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Old 07-22-2006, 07:13 PM   #1
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Rear spoilers........

Hey,
This is my first post. I have a question about rear spoilers. I've noticed alot of newer SUVs, wagons and vans have a rear spoiler now. They end with a sharp angle toward the front of the vehicle. Is this so that the boundary layer doesn't try to follow the down ward slope at the the rear of the vehicle (which it can't and ends up as turbulence?)? The reason I'm asking is I am the proud owner of a '91 Festiva (I've had it for a week!). The previous owner claimed 50 mpg with the A/C on going on a long trip. I'd like to improve the aerodynamics and thus the mpg. Will a upper rear spoiler help? I'm also thinking of a full belly pan and blocked grille openings, possibly rear fender skirts. Anyone know where I can get 12" Moon discs for my wheels? Thanks.

Kraig
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Old 07-22-2006, 10:06 PM   #2
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I honestly don't think a rear spoiler is going to do you much good at all, unless you're going to drive more than 200mph. In most cases they actually hurt the MPG of the vehicle. Ernie Rogers crafted a cool spoiler for his VW Bug and he noticed considerable gains (he said between 5 a 8 percent), but then again most vehicles don't have such a horrible drag coefficient like the VW Bugs do.

For the best gains I'd recommend the tings you were thinking of: rear fender skirts, belly pan, grille block.

In maybe a month I'm planning on constructing some rear wheel skirts for my car out of fiberglass. Hopefully they won't look too dorky for my wife to be ashamed to ride with me :P

When i think of improvements I usually look at cars like the Honda Insight as a guide. They are lightweight, have rear skirts, have a form of a belly pan, etc. THe Honda Civic Hybrid has the same (undertray). NONE of the cars with super high MPG have rear spoilers. That has to tell you something.

Glad to have you aboard!

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Old 07-23-2006, 04:42 AM   #3
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Naturally...I disagree. I've heard of ricer Hondas with the spoilers claiming better mpg at certain angles...though nobody want to beleive them.

Then there is the VW with the plywood wing.

My guess is that most are there for mpg gains...not racer decoration.

Seems that if they are up just high enough to catch some air...they cause a higher pressure plume of air to the rear that reduces the larger votexes and vacuum effect.

On a Festiva SW vortex generators would probably work.
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Old 07-23-2006, 06:43 AM   #4
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Honda put a lip spoiler on both the Civic and Accord hybrids to help reduce drag. Large high-mounted wings are designed to produce down force at the expense of drag, and I really don't believe that there's a way that they could do anything but that. However, the small spoilers work on a completely different principle. What they do is intterupt the flow of air from the rear of the car in order to induce turbulence, which works to reduce the size of the low pressure region behind the vehicle. Take a look at the rear end of a '04-'06 Prius and you see something similar.
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Old 07-23-2006, 07:49 AM   #5
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Hey Brick - do you happen to have a web source that explains the dynamics of "lip" spoilers (ie. not downforce producing wings)? I've been wanting to better understand them myself.

I'm not sure about the desire to induce turbulence at the trailing edge of a vehicle - seems counterintuitive.

My limited understanding of aero is that the cleaner the flow coming off the rear of a vehicle, the better. On any number of streamlined cars, any form of lip spoilers or "kick" in the bodywork at the rear is absent: VW 1L concept, Ford Probe V, GM EV-1, GM Precept, Dodge ESX, Mercedes Bionic, even the Opel Calibra...

If I had to guess (note: guessing!), I would have said that a lip "spoiler" helps to induce a less turbulent flow separation at an edge or corner, if it causes a cleaner separation than would otherwise be generated around a larger radius in the bodywork at the rear.

