Al Teague has been awe-inspiring. Burland at 422 with 50% power :eek:. If you ever go, take Plenty of WATER and SUNSCREEN (cover everything exposed and not exposed:mad:). The guys/gals in the pits are very friendly and approachable. The pits are so big that a bike would help.

What about spoilers for hatchbacks? I've noticed a few types:
- Type 1 - placed below the rear window
Attachment 1986
- Type 2 - above the window, extending above the roofline with a gap under it
Attachment 1981
- Type 3 - above the window, extending the roof horizontally of slightly down
Attachment 1982
- Type 4 - above the window, tiny
Attachment 1983

Here is a Honda with two spoilers, types 1 and 3
Attachment 1985

Which of these will help or hurt FE? I can tell that type 2 probably produces drag, but what about type 1? Does the tiny type 4 do anything?

1 will do nothing for aero, its just added weight.

2 will add drag

3 will help a very small amount IF it slopes down slightly

4 will make no noticable difference

I agree with Tim's reply.