Tuft Testing Smooth Wheel Covers
 

Tuft test #1

http://www.youtube.com/watch?v=UVpxq..._order&list=UL

Tango Charlie -

Great work! But I can't tell any difference.

CarloSW2

Yeah, and it might even seem that the smooth cover is slightly worse? Maybe some partial skirts that fill the leading and top gap are in order? And/Or, a few tanks to see how the FE does?

i think the air coming front the front fender flare plastic is foiling your tuft testings...

...and, you could simultaneously do your A-B-A tests by having one side have the stock plastic wheel covers and the other side have the smooth, pizza pan cover...that way, you're basically performing BOTH tests under identical (albeit RIGHT & LEFT sided) conditions on the same road, at the same time. Just have the "chase car" move from side to side.

The stock fender flares might be messing it up a bit. Or they are pushing air away from the front wheel. You might see a bigger difference in the rear wheels.


And tele_man, Tango has the alloy wheels, so there is no stock plastic wheels covers.

Hmm. A lot of reasons posted as to why the smooth wheel covers here aren't really worse than the alloys for smoothing airflow, despite the clear visual indication that they are. On this car the smooth wheel covers clearly disturb the airflow more than the alloys.

The car is the car, and the tufts are the tufts. If the car has slightly flared fenders then those are the conditions of the test. Many cars have those. I find this interesting because it shows that smoothing airflow along a vehicle isn't as easy as it appears it would be - even the most accepted of mods may result in a loss and not a win.

Are the alloys a "fan blade" type that might be sucking air into the wheel? That could be a factor in why the airflow downstream of the alloy wheel is less disturbed.

I think I see more tuft movement in the 2nd video.

Isn't it possible that more air on the outside (moving the tufts) means that less air is getting trapped inside the wheel well?

JMac
http://ecomodder.com/forum/fe-graphs/sig4777a.png

I was also like, :eek: and :confused: and :mad:

I did notice, though, that with the disc installed, the tufts around the perimeter of the wheel well in the nine o'clock to two o'clock positions, were more stable. So JMac, I think you may be on to something.

OTM, I like your idea of doing both sides at once. Wish I woulda thought of it, but it took a fair amount of arm twisting to get the wife to drive with me as it was. :D

I agree that the second video shows more movement of the tufts. Thanks for a very interesting test.

You can watch them side by side with the youtube doubler.

http://www.youtubedoubler.com/?video...uthorName=Qman

I do think that the tufts jump around more with the smooth wheel covers. I might have an explanation. It might be possible that the smooth wheel covers allow the turbulence to retain more energy.

Also note this is not a true ABA experiment. The first test was titled Test "A" . The second test was titled Test "2" . So technically you have done A2 testing ;)

The other uncontrollable factor (that would also be an issue with the one side smooth suggestion) and that is cross winds may have been different speed or different directions during the tests?

I will concede that this tuft test was deceptive; there was definitely a lot more movement on the second video with the covers on. However, uncontrolled environments are, as their name imply, unpredictable. No force was ever measured here, only tufts movements resulting from vectors that might or might not have been the same between runs. Every controlled test I've ever seen tells me wheel covers reduce drag, so I'll stick to SAE papers results to tell me what works and what does not for the time being.

The results were were quite entertaining to look at. Keep on the good work. You might want to give a couple coast down test a try.

Yeah, more tuft movement with the discs, I think. Especially further down the car from the front wheel - in the area near the end of the front door.

2 things I noticed

• 1. as mentioned, it looks like your getting more disturbance from the wheel well back with the discs installed, but it also looks to me like (from the direction of that disturbance) its coming from inside the wheel well.
• 2. the affect of the car doing the videoing, when they pulled forward a little, you could see the disturbance it had on the airflow of the vibe. I think to get a better idea of this would be to use a motorcycle as to have as little effect on airflow around the vibe as possible

.

from the looks of the shadows and such, I dont think there was much time between the two tests, and while I do understand that 15 mins is all that is needed for changing weather patterns, I dont think that happened here. but, we cant tell if there is any other vehicles on the road ahead, which would greatly affect the airflow.
lastly, you need to run the tufts all the way to the rear to garner correct wind patterns, as the rear most tufts seemed to me to have less movement in the 2nd vid than in the first.

To me it looks like this might be true, but in a more narrow range: 10 or 11 o'clock to maybe 1 o'clock. In the fan-blade wheel design is sucking air into the wheel, might that airflow then be forced under the car and out the top of the wheel well? Isn't the movement of air through the wheel itself a source of rolling resistance? Have you tested the effect on MPG? Sorry for so many questions. Thanks for posting the surprising result even though it frustrated you a little. Very interesting.

I'm with Daox here. A slight change in wind direction could have done that.

Just because the tufts moved more does not mean the smooth covers would create more overall drag. There are times when increasing frontal area lowers the Cd. Any change in the airflow pattern will change everything around it including in front of the car so perhaps that's what you are seeing there.

I kind of doubt those rims could blow air inwards. RPM is too low and outward pressure too great.

Still a good test and interesting to see.

Have you thought of rounding the trailing edge of your wheel well? I'd love to see how that smoothed things out.

Ha ha! Thanks for noticing my Paul Reiser-inspired attempt at humor. I wasn't sure if anyone would notice.

Neil, yes, there was a crosswind that day. I'm thinking this may very well have buggered my test. I only did one run north with the disc on, and one run south with the disc off. Then we went home. Since she doesn't read this forum, I'll blame it on the wife. I was trying not to inconvenience her too much. Lesson learned.

There were no cars ahead of us, I was careful of that. I tried to hug the side of the road to provide as much space between the two cars. dOsitmatr, which clip and at what time do you think you see the disturbance from the camera car?

