Rear spoilers........ - Gas Savers

Kraig · 07-22-2006, 08:13 PM

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

Matt Timion · 07-22-2006, 11:06 PM

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!

Matt

ZugyNA · 07-23-2006, 05:42 AM

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.

brick · 07-23-2006, 07:43 AM

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.

MetroMPG · 07-23-2006, 08:49 AM

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.

MetroMPG · 07-23-2006, 08:53 AM

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?

brick · 07-23-2006, 11:23 AM

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.

Kraig · 07-23-2006, 07:27 PM

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?

MetroMPG · 07-23-2006, 10:04 PM

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.

ZugyNA · 07-23-2006, 10:11 PM

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?

ZugyNA · 07-23-2006, 10:25 PM

Quote:

Originally Posted by MetroMPG

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.

Looks like the small spoiler keeps the TOP air flow going straight back and then the low pressure area at back is filled by air from the SIDES?

If you look at the Airtabs applications...that might be how they work...causing a straight sheet of air...a clean break?

I was assuming that they caused the boundary layer to follow around the curve into the lower pressure area behind the car.

If you look at how the Airtabs are made...look like they lift a stream of air in the center and force it out away from the surface plane to some extent? Between the Airtabs seems like there would be a lower pressure area that might supply air to the center??

Whereas the vgs I made induce trailing vortexes. By placing them 4" apart...the 2 vortexes from ajoining vgs counter-rotate and shouldn't interfere with each other.

My ZX has a small spoiler at back...so maybe the top vgs help to keep the air more down on it...while the side vgs help air getting around the back...or maybe hinder it?

Mighty Mira · 07-23-2006, 11:01 PM

I think that the jury on this is going to be out until we get enough people testing these things in a rigorous manner.

Kraig · 07-24-2006, 03:16 AM

So it would probably be beneficial to extend the spoiler down the sides of the hatch also...

ZugyNA · 07-24-2006, 06:15 AM

Here is a pic of the kind of vgs I use on the side...that are supposed to be similar in effect to the airtabs.

Far as I know...the air going into the center gets concentrated at a higher pressure than "ambient"....and the air going between them is possibly at a lower pressure than "ambient"?

Might be that the lower pressure air...fills in the large low pressure area behind the car?

These should probably be placed in a series 1" or so apart.

Here is the pattern used...found on the web...I used alum flashing material.

brick · 07-24-2006, 10:51 AM

You know, I'm starting to think that it wouldn't be that difficult to make an effective lip spoiler. I had a look at my girlfriend's mother's Mercedes E300 Turbodiesel (I think it's a '99) and that has one built right into the trunk lid. It's literally just a vertical lip formed into the top of the trunk, right at the rear edge. As those sources point out it seems like it's just a matter of optimizing size and angle. I think that the tougher part would be making it look good, and then reliably measuring its effects without a wind tunnel.

MetroMPG · 07-24-2006, 11:06 AM

I agree: a lip spoiler is an easy item to make, and the theory behind its purpose is solid (& logical, which makes it even nicer). As for angle, I think the safest bet would be to simply extend the plane of the trunk lid. As for size, I'm less certain, but the article mentioned 20 mm.

Quote:

Originally Posted by brick

think that the tougher part would be making it look good, and then reliably measuring its effects without a wind tunnel.

You can probably guess where I place looks on the scale of things

But you hit the nail on the head about reliably measuring effects.

Logically, a lip extension should improve aero. But the change will likely be small enough that on-road testing - whether coastdown based, or speed-controlled & averaged bi-dir. runs - may show nothing conclusive; the effect will likely be lost in the normal static of variability.

It's something I've been thinking about lately, since there are a number of other known aero tweaks I'd like to try, but whose individual effects (if properly executed) are known to be small. How to quantify...

MetroMPG · 07-24-2006, 11:09 AM

Detail of the integrated roof extension spoiler on my rear hatch (Firefly #1) that ensures clean flow separation via a sharp angle and small radius:

Above: note the hatch sides also have an integrated extension, to about 2/3 of the way down.

Why they stopped there is a question. The remaining of side-to-rear transitions are horribly large radii that would likely also benefit just as much from sharper transitions & cleaner flow separation.

brick · 07-24-2006, 11:42 AM

Quote:

Originally Posted by MetroMPG

As for angle, I think the safest bet would be to simply extend the plane of the trunk lid. As for size, I'm less certain, but the article mentioned 20 mm.

Sounds fair. The other car I looked at this weekend was my girlfriend's sister's '98 BWM 328 coupe. (What can I say, their father is German-born.) Anyway, that also appears to have a built-in extension that I can only assume is designed to do the same thing. The horizontal part of the trunk lid is simply extended ~20-30mm beyond the vertical part of the trunk lid below, tapering backward (err...forward?) to meet it. It makes a lot of sense now that I know what they were trying to accomplish.

ZugyNA · 07-24-2006, 01:15 PM

Quote:

Originally Posted by MetroMPG

Above: note the hatch sides also have an integrated extension, to about 2/3 of the way down.

Why they stopped there is a question. The remaining of side-to-rear transitions are horribly large radii that would likely also benefit just as much from sharper transitions & cleaner flow separation.