The absense of (or very small) radii in the side-to-back or top-to-back bodywork transitions on highly aero optimized vehicles is a very common feature, missing from most modern production cars. I figured lip extensions were an attempt to attain that kind of transition.
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Old 07-23-2006, 07:53 AM   #6
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Quote:
Originally Posted by ZugyNA
On a Festiva SW vortex generators would probably work.
Sorry to keep bugging you on this, ZugyNA, but could you please point Kraig to a credible controlled test showing the effectiveness of vortex generators at the trailing edge of a blunt hatchback-style vehicle?
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Old 07-23-2006, 10:23 AM   #7
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I'll see if I can find a reference or two later. In the mean time, I'll try to explain a little better how I understand it (which could always turn out to be wrong). First, I don't think that this applies to ultra-streamlined "tear drop" vehicles, around which air flows extremely well. Lip spoilers are more of a stop-gap measure to reduce drag on our conventional blunt-ended cars. On such cars, the air flowing over the back immediately travels downward as soon as the horizontal (or nearly horizontal) surface ends. This creates a trapped vortex "bubble" right behind the car. That makes for a zone of low pressure right behind you, effectively trying to suck you backward. Reducing the size of that vortex with turbulence from the lip spoiler reduces the size and severity of the low pressure region, thus reducing drag.
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Old 07-23-2006, 06:27 PM   #8
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spoiler

My intention would be as Brick says, to increase the aerodynamics of a blunt ended vehicle, not produce any kind of downforce. I can't believe all the blunt ended newer vehicles I see have a spoiler just for aesthetics (the factory isn't spending money on a vehicle just for fun), they must be trying for some kind of aerodynamic/mpg gain.

Will vortex generators help on a car shaped like Festiva?
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Old 07-23-2006, 09:04 PM   #9
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Jackpot!

Found an article dealing specifically with short, drag-reducing decklid spoilers (and by extension - pardon the pun - hatchback roof extension spoilers).

I believe I was on the mark when I guessed it was an attempt to deal with flow characteristics around the rear transition radii:

Quote:
In the no spoiler case, the low static pressure on the uppermost part of the rear panel (trunk lid) is associated with the airflow accelerating sharply around the radius here before separating slightly below the lip. This acceleration creates lift (on the horizontal part of the radius) and drag (on the rear facing part of the radius). But fitting the spoiler eradicates the zone of flow acceleration, resulting in higher pressure on the rear panel and less drag.
Source: Aerobytes: Rear spoilers part 2 (Racecar Engineering, The International Journal)

This would also explain the shape at the top of my car's hatchback, and around the upper half of the hatch glass, which I've always wondered about. It's a built-in spoiler, meant to force flow separation early to avoid the negative effects of having the flow wrapping partway onto the glass before going turbulent.

I'll get a pic and post it tomorrow.

Moral: everyone, go outside with your duct tape and cardboard and square off all your cars' trailing edge radii.
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Old 07-23-2006, 09:11 PM   #10
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Quote:
Originally Posted by MetroMPG
Sorry to keep bugging you on this, ZugyNA, but could you please point Kraig to a credible controlled test showing the effectiveness of vortex generators at the trailing edge of a blunt hatchback-style vehicle?

This pdf explains some stuff in detail:

http://techreports.larc.nasa.gov/ltr...-2002-3162.pdf


Would these truckers lie...fuel is a main expense??

http://www.airtab.com/welcome.htm


The design of the vgs I put on my roof were taken from some this guy made for an ultralight...in an email he claimed NO reduced range...but lower stall speed. Effective with no drag loss as far as mpg.

http://www.thirdshift.com/jack/firefly/firefly18.html


some plastic vgs:

http://www.markfelling.com/id384.htm


I think you are just going to have to break down and test them? They are not exactly difficult to make or test.

Check the research for airplane use?

All I can say is from the reading I've done...I think they would be MORE effective on a vehicle like the CRV....blunt with rounded corners....than on a fastback or sedan.

You can't just say something won't work (very popular on this site?)...you have to test it to say it doesn't work...then if it doesn't....you have to ask yourself...did I set it up right?
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