Orange, great minds think alike. I've been contemplating a wheel well insert with a nice radius-ed edge; something to emulate the first gen Insight. If painted black, it would be visually inconspicuous, too.

California, I have not calculated any gains/losses. My tank to tanks are just too varied to show this small of a change. I think the next thing I need to do is do some legitimate ABA tests. You know I'll post it when I do. :thumbup:

I def saw some disturbance in the first vid (more so than the 2nd) when the cam car pulled even/ahead a hair. I did notice you were trying your hardest to hug the edge of the lane :)

Heh heh. Reminded me of my gr11 math teacher who used to say "OK, anyone like to try equation number B? Number B?...crickets chirping....
Anyone?

Or is it "miserly loves company"?

The smooth wheel covers generally get better mileage because the wheel itself has less drag, since a spoked or unsmooth wheel acts as a crude turbine. So, it takes less energy to turn a smooth wheel in the air than a rough one, all else equal, which it seldom is.

Clearly, from the videos above, that's not the whole story, as tufts in video 2 show more turbulence along the side of the car, especially in the immediate wake of the wheel well.

Another factor that may be at work here is that since many wheels act as turbines and throw columns of air out sideways, they increase the effective frontal area of the car. We saw this clearly when a group of Porsche 944s went ratracing at high speed. Driving along wet roads, it was obvious that some wheels (on otherwise identical cars) threw out lateral columns of air 2-4' sideways, whereas smooth wheels like my D90 or the Porsche "sewer lid" types generated much less side flow, for less effective frontal area. Could be in the videos above, there is more turbulence in #2 because the sideflow hugs the car better, whereas #1 does not. It would be interesting to have tufts on sticks further out in the slipstream, to see what's happening to air flow 6", 12", 24" etc. out from the surface.

Also, how about repeat this test with tufts taped to the wheels themselves?

I'm not sure about that. Typical 18 inch fans turn at about 1,000 rpm on low speed and move about 2500 cfm. Car wheels turn at about 800 rpm at 60 mph. Wheels are probably not going to be as efficient at moving air as a dedicated fan, but I don't think it is out of the realm of possibility for a turbine design wheel to move 500 -1,000 cfm at 60 mph.

Of course we don't know if these even are turbine design wheels, but the effect of wheel design on airflow doesn't seem to be one that can be dismissed out of hand.

I can see how you might expect some airflow there except that I did some numbers and found that at 60 mph they will be fighting 5280 fpm and the negative (to your percieved flow) pressure differential between the underside of the car and the outside. Add to that a sub optimal relation to the air they are trying to move and the way they rotate and I think you can see why I was so doubtful.

I put some tuft a few days ago over the hood. They are very calm, moving by less than a 1/4 inch while there is nobody in front of me. As soon as there is someone then they move much more, up to 1 inch. Following a truck I got a situation I could see them being more or less calm alternatively, may be showing the vertical wave generated by the left side of the truck.

I have also being wondering about some hubcaps being "asymmetrical" when being on the left side or on the right side. May be some could improve the drag on one side while deteriorate it on the other side. With some cars it could be better to push some air under the car while with others it could be better to pull it outside.

Anyway, thanks for the test.

Have fun,

Denis.

chase car
 

In a wind tunnel,the car would be no larger than 5% of the test section frontal area.
It's possible that the proximity of the video chase car is actually affecting the airflow.
If the camera were stationary,videographing the Vibe as it drove by,it might reflect a little different flow behavior.Don't know.
When we tuft-tested the T-100 we experienced some chase car interference.
It is said that in Formula-1 Grand Prix,that at Monaco,when the cars go through the tunnel,30-feet overhead,the drivers can 'feel; the difference in handling.
Great images! Thanks!:thumbup:

lots of effort went into the tufts for sure, but I am more interested in seeing the details on how the pizza pans are attached! Are there some photos or vid of fabrication?

I may try something like this out on my electric Swift... and rather than tufts, I can pull some VERY accurate actual kWhr consumption over a set course at constant speed as I have some very accurate metering installed now.

Pizza pan wheel covers
 

How did you attach them to your alloy wheels? I have some aluminum racing discs that I want to attach to my spoked steel wheels, but haven't decided "how" to do this.

Great test! This is something I have been thinking about for a while for my vehicle. It could be that the openings in the stock wheel are causing just enough turbulence to mitigate air catching the wheel arch behind it. Just a thought. Definitely looks better without it. What does the A-B-A say though?

Dan and Buddy
I copied NachtRitter's process of securing locknuts inside the wheel's acorn-style lugnuts. You can also see a few more details in my Vibe build thread.

Tuft Disturbance
 

I just wanted to throw another thought in there to explain perhaps why the tufting looks more disturbed in the second (smooth wheel) video. The discs are covering the holes in the wheels, which would prevent any "venting" through the wheels. I think that the higher airspeed along the sides of the car (think bernouli effect of higher airspeed resulting in lower pressure) and the "compression" of air that passenger vehicles see underneath them would cause an outward movement of air through the spaces of the rotating wheel. I don't think that unless the wheel pattern is specifically designed to move air will there be any "fan" effect in either direction.

Therefore the wheel covers would prevent this outward flow of air. In the first video this outward flow is pushing turbulence away from the sides of the car, but when you reduce the amount of this flow the turbulence is closer to the sides of the vehicle and can be seen in the tuft testing.

As another post noted, there are many engineering papers that prove the worth of smooth hub caps. I think that we're only seeing a small piece of the whole picture which leads to improved efficiency.

i.e.; I need smoke.

and more runs both ways north and south.