If you look at the oil flow plots...looks like the gradual curves at the sides allow the air to flow into the rear area to increase pressure?

The spoiler they mention is about 3/4" high and at a 20* angle to the downsloping surface.

As far as testing various vgs and ideas....if ??????? can test and find a 1.5% gain with a V6 (?) from a row of rooftop vgs...what's the issue with testing....except for the time involved?

A lighter 4 cylinder would probably show increased effects from reduced drag.

Kraig · 07-24-2006, 02:24 PM

Spent four hours traveling for work today. It's amazing the number of newer cars with an aero device at the rear of the roofline! Magnum, Pacifica, Caravan, Matrix, Trailblazer, Beemer and Lexus Suvs etc. Must be there for a reason....

Kraig · 07-24-2006, 02:56 PM

Now that I think about it, the rear of the trunklid on my '86 TBird Turbo Coupe kicks up and then drops off sharply. Was this a aero trick Ford used on the car that is credited with starting the "aero" design trend ('83 TBird) in the U.S.?

MetroMPG · 07-24-2006, 06:26 PM

Quote:

Originally Posted by ZugyNA

As far as testing various vgs and ideas.... what's the issue with testing....except for the time involved?

That's it, basically: time.

I'd just rather focus on things that have a known effect for my hatchback car, before looking at things that may have some effect, but for which there's precious little technical info available, and no evidence (that I've seen so far) that they would work for my style of car.

Just to be clear, ZugyNA, I'm not debating that VGs work, when properly situated on a 3-box vehicle, with the right angle of the rear glass. And there seems to be little doubt that they help at higher wind speeds in aeronautical applications.

But back to cars - aside from the specific 3-box vehicle application, I just haven't seen anything more... rigorous.

There are no details of controlled tests anywhere on the airtabs website. When you look up "tests", you get "testimonials" - not at all the same thing. They mention a UK controlled test, but offer no details about it.

I'm not opposed to the idea of trying them out for the heck of it, and my mind is open to the possibility that they may have some effect on the back of my car. But I have half a number of other things I'd rather do first, is all.

Sorry for jumping on you on this topic!

MetroMPG · 07-24-2006, 06:30 PM

Quote:

Originally Posted by Kraig

It's amazing the number of newer cars with an aero device at the rear of the roofline! Magnum, Pacifica, Caravan, Matrix, Trailblazer, Beemer and Lexus Suvs etc. Must be there for a reason....

I had the same thing happen today too. Trunk & roof "extensions" are everywhere, aren't they? And now we know why.

MetroMPG · 07-24-2006, 06:42 PM

Zugy (and others): Earlier today I e-mailed the company that sells airtabs asking about the UK study, and lo and behold, I just heard back from them. They attached a copy, which i haven't read yet. But I will have a look and get back with more info.

I may also move these posts into a new thread, since VG's aren't exactly on topic here in "Rear spoilers". (Sorry for hi-jacking your thread, Kraig.)

Kraig · 07-24-2006, 07:35 PM

No problem. I'm new to this stuff and looking to learn. I want to make a "hypermiler" with a Ford emblem on the front (I know it's a Ford/Mazda/Kia). Any advice and/or hints are appreciated.

brick · 07-27-2006, 12:24 PM

Here's something to talk about. What do you guys think would be the best way to fabricate a lip spoiler for the trunk lid that doesn't look like a complete disaster? (Read: no cardboard or duct tape

)

If the spoiler can be straight, I was thinking about finding someone with a metal brake to bend something simple out of aluminum sheet. It could be done with two or three bends depending on whether you want it to wrap around the back side to be extra-secure or just sit on top of the lid. The only problem with this is that if you want to form the spoiler to a lid with a curved edge (in the horizontal plane as well as vertical), the brake wouldn't be able to form the metal in the second dimension. So for me that's out unless you guys know of some magical technique that I'm not aware of?

Another option would be to form it out of multiple strips of aluminum, cut to match the curve and welding or brazed to make it whole. However I've never welded or braized so I have no idea if that's really a viable option.

And then we get to other options that get a little more creative. For example, I do have several sheets of 6mm coroplast. Maybe I could find a way to cut and form that stuff to the shape that is needed? Perhaps some creative handywork and covering material could produce something that doesn't look like I made it out of coroplast and stuck it to my trunk lid?

Fiberglass? Wood?

Just looking for ideas.

SVOboy · 07-27-2006, 12:37 PM

I would suggest fiberglass. Luckily I don't really have this issue as there are little wings mand for the crx that match the body to extend the flow and blah blah blah, based on talking about it and looking at the little images of shapes in airflow it looks like it would work.

But yeah, I think make up a coroplast model and then fiberglass it. Coroplast is bendable with heat and all that, so yeah, not to mention cheap and smooth.

Mighty Mira · 07-27-2006, 04:05 PM

A DIY on fibreglassing or a link would be neat.

omgwtfbyobbq · 07-27-2006, 04:58 PM

What about paper mache? It's durable, sandable, and paintable, if your mixture's right.

ZugyNA · 07-27-2006, 07:46 PM

Why a VW needs a wing?

http://www.mulsannescorner.com/aerod...wbeetle00.html