List of aero mods

[MetroMPG]

[This thread will be a work in progress.]

[kickflipjr]

Good list metro. I don't think you mentioned side skirts.

[MetroMPG]

I though side skirts were more of a downforce producing mod than a drag-reducing mod. (Combined with a low front air dam they reduce the pressure under the car.)

I've only ever seen them mentioned in handlin/racing applications.

Could be wrong - anyone have any references?

[kickflipjr]

Yeah that picture above definitely has some downforce characteristics in it. I always thought they would benefit aerodynamics because they prevent air from coming in through the side of the car.

Example 2

[BluEyes]

The one in that pic is probably more for show than anything else.

One thing to note the photo - the side skirts would be better if they incorporated a rear tire air deflector. It looks like it tucks in right before the rear tire, directing the airflow right at the tire

For an aero-minded side skirt, it should not extend any lower than the lowest point of the vehicle. That way you are not increasing your projected area in a cross-wind, but you will keep the crosswind from directly hitting any exposed parts - kinda like an air dam for the side.

[Peakster]

How about headlamps that fit flush with the car's body:

geo_metro_automata_klimas_3.jpg

Vs.

IM001039.JPG

Do you think there's any real aerodynamic improvement? To tell you the truth, I like the square headlamp style better (in terms of looks).

[MetroMPG]

I'd say it's likely one of those "tiny but real" improvements. Partly because it's not a uniform bucket (unlike, say, a hole in the matrix of a vertical grille which you could block from either side for the same effect).

I'll add it to the list.

[skewbe]

Originally Posted by theclencher

...but I read that headlight buckets "pile up" dead air in front of them and the rest flows well so no aero penalty...

That was just some marketing tripe. Of course there is a penalty. Leave all your other windows closed and pop out your windshield, do you think your MPGs will notice?

[skewbe]

Originally Posted by Peakster

How about headlamps that fit flush with the car's body:
...
Do you think there's any real aerodynamic improvement? To tell you the truth, I like the square headlamp style better (in terms of looks).

your metro has the same hazy plastic headlight symptom that mine does. It looks a lot better when they are new.

[Silveredwings]

Originally Posted by skewbe

your metro has the same hazy plastic headlight symptom that mine does. It looks a lot better when they are new.

I had that symptom on my passat. I wet-sanded them with #400 emery paper, then #600, then Novus #2 (fine) polish and then Novus #1. They were crystal clear. It works on taillights that get foggy too.

edit: use masking tape to protect the paint.

[omgwtfbyobbq]

Originally Posted by skewbe

That was just some marketing tripe. Of course there is a penalty. Leave all your other windows closed and pop out your windshield, do you think your MPGs will notice?

Sure, but if covering the rear wheel wells only increases FE by a couple percent at ~50mph, how much can these do? A fraction of a percent? Very hard to discern from background noise.

[Peakster]

Originally Posted by Silveredwings

I had that symptom on my passat. I wet-sanded them with #400 emery paper, then #600, then Novus #2 (fine) polish and then Novus #1. They were crystal clear. It works on taillights that get foggy too.

edit: use masking tape to protect the paint.

Thanks for the tip! I'll try that out. I'm tired of my car looking like a bloated walleye fish.

[Silveredwings]

Originally Posted by Peakster

Thanks for the tip! I'll try that out. I'm tired of my car looking like a bloated walleye fish.

It depends on whether the oxidation of the plastic (your car's cataracts) is only on the outer surface, or is milky all the way through.

It's usually the former, and it'd be pretty hard to make it worse with this procedure.

If you do it, take before, during, and after pictures so you can make a DIY article out of it.

[skewbe]

Originally Posted by theclencher

Apples n' oranges. Re: your window tricks: all that results in air flow past un-aero surfaces resulting in high drag. The headlight bucket with almost no flow-through (just a bit around the gap around the sealed beam) is different. I don't think Popular Mechanics would simply regurgitate marketing tripe but I suppose it's possible.

I said leave the windows CLOSED, no trick except to try and describe a larger air bucket.

I suppose parachutes are really streamlined once they fill up with air too

[The Toecutter]

In some cases, having a pocket of air form can actually reduce drag. Look at pickup trucks and the effect of having the tailgait closed versus open.

I doubt this would apply to the Metro's headlamps, but the only way to know with 100% certainty is to test it in a wind tunnel. Testing it for fuel economy in the real world, you may get results that aren't statistically significant.

Since we don't all have access to wind tunnels, those here follow a general rule of thumb that won't yield an optimized design, but will give large and measurable improvement in drag if all of these modifications are taken cumulatively. Basically, this rule of thumb is to make the car as smooth as possible with as few gaps, projections, and holes as possible, and have the rear end be as close to a 6:1 fineness ratio as possible. This may not give the best design, but in cases like Basjoos' Civic, Phil Knox's T100, Darin's Metro, and other vehicles, the results are huge. Some of these aero modifications may have actually added drag, but the overwhelming trend is that they have reduced it.

