underside air flow
 

https://www.gassavers.org/garage_imag...1yboonm54a.jpg
I've added some foam board and duck tape to the underside of the front bumper to create more of a slope so air will go under the car better.It's covered back as to about 10" behind the radiator still sloping to block air off my oil pan for aerodynamics and to keep cold air and water off the pan as well. Should I try to keep the air flowing under the car and out the back or divert it out the sides? I was considering some homemade side skirts but didn't know if it helps or hurts.

This is an 02 Cavalier BTW/ I'm gonna be made fun of anyway for the red hub caps, so no need to stop now.

I don't think anyone here will make fun of your for your red hubcaps ;)

Interesting question. I'm going to guess you want to divert it out the back, and not out the sides, but I have no idea why.

The aero issues discussed here are always interesting....

I was thinking out the back too. I've got the hypermiling techs down pretty well with the scan gauge,so Aero and weight reduction is all I can do now.

Was in Wal-Mart today and saw this nice roll of aluminum window screen;48"x84". I think I've found the material for the rest of underside enclosure. I built up my homemade duck tape air dam this morning and the car seems to handle better. Drove a little faster than normal testing it and still maintained 35mpg. Later trip to store got SGII tank back up to 38mpg for 100miles so far.

How about this:

"Vehicle: 2006.5 Jetta TDI DSG
Vehicle Mileage: 1750 Miles
Speed: 72.4 +/- 0.1 mph (GPS)
Temp: 60 degrees F
Wind: Out of the N.N.E.

With VG's Under Front Bumper:

Run 1 North: 46.6 mpg
Run 1 South: 47.2 mpg
Run 2 North: 45.0 mpg
Run 2 South: 49.2 mpg
Average: 47.0 mpg

Difference from Baseline: +2.6 mpg"

Good numbers; I had to look up "VG" and read a little on it. It seems every graphic I've seen involves a car with a spoiler raised a foot or more off the trunk lid;which I don't have and don't plan to get one( not worried about car leaving the ground at high speeds).
The VG's seem almost too easy of an answer; making some extreme aero work I've seen on the site a big waste of time.I'll read more on the subject once I've done what I've already planned. I tend to watch out for people quoting Machiavelli; though " The Prince" was a good read.

couple of pictures of air dam and under screenhttps://www.gassavers.org/garage_imag...px6idsru7p.jpg
https://www.gassavers.org/garage_imag...2k33tmb9n2.jpg

aluminum window screen
 

I've finished covering the entire underside of my car with window screen. This is touching the exhaust components in several places and has not burned nor melted. I plan now to paint the screen in some places to reduce friction more,leaving other parts near the exhaust unpainted for exhaust cooling. I know I'll have to cut a small hole when I change the oil ( filter is reachable from the top if I remove an electrical component from the fire wall and replace). Just drove 20miles from a cold start and managed 40mpg on 45/55mph zones.

Hateful -

EDIT : Nevermind my melting question below, I see from earlier posts that it's aluminum window screen. Won't melt.

Interesting. Is this the "normal" plastic mesh or some kind of metal mesh? The plastic would be easier to install. I wonder why it hasn't melted. If you find it does melt, you could have spacers and/or metal mesh on areas near the cat.

And the paint will fill the little holes in the mesh. Never thought of that ... :thumbup: .

CarloSW2

Wal-mart had fiberglass and aluminum screen so I picked the aluminum( took 2 7ft rolls).I expect the motor heat will last longer when parked.I used tie wraps to anchor it to anything I could find ( drilled a few holes for tie wraps also).

What the price for that?:thumbup:

It was about $15 for both rolls. each roll 48"X84".

What kind of MPG gains are you getting so far with the underbody condom you've made?

It's hard to say since I've added to my duck tape air dam also. I ran some errands with some highway miles and averaged 44mpg for the entire 40mile trip.Last tank averaged 38.4mpg I have noticed that the engine retains heat better between trips and warms up faster as well.

That's a pretty good idea :) What are you doing to make sure it doesn't flap around?

I really like this actually, while it won't give the benefit of a non permeable surface, it more than makes up with super easy installation :thumbup:

I drilled holes in the ridge of metal along the sides and used tie wraps to tighten it and well as duck tape on the metal and the screen to keep the screen from ]breaking( holes through tape and screen. I've also bought some spray paint to make the screen less permeable( only one can so far, three more when it's not so windy) The flaps on the bottom edge of the air dam do fold back hitting the tire at highway speed but no harm done; I can always add more red duck tape when it wears down.https://www.gassavers.org/garage_imag...pcwmg707o4.jpg

Slick. Pun intended.
I really like that the aluminum should hold up well and looks like a really easy install. The whole thing is a lot like the old aircraft cloth that gets painted with "dope".

Anyway, let us know how the paint on screen material holds up. If it cracks and leaves holes then maybe latex paint would be the way to go - but you'd have to brush it on; I don't think it comes in spray.

the plane doping is what gave me the idea; I was watching an episode of History Detective on public TV about a civil war hot air balloon where cloth and doping were used.

