rear wheel skirts...
 

for the people who have done them, how much of a gain is there? I'm pondering making some for my tercel...

I have no fe instrumentation... only calc fe tank-to-tank... had skirts and other aeromods on too for quite a while. Couldn't detect change that could be attributed to aeromods.

Not saying it's impossible. Saying results are so small- if any- you'd have a hard time noticing it. And an even harder time proving it.

Do it if it makes you feel good, or are curious. Just don't expect any miracles.

They looks like they're good for around a 3% increase.

Testing grille blocking & wheel skirts: +5.7% improvement - MetroMPG.com
http://ecomodder.com/forum/showthrea...-camry-84.html

Hi,

On my xA, the front grill blocks and fog light covers added ~10-12%, and the smooth wheel covers added ~6%, and yes the rear wheel skirts are ~2 to maybe 3%.

so it looks like its minor...i'll worry about doing a partial under tray first...that should make a bigger difference

i also need to smooth out the rear from the rear axle beam to the bumper

It depends (TM)...

If the rear wheels are right at the back corner (less bodywork aft for detached flow to reattach on), I think skirts will have a bigger impact.

Also depends on the size of the vehicle's tire/wheel arch gap, the rim design (unless you've got smoothies on), wheel offset...

I agree with Frank: for a normal car, you likely won't be able to see the results over the usual tank-to-tank noise.

Also see:

http://ecomodder.com/forum/showthrea...-camry-84.html

I have skirts on my Echo and Scangauge instrumentation. I made before and after, two-way tests on a windless day. It looks to me from the scangauge data that the improvement was about 1.4%, near as I could tell. Might be worth more on a Metro, as someone above mentioned. I wouldn't be able to see that small change on a tank-to-tank basis.

As for the undertray it seems as though the amount of benefit is dependent upon how high your car is and how rough the underbody is. If your car sits low to the ground already then the undertray may not be that effective. However, the higher it sits and the rougher the underside then it may have more of an effect. That's what I gleaned from reading a VERY long thread on the subject. If I misinterpreted it then by all means let me (and everyone else) know.

PS When doing your aeromods try to do the ABA testing so you have before and after mod baselines for comparisons. :thumbup:Thanks.

One good thing about the under tray vs other mods, even if it's not that great of an aero-improvement, it helps to keep your engine bay clean, which is nice for when you're working in there.

Also, a clean engine could be running more efficiently than a dirty one. The grease and grime kind of holds the heat into the engine, in the wrong places, IMO. I prefer to keep it clean. A bottle of simple green and some water will get you a long way in making the underhood area a little more tolerable to mess with.

Not sure I'd spray Simple Green in the engine bay. It softens some polymers. RC vehicle guys soak tires in SG to make them softer and gripper.

I don't really see an issue... you're not just randomly squirting SG all over the place, you're concentrating a 20:1 (or weaker, it's pretty powerful stuff) mixture at the areas that actually NEED to be cleaned... you can clean soot, grease and oil off the hood/sheet metal surfaces/painted surfaces with as little as 100:1 mixtures of SG/Water. It's not like anything is "soaking" in it for any length of time... you're rinsing it right back off in most places.

There is an SG for automotive purposes, if you're truly scared to use the real stuff... although I'm pretty sure it's the same thing with a different label.

Anyway, a simple google search will yield 100 or so dead-on results showing proper method, tips/tricks, and other parts of cars that have been successfully cleaned with SG, some people have used it for years and years (myself included, since I got my first car, almost 10 years ago) without any problems at all.

heres some pics of my car, what do you think i should do to it to help it out without going extreme...i'm debating in my head about a mirror delete

http://c4.ac-images.myspacecdn.com/i...f3795f7d5b.jpg
http://c1.ac-images.myspacecdn.com/i...d2fed9fd88.jpg

the car has 185/60-14s now...i plan to move to 175/65-14 LRR tires for daily use after these tires wear out, the car is going to be lowered a tiny bit more in the front, the rear is coming down around 1.5...i don't really want to get rid of the rear spoiler, its got an LED brakelight & i like the way it looks

also getting a new O2 sensor today along with a new fuel filter (original one is still on there) i keep throwing a code for the sensor, the car runs rich, i get tons of soot build-up on the back of the car, so i'm hoping this & disconnecting the battery should solve that

Isn't losing the spoiler kind of an apparent first step?