From what I've read, there are two main methods to optimize a car for aerodynamic efficiency if you have access to a wind tunnel.

These are detail optimization and shape development starting with low drag shapes.

Detail optimization involves starting with a concept based entirely on style without yet having any regard to function. From there, areas such as spoilers, mirrors, grilles, windshield, bumpers are optimized piece by piece to reduce drag, reduce lift, aid cooling, or other functions. You basically start out with a bad shape and end up with something better than you started with that remains true to the basic stylistic design and is very hard, if not impossible, to distinguish from the original stylistic concept with the naked eye.

Shape development starting with low drag shapes involves starting with a basic concept shape, as opposed to the concept rendering of a car. The shapes include the Jaray body, Schlor, Kamm back, Torpedo, half-body airfoil, Reid body, and others. In contrast to starting with a fully stylized car with high drag, these basic shapes don't include any stylistic or functional details. Just basic low drag shapes. From there, these shapes are built into a car piece by piece with optimizing every last detail.

The folks at gassavers have no access to a wind tunnel. They can't custom build spoilers and parts with a garuntee that this shape will produce the lowest drag possible for their application. They are stuck with a car that's already been built and optimized according to the sub-par specs the auto companies decided was good enough to sell. So they don't do either method. Instead, they start with a crappy shape and try to turn it into a streamlined shape. Overall, it works. Even if it may not yield the lowest possible drag given that some modifications won't be optimized or may increase drag, the general trend is very clear. It's sort of like a reverse shape development, only you start with a high drag shape and move toward a low drag one. I guess we could call this the hobbyist method. It's not the best, it's not very fancy, but it works.

[BluEyes]

Great points Toecutter, wind tunnel access would be our holy grail for aero mods, but it is expensive.

Here are the pointers given in the "Wind Camp" article from the Mar 2007 issue of "Hot Rod", where they actually do take a car to a wind tunnel to better optimize it for Bonneville racing.

5 easy mods that almost always work:

-Lowering the ride
at least 20 counts of drag per inch

-Grille block (yes, in a performance mag!)
15-30 counts of drag and 50-100 counts of lift

-Front air dam (big one - from the front bumper down to the ground)
20 counts less drag, 50 less lift. more effect the taller the car is

-Seal the back of the cowl (stop airflow going under the hood at the rear)
10-20 counts of drag, 50-75 counts of lift

-Remove the outside mirrors
10-20 counts less drag.
*Except in '94-up Chevy Caprices where the Cd actually increases without the mirrors

One "count" of drag is .001 Cd, but they add up quickly


Also, on their list of "aero stuff that really doesn't matter":

-Wax
smooth wax shows no difference to a bumpy spray-can job in the wind tunnel

-Golf-ball dimples
works for small spinning spheres. Cars need other ways to delay flow seperation

-Taping seams
They tested it on a '70's Camaro and it made no difference.

-Smooth rivets and hood pins
Apparently Howard Hughes was wrong.

-Dropping the tailgate
But the wind tunnel guys did say that extended cabs and crew cabs are more aero

-Windshield rake (after a point)
Once it is past 45*, the wind tunnel guys said don't bother. The mag tested it on their Camaro (the '70's models are around 45* rake) and it made no difference.


BTW: if you want to wind test your own car, go to www.a2wt.com The price is $345/hr for the first two hours and $490/hr after that. But in probably a days testing, they were able to take their Camaro from a .497 Cd to 0.201



I'll also add one more of my own - fill the gap between the bottom of the rear bumper and the trunk well (or fuel tank if it's a RWD). My car had dirt and gravel up in there. If small rocks get trapped, air certainly will. If nothing else, it will keep that area clean and less prone to rust.

[Peakster]

Originally Posted by MetroMPG

- reduced ride height (3 inches is often quoted as optimal)

I'm seriously considering to lower my Geo. My old Fiero had a ground clearance of around 4.5 inches (with the tiny P185 70 R13 tires) and wouldn't mind if the Geo was the same.

324890 Full.JPG
VS
417096_114_full.jpg

What is the least expensive method of doing so (without throwing out alignment to the tires)?

[skewbe]

I'm just about there myself peakster. I read many ideas about it on teamswift.net and even picked up a couple cutoff wheels for my angle grinder just yesterday for this purpose.

I'm going to go with cutting down the stock coils since that will accomplish the result and several people have taken that route. I've got a spring compressor and will try and do it on the car (without taking everything apart). How many coils, and from which end? I havent sorted that out yet, sorry. I need to take a closer look at things first.

