.

theclencher -

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

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.

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:eek: 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.

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.

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.

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.

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.

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.

This happens on anything with flow moving around it - the transition point can be found with the Reynolds number equation set to the transition Reynolds number - then solve for the characteristic length. Everything after that point can be considered a turbulent zone and the boundary layer starts getting bigger :(

No so easy on complex shapes like cars -- so experimentally, you can use tuft testing or smoke testing :)

I'm considering closing up the front wheel arches on my Insight, something like the ice in the first picture.

Attachment 328

After looking at the other two pictures, it looks like my wheel skirts add drag to the car

Attachment 329
Attachment 330

I like Houston Bill's idea of using a video camera to record an analog gauge for coastdown testing. It would permit you to remove most of the human variable of coordinating the stopwatch with a moving target (needle). Tougher is ensuring you're always starting the coastdown from the same speed. This could be accomplished with cruise control & pressing "cancel" at a predetermined point though.

Yep: to me it says that aside from a fully tapered boat tail, there's no magic bullet in aerodynamics. Successful aero is the sum effect of a lot of small, but effective, individual elements.

Can we conclude that from water/debris flow patterns?

When I look at the bottom pic, I think we're seeing the effect of water getting channeled upwards and backwards in the skirt's forward seam, and reaching some point where some factor (volume of water?) causes it to spill out of the channel and continue being swept rearwards.

They aren't that important compared to under/cooling/rear end flow, but it's still something imo. The trade off of a count or two of drag just isn't worth alienating buyers in the eyes of anyone who designs cars. As for being able to tell from coast down tests, from my back of the envelope *guestimations, a ~mph change in relative speed at ~50mph will result in about the same impact on a coast down test than a reduction of drag from .3 to .29 will. Which implies to me that small items like mirrors, windows up, wheel skirts, etc... will be extremely hard if not impossible to test using a coast down. There's going to some speedometer error, timing error, a likely difference in wind speed. The coast down test would need to be from such a high speed, that it would be needlessly dangerous, if not impossible.

*Going from ~49mph to ~47mph at standard everything is analogous to going from ~22m/s to ~21m/s. In a car with A=~2m^2, and Cd=.3, a drop of 1m/s results in 378N compared to 415N. A reduction in drag of a count, so Cd=.29, results in ~403N of force, which means that a 2mph speed error results in 2.5 the difference that a 1 count drop in drag does. Or that a 1mph speed error at ~50mph will have the same impact of wheels skirts, or a mirror delete, or rolled up windows. For the DIY'er, it's pretty much impossible to work around that level of noise.

The only consistent coast down test I could ever get to work right was to coast down a hill where the car reached terminal velocity. When I experimented with the aero mods I did to my car they all increased my terminal velocity and I could repeat it as many times as I wanted going back and forth with the mods. I could start at the top of the hill within a minimum speed and at the same point on the road I was always at the same speed. I could start at the top of the hill faster than the terminal velocity and the car would still slow down to that speed.

The wheel skirts picked up about 1-2mph on my terminal velocity. Around the same amount as the passenger mirror if I remember it right. So they don't make a huge difference but they do help some. Swapping front bumpers made a much bigger difference, almost 5mph. Rear swift wing on the hatch gave me just a tiny improvement. The needle would sit on the other side of of the mark so maybe .5mph

With all my aero mods I had a hard time reaching terminal velocity at the same point, the car was still accelerating and I was not able to get more speed at the top really due to having to climb the other side of the hill. I need to add nitrous or something to get more speed at the top of the hill :eek: :cool:

The other big hill I had to go over I could coast down and would always hit 55mph at the same spot on the road. I could change something and the car would vary from that spot where it hit 55mph and I could tell if it was better or worse. But that method was nowhere near as consistent as the first hill where I had a long downhill run.

Yeeeesssssss. Lemme see. When I'm coasting down, I notice a significant difference depending on passengers, temp, etc... So it seems doable. We'll have something (All in metric or we crash into Mars) like Weight(Crr)+.5(ro)(Speed^2)(CdA)-Weight(gravity)sin(theta)=Force, where theta is the angle corresponding to the grade. A 10.5% grade is a 6 degree angle, so that means the potential energy component of a 14,500N car is about -15,000N. Assuming Crr=.015, the rolling friction coefficient is ~220N, so, for a car with CdA=.7m^2, the fluid friction coefficient must equal ~12,800N. Plug'n'chug, and we get the Speed is ~122m/s. Way fast. But not surprising considering the grade. Since we're sane, we leave it in gear, and end up going much slower since the spinning engine/trans provides drag. Lets say in gear we end up going 38m/s (85mph). This means we have ~1,240N from air drag slowing us down, and the other ~11,560N comes from the engine/trans spinning at whatever rpm. Lets say we drop the CdA from .7, to .68. Now we *still expend (not exactly) ~1,240N for fluid friction, but our speed increases in order to do this. With the .68 CdA we're going ~38.6m/s, which is a ~1.3mph increase in terminal velocity. Granted, there's still the tires slipping a bit, and the *engine/trans drag probably isn't linear, but it's still a nice result imo. I bet a much nicer grade (~2-3%?) with the car in N would yield similar behavior.