I just filled the tank again. This time I got 502.1 miles on 12.894gal..I drove 125miles today,up to 70mph at times but mostly around 63 to 67.It was very windy,but warmer today. The screen is keeping the water temp to 130degrees after sitting in 40degree weather for 2 hours.

How about getting resin, one which can withstand some heat and cover up the mesh. Sort of like fiberglassing, you have your form, then you cover the form with fiberglass and then add resin.?

I know fiberglass is very light, but can it resist heat?

I myself have thought about adding aluminum underneath..mainly for a nice clean look.

I've used a few cans of spray paint, but anything else I apply is going to be with a brush. Anything that repels water will do. Fiberglass might be a little thick as I only need to fill in the tiny holes in the screen. I'm guessing the heat from the screen would create a barrier against the cold air and keeping the entire car warm for longer as the wind is not cooling the underside when parked,as much as without the screen.

Now that's a good one. :D

I was thinking that you might be full of beans with your window screen idea...but it might be a really effective material to use. Why?

* fire proof

* should smooth airflow UNDER it...but also allow some of the air to escape DOWN that flows over the engine/trans...but forces it to "seep" thru...not forming local eddies?

* relatively inexpensive to use...light weight

* not so sure duct tape will hold it very long though...maybe sheetmetal screws and the plastic washers that are used with the nails to hold felt roofing material down?

Hey there Hateful,

How are the aluminum screening and the paint on it holding up? Been on there nearly a month now.

The duck tape had started detaching from the screen in places but held well to the metal pinch weld along the sides. I've been adding more tie wraps to replace where the tape was holding the screen on. Still the tape keeps the tie wraps from tearing through the screen. No heat damage to the screen. The water temperature goes between 195 to 206 degrees, so I've been holding off on adding more paint.

You might want to consider stitching the screen with something like nylon string and a needle (fishing line?), instead of a few zipties

After a summer spent playing with a coroplast-and-duct tape air dam, I'm not using any more duct tape on aero mods. Either it comes off when you don't want it to, or it leaves a mess when you pull it off deliberately.

I still use it in semi-protected locations and it does make sense for protection under zip-ties since there, sticking doesn't matter.

Try gaffers tape. sure it's $8-15 a roll (depends on where you get it) but it lasts a long time and doesn't leave residue. 10x better than duct tape. Get it at theatre supply stores

EDIT: don't reccomend for use in permanent installation, still use fasteners for that.

I'm still going to use screws, nuts, and other such fasteners whenever possible. Unless maybe it's a temporary test situation.

I like this idea I may just give it a go on my wagon

Oh, oh, oh! I work at a rigging shop, and I have one thing better for that:

galvanized wire. At our shop we have 1/16" galvanized steel wire, and it'd be ideal for "stitching" up screening like that. From what I remember, it sells at our shop around $0.80/foot; a small mom & pop store might likely sell it for more.

Such wires are often sold at hardware stores as well (look in the section where they sell ropes and chain by the foot). The only issue would be tying off the ends -- normally we use aluminum oval sleeves.

Hrm, this is making me think that I should try out the screening on the bottom of the car here...

Safety wire could be had easily too.

It's stainless, and usually you buy it by weight. Stuff smaller than .020 usually fractures a little sooner than stuff that is .032 when twisted.

Wire sounds good. I've used stainless wire for household repairs. Galvanized is probably OK depending on how long you want to keep the car.

1/16" sounds kinda heavy to me but you'll see what you like when you're in the hardware store. Just remember, you have to bend it and cut it and thread it thru screen holes.

Forget the paint near any heat source and stay with plain aluminum screen, which is fireproof and a superb conductor of heat, good where the screen touches a hot exhaust pipe or muffler. You want it to breathe so as not to trap heat under the car.

Use several pieces of ~1/8" wall thickness 90 degree aluminum "L" angle stock as stringers under the screen to hold it taught and in place, with the stuff oriented parallel to the long axis of the car. This will act as flow fences, channeling the flow rearward and lessening turbulence. In effect, this will act the same way as the channels seen on the roof sides of NASCAR racers.

Flow fences, btw, are an excellent, simple, and cheap way to keep the airflow rearward rather than degrading into eddies or backflow. Old MiG fighters used this trick a lot.

Another possibility for areas free of heat and fire hazard would be to use dacron or mylar, fitted and stretched tight. Once so fitted, use a heat gun to shrink the dacron or mylar to an exact, smooth and tight fit. That's how they do it on modern fabric-covered aircraft wings and fuselages. Once the dacron is taught, they dope it to preserve, waterproof, and seal. Kayaks and some canoes also use this method. However, it's flammable. Being tight, it will probably be drum-like, and make a booming noise.

Just thinking about how I'd attach screen and came up with the following...

cut the screen about 1/2" oversize all round...
Unbend some wire coathangers, and wrap them around a nail every 6" or so to make loops..
Wrap the edge of the screen over the coathangers and sew it down with wire..
Use the loops in the coathanger to fasten screen on with self tapping screws or zipties.

Methinks that would be fairly durable.