Do wheel skirts really work?

[theclencher]

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[cfg83]

theclencher -

Originally Posted by theclencher

I think Bill is on the money.

As for why aren't skirts in general use: they are extra cost and weight, they interfere with checking tire pressure, they interfere with tire changing and other wheel service, and last but not least, some of the lowest drag cars in existence don't have 'em!!! So they must not be that critical.

And maybe they're not in fashion anymore? While I have nothing against them, I have always identified them aesthetically with great big Detroit-dinosaur cars.

CarloSW2

[omgwtfbyobbq]

Originally Posted by theclencher

And, the aero benefits are dubious.

I don't think there's much of a question about the benefits. It's just that they're small, and generally not considered worth alienating a buyer. It's kinda like a full boat tail, sure, it may increase efficiency substantially, but the drop in relative appearance, utility, and maneuverability in tight spaces isn't considered worthwhile.

[zpiloto]

Originally Posted by theclencher

Ford Prodigy: .20 Cd (With big wheel wells and open-spoked wheels too! And look at all that ground clearance!)

Very good points. Do you have anymore pic of the Prodigy? I find it hard to believe that it has a .20 Cd. Looks sure are decieving The front end looks like a MAC truck and it sits high enough to go 4 wheeling. Although it looks like they removed the mirrors.

Edit:
This car was made in 1999 and got 70 MPG. Why the heck is it not on the road. Looking at it you would think that it was just another sedan. Ford blew it on this one. Same space as the Taurus but 2400 pounds.

[MetroMPG]

Originally Posted by theclencher

Intuitively skirts make a lot of sense for lowering Cd. Reality seems a bit different though. I did a skirt/grille block/mirror fold/wiper remove college paper on aero and failed to find A-B differences in coast down tests.

Which raises the question: what's the resolution of coast down tests? Would it be able to detect, for example, mirror removal?

I haven't tried any, but it would be interesting to see which had a smaller margin of error: a steady-state SG test, or a coastdown test.

I guess the answer depends on how well the test is designed, the experimenter, and the measuring equipment used for the coastdown.

[Bill in Houston]

Originally Posted by MetroMPG

Which raises the question: what's the resolution of coast down tests? Would it be able to detect, for example, mirror removal?

I haven't tried any, but it would be interesting to see which had a smaller margin of error: a steady-state SG test, or a coastdown test.

I guess the answer depends on how well the test is designed, the experimenter, and the measuring equipment used for the coastdown.

I had an idea on coast-down tests. Someone mentioned difficulty in getting consistent, accurate, believable readings. And I can see how that would be rough, even with using a stopwatch with a multi-lap function. So my idea was that to time the coast-down, you would aim a video camera at your speedo, and then load the video into your computer and use the time stamps to measure your time intervals. A camera on a tripod in a vehicle takes surprisingly good video. Anyway, using the video would help remove a lot of your measurement error, it seems.

As far as whether skirts work, they definitely must work, or Honda would have left them off the Insight. The front fender well of the Insight is also optimized in a way that none of the other photos posted showed. Something to do with how the back edge is shaped. The Insight guys were willing to do a few things that the Prodigy, Loremo, ESX guys were not. In my opinion, anyway.

[Peakster]

Originally Posted by Bill in Houston

As far as whether skirts work, they definitely must work, or Honda would have left them off the Insight... The Insight guys were willing to do a few things that the Prodigy, Loremo, ESX guys were not.

I agree. Next time you're driving on the highway when it's raining, take a look at the wheel wells of the cars driving beside you. There's lots of misty-water being spewed out of them. If you take a close look at the Loremo, there's virtually no wheel well space at all, so the wheel and tire in itself creates a sort of wheel skirt. I would think the larger the wheel arch, the more benefit to Cd there would be with wheel skirt application.

Also, check out the dirt accumulation from the front wheel arch on my car:
Attachment 322
A pretty good indication that a low pressure area is being created.

[trebuchet03]

Okay, I've been searching in academic journal databases for information on wheel fairings, skirts, fairings - whatever you want to call them.... Because of that, I can't link full text (if available) - so, for the most part, you're going on weather or not you trust that I'm representing the cited articles....

Journal of Sound & Vibration; Jun2006, Vol. 293 Issue 3-5, p910-920, 11p
"Noise control design of railway vehicles?Impact of new legislation."
One thing tested were train bogie skirts. The result was less transmitted noise and better aerodynamics. Unfortunately, full text is not available at the time - so I can't give a quantitative value. These were conventional rail trains which, while not high speed, I'd expect these vehicles to travel faster than our vehicles

The same journal - different article published a few years earlier
Journal of Sound & Vibration; Oct2003, Vol. 267 Issue 3, p709, 11p
"Skirts and barriers for reduction of wayside noise from railway vehicles?an experimental investigation with application to the BR185 locomotive."
Again, no full text But showed a 2-3 decibel drop in noise. Not exactly related, but it shows a bit of validity to the first citation above.