So, my BS seems to match up with Coyote X's experience. As long as we find a smooth enough, or steep enough highway hill, it looks like those of us w/o scanguages can figure out what improves CdA the most. Lothar approves! :D

I should add, that imle it's way easier to notice what the top speed on a decent is because highways tend to have pretty even grades, and the top speed will be held for at least a few seconds. A coast down test otoh, requires the driver to start coasting and a specific point and specific speed. Then accurately give the speed the instant they get to the other location. Or, make note of the location where they see the speed. Either way, there's more room for human error imo. Whereas with terminal velocity, we don't do anything except keep on eye on the speedo for a top speed, which should be present for at least a few seconds. :thumbup:

10% is a bit steep. The hill I was using I hit ~75mph in N with the windows up. On a side note I am now driving a state car to work, 03 or so Taurus and it is more than happy to go past 90 on the same hill :)

Terminal velocity vs. Coast down
 

I think that we are talking about two seperate flavors of test:
1- Coast-down: The car is accelerated up to, say, 62 mph. Car is placed in neutral. Time intervals are recorded as the car goes through 60 mph, 55 mph, 50 mph, 45 mph. The times are put into a calculator and a combined Cd and drivetrain/rolling drag are spit out.
2- Terminal velocity: The car is driven down a hill with a consistent grade, and the terminal velocity is measured with the car in neutral. Since the effect is small, repeated trials and strong downhill acceleration (before putting the car in neutral) may be required.

I have not done the math on the coast-down test to see if it has the possibility for better resolution, but I know that one problem with the terminal velocity test is that a 3% reduction in Cd will only change the Vt by 1%, which could be pretty hard to measure, as byobbq was saying. Also, it's just plain hard to know what the Vt is, since you can be within an mph or two of it, and feel like you are topped out. Best to approach from above and below to try to converge on it.

As you guys know, both of these tests are going to be sensitive to temperature and crosswinds, as well as changes in rolling resistance.

Lots of fun.

I think that you would start from above, and time as you coast down through the speed range. I think...

Originally Posted by MetroMPG
I think I tend to agree. Maybe I should tape the seams and drive through a few puddles:)

This one throws an interesting twist in the aesthetics vs. style debate:

https://www.motorsportcollector.com/M...san/24p1L.jpeg

Nissan R89C with rear wheel skirts

I doubt these are on for styling reasons. That said, if they were useful, why weren't they adopted by every race team? Was the theoretical speed gain offset by the extra time the pit crew needed to remove/reinstall the panel for a wheel change? Brake cooling issues?

Inquiring minds want to know...

i suppose basically there's nothing wrong with wheelskits themselves, i can't see how they might directly have a negative inpact on drag BUT.
appart from practical issues such a tire maintenance and dirt accumulation (possible corrosion! ) there's also this: for stability reasons most cars seem to have an as broad as possible wheelbase. the instalation of wheelskirts on a lot of current cars would either require the rear wheels to move inward or the skirts to buldge around it. since the desired aerodynamis shape is tapered towards the back, option one may seem the best solution. however you would take the rear wheels out of line with the front ones... i don't know how that will effect road handing, but you'd still add frontal area, since the wheels would not be inline anymore. and possible extra rolling resistance when dealing with foul weather. all these things might counter any benefit generated by skirts...

writing this i suddenly realise a few cars did employed this, even as early as the mid 50's! the citro?n DS for example! but than again the car was revolutionarry in many aspects...even today it still has a futuristic look to it. (in the event of a flat tire the skirts could not be removed but with the aid of just a small pillar and due to the special hydrollic suspention the car was able to lift the wheel that was to be replaced of the ground and drop it out of the wheelwell... talk about advanced!)

finally there's one thing i really like about my car's design is the rear wheel whell...
https://www.gassavers.org/garage_imag...9nugbr9aie.jpg
the shape actually seems to be dictated more by a desire not to desturb the airflow than to echo the front wheel arch (wich is HUGE for the littel wheels but the car leans over quite a bit while conering fast so i think it needs the space sometimes) anyway it's far from a wheelskirt and the tires stand to far out to extend one further, but i thought it was worth mentioning.
afterall it's the little things that matter right? (don't look at the mudfaps :o )

Wow... I too wonder how fast their wheel changes are/were.

https://www.recumbents.com/streamliners/apvector.jpghttps://www.recumbents.com/streamliners/apvectop.jpg
Held the speed record in 1980 @ 63mph (look at that monster chain ring)

https://www.recumbents.com/WISIL/raci...strange_sm.JPG

You've seen the HPV I'm working on.... From CFD modeling, we know fully enclosing our wheels is beneficial. Empirical testing is a bit more difficult (I don't think anyone is going to cut into our wind tunnel model). Unfortunately, this year spacing is an issue so, we're compromising with wheel disks and minimal wheel arch :P

You can also look at almost every soapbox car -- they will have disks.