[hotroddr]

^^You should start a new thread with this question because the answer will probably take a few replys. It is possible to cut the springs in place but you have to be very careful. you will not want to try to use a spring compressor on the car with the strut on the car as this will be a MAJOR pain. As long as you keep it minimal(1-1 1/2") you can get away with it on stock struts but as soon as you get lower than that you need a firmer shock to control the suspension otherwise you will get a bouncy ride. Start with cutting one coil and see how it looks from there, then if there is not enough travel trim the bumpstop a little. I usually cut from the bottom of the spring but it depends on what the bottom of the spring looks like. If it does not taper much in order to sit flat on the perch, you can cut from that end. Post a picture in a new post of your struts/springs front and back and your questions can be better answered.

This subject needs a new thread to fully discuss so this is all I will put here.

[SVOboy]

For me, at least, using spring compressors on the car is almost impossible. I gave it about 5 seconds worth of try once and then just pulled the strut and did it seperately. I assume the geo has a similar set up to mine...

[irsa]

DO NOT CUT THE SPRINGS! It is dangerous as the spring will no longer be captive in the strut, or spring perch. Also proper lowered springs have a different spring rate so while lower, they have a similar real travel to standard springs. Cut springs, especially if cut with an oxy, tend to be weaker due to heat stress which means they will bottom out quicker. Spend money on proper lowered springs, which will also benefit in that your car will handle better so less throttle useage may result around corners.

[ma4t]

That's it. I'm taking off the luggage rack. I have never used it since I got the thing, and it makes washing and waxing more difficult.

MA4T

[BluEyes]

Originally Posted by theclencher

Baloney!

Actually, Irsa has a few good points. Cutting springs with a torch is the worst way to do it. Unless you are very good and can keep the heat to a minimum you do run a very real risk of ruining the springs temper and making it lose its springy qualities. If at all possible, use a cutoff wheel.

Personally, I would not want to cut stock springs for anything more than a mild drop. While cutting springs does increase the springrate somewhat (if you avoid heat issues) it usually does not increase the rate enough to match the lowering. The result is a very bad ride due to the suspension bottoming out on the bumpstops because the springs are not stiff enough. Besides being hard on the chassis, bottoming out the suspension can lead to a loss of controll which really sucks. I can't say that I've seen any stock springs where you would run the risk of them falling out if you cut a reasonable amount off though.

[irsa]

Yes alot of people have cut springs with no ill effects. However, I personally know people who have had serious issues with cut springs. Like every modification you have to be prepared to accept the risk. Naturally if you attempt to cut worn out springs you will have more problems then with newer springs. I stand by my statement that is not a good idea.

[slurp812]

Old school low rider guys would jack the car up, and heat the springs cherry red, then drop car, and it would be lower. How much? not sure, but it would be lower!

[BluEyes]

The springs also lose alot of their properties as, well, springs. Not only is the car lower, but the spring rate is drastically reduced because the careful heat treating that goes into making springs has been ruined. The result is typically a car that bottoms out often, leading to reduced control in emergency situations. Definately not a safe way to lower a car.

[Danronian]

For my VX would removing the mudflaps be a good thing to do?

[lunarhighway]

i noticed both an increase in FE and dirt accumulation on the car. i'm considdering makeing some small flaps to cure the later again, but big mudflaps should definately be removed if you go for FE. it's hard to tell how much it helped as i can only compare tank to tank fillups.

[Danronian]

They're not too big, but I agree, without them, the car gets dirtier a lot faster. The main reason I went with them is to keep the dirt, and air from going into my side skirt.

On my VX I installed some thicker sideskirts from a 96-00 civic, and the front of the sideskirt was not covered completely by the factory front inner fender lining. I was afraid air might flow into that opening, making some type of whistling noise, and I was afraid snow and dirt would get packed into the hole. The mudflaps seemed to cure all of these problems, but I was wondering if they drastically affected the flow of the air along the bottom of the car or not.

[wireless0ne]

Originally Posted by skewbe

I'm just about there myself peakster. I read many ideas about it on teamswift.net and even picked up a couple cutoff wheels for my angle grinder just yesterday for this purpose.

I'm going to go with cutting down the stock coils since that will accomplish the result and several people have taken that route. I've got a spring compressor and will try and do it on the car (without taking everything apart). How many coils, and from which end? I havent sorted that out yet, sorry. I need to take a closer look at things first.

there is a bouncing problem with cutting factory springs, after market springs increase their rate pressure under load, many coils at the bottom few at top, factory springs dont, 1 link cut is approximately 1" drop, the more links you cut the more dangerous the bounce.