Hi,

A flush upper grill block would be at the top of my list, and tape over the holes in the wheels (or are they hub caps?). Seal up the front hood gap with foam gasket, and try to move the antennae to the inside of the car.

i already have the grilles blocked

i'm pondering the antenna move...not sure yet

taping the wheels...eh, no no no

but i need to seal off some gaps tho

thanks for the suggestions

malibuguy -

For the wheel skirts on my SW2, I saw between a 1.3% to 4% gain :

http://ecomodder.com/forum/showthrea...-pan-4472.html
http://img507.imageshack.us/img507/3...sidevo2.th.jpg

Because I am conservative in my estimates, I only claim a 1.3+% gain.

I have a big advantage because most of my connecting parts are plastic. I can drill into pieces of my car very easily, and I don't have to worry about rust. My personal rule is to have "reversible" mods, so the holes I drill are not where you can easily see them.

I have been thinking that my mod is probably better as "infrastructure". If you look at the picture above, you could cut out the center and use the surrounding "frame" as a base to attach varying skirt designs :

http://ecomodder.com/forum/attachmen...1&d=1235672868
(the dots would be fastener locations)

The cool part is that the skirts could be attached with "quick release" connectors like dzus fasteners. You would have full access to the tire and you could more easily try different wheel skirt designs.

https://www.pegasusautoracing.com/pr....asp?RecID=758
http://ecomodder.com/forum/attachmen...1&d=1235673272

CarloSW2

.

nice writeup, i def want easy access to the wheel, & be able to easy remove them for servicing at the track

On that car, not necessarily. Just guessing (I know that's dangerous when it comes to aero), but I don't think that car has reattached flow on the end of the decklid. It's too short, and the rear glass is too steep. So effectively raising the decklid as that "spoiler" does, may be helping a small amount.


Since the "extreme" qualification is subjective, and only you know the answer to what's "going extreme", I'd just suggest going through the aero mods section in the 65+ efficiency mods list and pick out the projects that meet your criteria.

i'd say lose the racing stickers. they're probably eating 2 - 3 mpg. ;^)

Agreed, "eyeball aero" is not good science. Buuuut...

I think dragging a spoiler through turbulent air is still dragging something that otherwise isn't there.

I intuitively agree with Daox' assessment (# 25 of this: http://ecomodder.com/forum/showthrea...rag-598-2.html ) :

"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."

I recall reading somewhere that Fieros with factory spoilers gained a few counts of drag, and spoilered Countaches lost a noteable amount of top end speed.

And from 60+ Vehicle modifications for better fuel economy - EcoModder.com :

Raised wing type rear spoilers
Sure, the dealer or that go-fast kid on the corner might've convinced you that spoilers are good for downforce, but in reality, most are merely a cosmetic addition - one that is more likely to add drag than it is to do anything meaningful for handling.

Take it off and your car will not only look stock, but it'll have better aerodynamics.

It isn't always obvious whether a particular spoiler style is an aero help or hindrance.

I've tried to cover as much as possible from the rough parts of the underside with a tray on my car. Less for mileage improvements, although they are, but for a better handling at speed.

Original: small undertray before engine, entire lower side of the engine bay open to air, floor mostly flat from behind the front axle subframe to rear axle, save for the exhaust tunnel.

Afterwards: full undertray from a Seat Leon 1.9TDI, from front bumper lip to behind front axle, two Coroplast additions to seal entirely the floor before front wheels, aero shaped deflectors in front of the wheels patterned on those from a SLR McLaren, rear undertray from rear axle to the middle of the spare wheel well, two small undertrays behind rear wheels to connect the bumper lip to spare wheel well, two boat-tail fins behind rear wheels.

Patterned everything on factory undertrays from economic version of Skoda Octavia - green in the picture. Did not use the undertrays in the middle of the car, between axles, as the floor is mostly flat there.

