Tuft Testing Smooth Wheel Covers

[orange4boy]

Originally Posted by Tango Charlie

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

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?

Orange, great minds think alike. I've been contemplating a wheel well insert with a nice radius-ed edge

Or is it "miserly loves company"?

[Otto]

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?

[instarx]

Originally Posted by orange4boy

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

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.

[orange4boy]

Originally Posted by instarx

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.

[groar]

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.

[aerohead]

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!

[dtbaker61]

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.

[Buddy]

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.

[bgfshrcr]

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?

[Tango Charlie]

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.