Journal of Fluids & Structures; Jan2007, Vol. 23 Issue 1, p85-100, 16p
"Multi-resolution analysis of the large-scale coherent structure in a turbulent separation bubble affected by an unsteady wake."
Unfortunately, not full text (just my luck :P) They used a spoked wheel to generate a wake. Also, "The unsteady wakes decayed faster in the system with CCW rotation than in that with CW rotation." But the abstract doesn't say which direction the flow was coming from...

This one is more of an engineering magazine than an academic journal....
Professional Engineering; 5/11/2005, Vol. 18 Issue 9, p42-42, 1/2p
Queenslanders' bright idea for commuters.
I can't justify anything here - other than their claim that their wheel fairings helped. green Car Congress had an article about them and their solar-electric hybrid

It's interesting that I can't find anything that directly tests wheel fairings on cars... I'm sure the automotive industry has, but I doubt they will release it to public domain :P Here's a great one on cycling as it also makes a comparison to engine load/strain (you know, lactic acid build up). This is an academic journal - even though the title doesn't seem like it would be.

Sports Medicine; May2001, Vol. 31 Issue 7, p559-569, 11p, 6 charts, 1bw
Improving Cycling Performance: How Should We Spend Our Time and Money.
With Faired aero wheels, a novice cyclist sees about a 82 second advantage in a 40Km Time trial. A trained cyclist is 67 seconds and an elite cyclist is 60 seconds. Then comparing weight to aerodynamics. Using a non aero wheel that was 500g lighter, the aero wheel was better on a 3% grade. However, for the 6% grade test, the aero wheel was NOT better for the novice and trained cyclist cyclist, but it did give the elite cyclist an advantage. Finally, on a 12% grade, the lighter non aero wheels were beneficial for everyone. Keep in mind, 500grams on a performance road bike is a BIG deal :P The article has some rather interesting wind tunnel pictures too

Sports Engineering; Nov99, Vol. 2 Issue 4, 6 diagrams, 1 graph
Formula 1 car wheel aerodynamics.
This is where it gets interesting - an F1 constraint is open wheels. So this is how they adjust for that.

Here's what is said:
"Experimental drag measurements were carried out on a 40% scale rig representing the front right-hand quarter of a generic Formula 1 car, with features such as the front wing and car body modeled accurately to generate a suitable flowfield around the wheel. "

"The wake of a Formula 1 car wheel was found to
consist of a significant region of separated ?ow.
This region is formed by ?ow separating from the
crown and sidewalls of the wheel, which recirculates
into the convergent region of the lower
downstream portion of the wheel. The shape of
the wake is influenced by horseshoe vortices shed
from the wheel, and the overall wake symmetry is
affected by aerodynamic features of the car as a
whole."

"The aerodynamic drag of the wheel is signifcantly
affected by the symmetry characteristics of the wheel
wake. A more symmetrical wake appears to give a
reduced drag. This symmetry is strongly dependent
on the vortical ?ow shed from the front wing."

So if you can get figure out how to get the flow to move around the wheel -- you're in a good position.

-----
I'll keep searching for testing that was done on this exact situation but as of now, I would say that the claim to a benefit is plausible. Especially if you do tuft testing and see all those little threads get sucked into the wheel well.

[landspeed]

I suppose the frontal area also counts, because the Cd is the drag for the shape, while the CdA is the actual drag for that size car .

So, the insight might end up with better aero in the end due to smaller frontal area than the other cars.

Another thing not really considered is the effect of wind speed. I know that, on aeroplanes, the faster you go, the flow seperates earlier from the aerofoil. So, maybe this also happens on cars?. It might be that, at 25mph, my car has good aero from the rear hatch, while at 50mph, the Cd increases (in addition to the actual drag increasing), as the air is flowing too fast so it seperates at the roofline.

[Bill in Houston]

Originally Posted by theclencher

Insight skirts: maybe Honda was making a statement with them?

Insight guys doing things others didn't: yes, but the Cd numbers say the others still beat them! I wonder how accurate the Cd ratings are? I know there is tunnel-to-tunnel variation but I don't know how much.

Statement? Some element of it, sure. As radical as the Insight is in every other way, there was not much of a marketing risk in adding them, and adding them probably helped the car get on more magazine covers.

I think that there is a lot of variability in Cd measurement. Tunnel to tunnel, and manufacturer to manufacturer. For instance, was it measured with the car empty/dry? Or was the car loaded to GMWV? If loaded to GMWV, did they try shifting the ballast around to get the f/r distribution that gave the min Cd, or was it in a location that made sense (i.e. sitting in the seats). What were the tires inflated to? Recommended inflation, or sitting on four flats? Or front two flat, rears normal? Moving floor wind tunnel, or stationary?

Anyway, I am just agreeing with you in a very longwinded way.