Most aero mods have minor effects, but they can be felt. First, fuel mileage improved visibly when running long distances at steady speeds. Then, wind noise diminished. Third, handling was slightly better, car is more steady in a turn or at speed.

Autospeed report reduced stability with the McLaren-style wheel spat.

Better overall, or specifically with the wheel spats?

May we see this?

1. There is some small amount of front end lift. Moving non-essential parts from front to rear also lightened the front and contributed to lift.

But, as Dick Shine proved with his track Golf Mk 4, some front lift balances better the nose-heavy chassis of the Golf Mk 4 / Seat Leon Mk 1 / Skoda Octavia Mk 1 / Audi TT Mk 1. In fact, Shine Golf Mk 4 is raised by the front a ridiculous amount, maybe 2 inches taller than rear. Something like this can't be done just with aero mods.

2. The more aerodynamic a car is, the more vulnerable to side winds at speed it will be. Aero vortices around "brake" the car in a straight line, but they also "brake" the side to side movement in the wind. Track cars have miserable drag, Cx over 1.0, they are designed for downforce, not straight line runs.

In a road car with just undertrays, spats before wheels, sealed body gaps and sealed non-functional holes in the front, instability in side winds is too small to matter. It takes a bodywork like a Tatra T77 to become dangerous.

You said track days ... what form? Makes a difference on what is good V bad. Like everyone has stated test test then test some more. Tuft tests to determine if it is a spoiler or a factory aero mod. If your into drag racing then converting to a Bonneville spoiler could be an option for down force and aero+. I saw many last time to the drags in Utah.
If road corse then any down force at any cost may be worth the inproved times look at time attack cars they use this principle.
Aerodynamic visualisation fluid( clay disolved in water) will also show the points were the air separation is happening and then were reattachment occurs... like how after a snow storm your car is covered in road spray that almost always has the same patterns .
Aba on a set corce preferably 3gal or more to make the filling discrepancies (.001-.01) a smaller % value (makes for less noise in your results). If posable data log through a Obd2 dongle device. Useful at the track too.

stability
 

The 1992-98 McLaren F1 had intentional 'lift' designed in.It was only to be driven by veteran race drivers and had straight-line speed advantage.
A long-tail version of the F1 was also raced and it produced downforce as a concession to speed.
The Kamm-designed 1953 Cunningham C-5RK couldn't be matched for speed at the 1953 24-Hours of LeMans.It led the entire field on the first lap but it didn't necessarily handle the best through the curves and didn't finish the race.
Stability is a quirky subject,with lots of conditions and caveats.
Today,all super-high-performance cars are 'computer-controlled' with active aerodynamics engineered in to balance speed vs stability.

skirts
 

There's probably some data in the seminars.
The 1938 Adler-Jaray was tested for skirts:
* no mods = Cd 0.38
* front skirts only = 0.37
*rear skirts only = 0.34
*front and rear = 0.33
-------------------------------------------------------------------------
At MIRA,the Audi A2 was tested with rear skirts.showed a delta-0.001 drag reduction ( on a car with normal Cd 0.288)
--------------------------------------------------------------------------
Hucho says that the better the flow leading up to the rear skirts,the more effective they become.No truly-low-drag cars are without them.
Wheelhouse volume is going to affect them.
Underfloor condition will affect them.
Presence of 'taco' fairings will affect them.
Ditto actual wheel fairings.
Or combination of any and all.
Very conditional !:)

Stability has also been improved by fitting Yamaha Body Dampers front and rear. They take away the vibration which escapes filtering by suspension.

None of my references support that assertion. In fact, the car had measurable downforce - 136kg at 240 km/h.

I do wish people wouldn't post stuff that isn't true.

isn't
 

Take it up with McLAREN.

Happy to quote my references. What are yours?

But honestly, to believe that the McLaren F1 had 'intentional lift designed into it' is absolutely absurd. As soon as I saw that had been said, I knew it was wrong - and it took less than 5 minutes in my library to prove it.

reference
 

My aero library is about 60- linear feet, so it may take awhile. I distinctly recall that the F-1 porpoised, and was a hand full to drive. It was reported that only the 'long-tail' had any significant downforce, which would be handy on the Mulsanne Straight at Le Mans, especially in a rain event.