I'm not saying they're beneficial or not beneficial. But there's obviously exceptions to the rule. What that rule is (no skirts are better or skirts are better), is what's in the air :P

-------
For flow visualization..... If anyone wants to build a small water tunnel and make a sectioned model (with spinning wheels)

Trebuchet - now I understand why you're enclosing the front wheels - well.. if your machine is anything like the ones pictured.

You have drastically constrained steering angles. Which means you can locate the wheels very close to the rider's legs, and you don't have to worry about interference with the bodywork when "turning".

Crosswinds and Dirty Water
 

Looking at the pics of the grimy road marks on vehicles leads me to wonder about airflow patterns that aren't head-on (like pretty much most air).

Around here, it's windier than Chicago, so I'm almost always getting hit with wind to different angles of the car. So from what I understand, if you're driving at a 360-degree heading, winds from 270 to 90 degrees, at highway speeds, is mostly vectored to a front corner and then follows with the car (with most of the extremes of that radius, and winds further down the car, pushing on the flat side of the car -- and rearward winds [90 to 270 degrees] being negligible except again on the ends of the spectrum and further up the car).

Point being, all of this head-on testing only similates driving down a highway with zero wind coming from anywhere but straight ahead.

Shouldn't the car be put on a turntable to see how it fairs with wind vectors?

RH77

Good points, Rick. Cd figures do represent a head-on calculation. Cars with the same Cd & A values (but different shapes / aero detailing) could see different results in crosswinds.

Youu got it :) Our design different - but the same general idea. Our wheels will be flush with the fairing (hopefully - we'll see once manufacturing is done).

Our only steering constraint is that we must be able to achieve a 25' turning radius or smaller. But, because our CG is low, we can place our front wheels a little closer thus allowing the rear wheel to be a little closer without sacrificing stability..... All of that means - to make that 25', we need less of a steering angle.

Cool.

So your front wheels will be exposed & disc covered? GoOne style?

This sounds like a really fun project.

So just drive really, really fast and drag the apparent cross-wind up closer to on the nose. :D

Yep. Minus the suspension :P

https://www.primitiveengineering.com/gas/fairing.jpg
mind you, this is not the most up to date version - but the overall shape has stayed the same.


I really wish I had the money -- I'd buy a model of my car, build a water tunnel (which can be done cheaply with decent flow visualization results) and test :P You can even tackle the cross wind scenarios :D I didn't even think of those situations until now :P

Rudder
 

I'm constantly complaining, with the crosswinds, that I need a rudder.

I've got it -- 4-wheel steering, and you aim the front of the car into the wind, and drive "straight". Gusts, however, would be a problem :p

RH77

I think you just found the next competitive motorsport! It's got to be harder than drifting :rolleyes:

Perfect!
 

Perfect! Where can I get one? :cool:

Poland I'm told :P Only a limited number manufactured.... 1 :rolleyes:

Generally, during a race the tires are changed often, the skirt would increase the time to change the tire significantly. The gain in speed/mileage with a skirt is less than the loss of time during tire change.

I've always had a thing for these vehicles, and therefor have done a little bit of research into them. The rear wheel skirts did increase top speed, but there was not only the problem of having to remove them in the pits, but also the fact the tires would run hotter than if they were exposed, and of course the hot brakes, but I'm sure these had their own cooling ducts. I think one of these cars would make a GREAT FE car if you just remove the massive engine, and the massive cooling ducts. The rear spoiler could go as well, and you'd have a VERY clean car. They share a lot of aerodynamic principles with the insight, radiused front wheel wells, covered rear wheel wells, very little detached flow, No A pillar turbulance. The use of Naca ducts is great too! Super light, carbon fiber bodies, and spaceframe chassis. MMM i bet you could get well over 100 mpg with that setup.

Another strange thing is these cars were basically Unlimited as far as modifications, they were on a fuel ration system. They would generate ridiculous power and top speeds in practice session and qualifying, 1000 hp, +200 mph! So fuel economy was actually important to these racers!

I find that exhaust placement very strange on these cars, I see no benefits to routing it to the side of the cars. My perhaps it's to push the laminar flow away from the turbulant wheels? as it is a feature shared on several other group c cars, skirted, and non.

Wheel skirts are definetly different for every car. Mine are HORRIBLE to begin with. Therefor adding just half wheel skirts in the rear netted me a an avg. of 1-2 mpg better at 60 mph. A whole 10% gain! I think test after test has proven them to be good. I"m a little confused as to this topic?

Cool, info red91. Thanks.

Are you sure those are exhaust and not cooling outlets? I've seen a few photos of these cars racing, and if they were exhaust outlets, I would have expected to see some soot/crud on the bodywork aft of them. But they're clean in the pics I've seen.

It's been a cold rain all day today. I noticed looking at the cars as I was driving around that there is significantly more spray from the front then the rear wheels. Which would make me think that some kinda of coverage on the front would decrease drag more than on the aft wheels. Has anyone run across any data that reflects that?