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

[MetroMPG]

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?

[MetroMPG]

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):



(source)

vs. original model with higher cD:

[3lr]

Originally Posted by Daox

Also, I have noticed that on a rainy day the back windshield stays completely dry at highway speeds.

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?

[3lr]

Originally Posted by Coyote X

I would think a basic coast down test of some sort would work.

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!

[tasdrouille]

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

[3lr]

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

[bobstuart@sasktel.net]

[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.

[aerohead]

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

[Daox]

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

[CobraBall]

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.

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.