UPDATE

As of July 2020, the underbelly of the car had been modded as such:

1. Plastic undertray from front bumper lip to just after front axle subframe, at the front end of the catalytic converter. It can't be flat, due to engine oil sump and gearbox, what matters is that it's quite smooth, no bits and bobs dangling in the airflow;

2. Aluminum sheet rear edge on the undertray, where it approaches the hot exhaust and catalytic converter;

3. Full undertray from rear bumper forwards to the well of the rear axle;

4. Flat plating as tightly as possible around axle well, leaving the exact space for the axle to move up and down;

5. Aerodynamic exhaust muffler (part of the Milltek exhaust. Milltek muffler is oval in section and with a smooth surface, unlike suitcase-like and ribbed factory muffler);

6. Aluminum sheet flat plates around the exhaust bend, below rear seat. Around the pipe, not over it. Exhaust needs to radiate heat away. There is half-inch of free play between plates and the pipe;

7. Aero (boat prow like) deflectors before front wheels, as the above said SLR McLaren "boat prows";

8. Boat tails behind rear wheels;

9. Front control arms plated over, to achieve smoother lower surface. Golf Mk4 / Leon Mk 1 sheet steel control arms are pretty good for this, since they are smooth from factory. Audi TT Mk1 / Golf R32 cast steel control arms are not smooth, and they are also heavier, with bigger and softer bushings, and they shouldn't be used;

10. Factory flat plates over rear control arms have to be left in place. They are officially called "stone guards", but they are too flexible to claim to protect anything from stones. They had been designed to flatten the surface of the rear axle around the fuel tank and keep the airflow straight.

Materials

All trays have been made from 2mm Coroplast, only the parts near exhaust from 0.8mm aluminum. They were assembled by screws or small bolts wherever possible, glued with strong Mamut Glue where there was no way to screw. Glue hardens itself like RTV silicone, holds even better than small sheetmetal screws and dampens vibration better than silicone. Obviously it has to be used on those parts which are hardly probable to ever be removed for mechanical repairs, throughout the car's life.

Exhaust tunnel has to be left alone. Milltek runs much cooler than factory exhaust, but still it needs air to cool itself. Factory muffler surface may heat itself to 140-145ºC (284-293ºF), as measured with a multimeter and a probe. Milltek may be touched unpleasantly by hand after a highway run, may burn hand after city run, but it's nowhere near factory.

Ground effect

Car is pretty low to the ground, minimum ground clearance is in the 80 mm (3.15 inches) range, in front, higher in the rear, with deep side sills. This means, if the highway is very smooth, there is some stability gain once the entire belly had been smoothened as quoted above. Since the highways are often not too smooth, this is not very important.

Cooling

Engine coolant temps in the 85-88ºC (185-190ºF) highway in cool weather, 90-93ºC (194-199ºF) hot weather, 93-96ºC (199-205ºF) city driving, occasional jumps in the ranges where fans go into full speed (over 102ºC/216ºF) in stop and go traffic. Intake air temps as read at the manifold 13-15ºC over atmospheric temps.

Comparisons

As Audi had found out by experiment on an A4, smoothening the underbelly as much as possible may give a delta Cd around 0.024. I've left alone the effect from flat plates front to rear when adding up, on the sides of the exhaust tunnel, since the floor had been already smooth there from factory. Also, their A4 car had already small plates before front wheels. Leon didn't have them from factory, when a Cd of 0.32 had been calculated and published. SAE paper 2004-01-1307 says no deflector before front wheels contributes 13.1% to total car drag, this is why all post-2004 cars have them. Also, there was no quote on how much delta Cd we can gain by closing all useless bumper and body gaps. By useless I mean "which obviously do not feed into various radiators of the car".

This means I might have been lucky enough to achieve a delta Cd of more than 0.025, and therefore cross the barrier of "very aerodynamic cars" set at Cd 0.30.

Road testing

Stability at highway speeds has proven itself pretty good until now. By moving all unnecessary parts rearwards, the front/rear weight ratio has improved by 55kg (121.4lbs), which contributes to superb turning ability, besides suspension and tracking mods.

Milltek exhaust produces less vibration and noise than factory exhaust. Engine airflow measured by mechanical pressure gauge and Torque 7% better than before, which should give a peak hp over 250. Not bad for a factory turbo.

Fuel economy may be in the 24-28 mpg in fast driving, mixed city-highway with moderate traffic, 29-30 mpg in highway driving at constant speed.

(If doing everything possible to hypermile, it may be 36-37 mpg, but this defeats any reason to buy a turbocharged gasoline engine in the first place.)

PS Mr. Julian Edgar gave me the idea to make the aero mods, when I started to test the aerodynamics back in 2011. Nowadays, compared to 2011, I'm 20hp higher, 16lb/ft higher, but also on the second fuel pump, second alternator and third clutch in the car's life :)

On topic: how does all those posted above have anything to do with skirts?

Three problems when dealing with skirts:

1. Are they effective at all?
2. How to fit them properly to stay still at 60 mph?
3. How to remove them easily when wheel needs to be changed?

Answers:

1. Yes :)

2. Use the strength and flexibility of rubber polymer glues.

That is: pick some 2 to 3 mm thick aluminum strips. Bend the strip with pliers into shape. Drill and make a thread into one of the ends of the strip.

Glue the non-drilled end with polymer rubber glue (Mamut from Den Braven, in Continental Europe) to the inner lip of the metal fender. Not into the plastic fender liner. If the wheel well was a clock face, use strips at 12 o'clock, 10, 2, 9 and 3.

Bolt the plastic fender skirt to the drilled end of the strips.

From that point on, a fender skirt has little vibration (polymer rubber glues are about the consistency of tyre rubber when fully cured) and rather good resistance to shock. Which is much better than drilling into the metal fender and/or fit some weird contraptions as struts and springs.

Made me look. There was a feud spilling through various threads during 2020-05. The antagonists still post, just not in the same thread.
Not familiar with [polymer rubber glue (Mamut from Den Braven, in Continental Europe)]. What I like currently is foaming Gorilla Glue. It's a good gap filler and you can remove it with rubbing alcohol, making it a temporary fastener.

effective, stay still, remove
 

I have only one source with a dedicated skirt investigation which dates to 1936. An Adler-Jaray was improved from Cd 0.38, to Cd 0.33 with the addition of skirts on all four wheel openings.
The front skirts were responsible for most of the benefit, 0.04.
Hucho mentioned in the past that, rear skirt performance could be compromised if the flow coming from the front of the car wasn't already cleaned up just as with a diffuser.
----------------------------------------------------------------------------------
Keep an eye on the skirt panel at speed. Flat stock, used as a reinforcement could deform under aerodynamic forces. Some have bonded carbon-fiber arrow shafts to the inside surface of skirts to mitigate aeroelastic deformation.
------------------------------------------------------------------------------------
The first-gen Honda Insight uses registration pins to engage slots for the top of their rear skirts, and a type of 1/4-turn Dzus fastener to secure the bottoms against the body. Very easy on and off. If you cant find Dzus fasteners locally, they're available through racing catalogs. I got mine from SUMMIT Racing.

Mamut High Tack Glue:

https://fixa3.pt/2523-large_default/mamut-glue.jpg

Patented by Den Braven, rebranded as Bostik after Bostik took over the original company.

It's not a temporary fastener, after the first 3 hours in summer temps it holds pretty well. Almost fully cured after 1 day. Has the consistency of tyre rubber, does barely flex by hand, unlike RTV silicone. Almost impervious to weather or solvents, it can only be removed by cutting with Stanley knife blade.

Most likely in the US or other continents than Europe there are other glues with similar ability.

The logic of polymer rubber glues: bolt holes should not be drilled at all, if possible, in places where aero plating is done. Underfloor, wheel arches, axle subframes, under the rear well for the spare wheel. Rust which starts there devours the steel in a few months. So we bond ("weld by glue") a frame of drilled aluminum or hard plastic bars and bolt the sheet to them. Sheets of ABS or aluminum are usually of negligible weight, but glues hold as well as 20-22kgs pe square cm (which translates to >300lbs/sq inch), so they wont break and fall, regardless of speed.