Steelies or Alloys?

[aerohead]

Originally Posted by ebacherville

so what your saying is I need to build a big treadmill in my wind tunnel... ill get right on that

I've got an older SAE paper from some of the big-dogs.They did tunnel testing with and without the moving ground floor and results were so close,they deemed the added expense and trouble not worth the effort.The difference in drag associated with the stationary vs. rolling wheels could hardly be discerned from the signal-to-noise ratio at the tunnel floor.

[ttoyoda]

aerohead,
Thanks for the info that the difference is small. Do you know if they were testing changes with the tires rotaing, or the floor boundary layer, or both?

Chris D.,
The velocity profile of the oncoming air is different in the two cases.
When a car moves thru still air on the road, The velocity of that still air is zero (obviously) no matter what the height from the road. The car moves at velocity x thur an air stream of uniform velocity zero, no matter what the height of the air above the ground is.

When air is blown at velocity x by a stationary car, the air velocity is as desired (velocity x) at some distance from the floor, but the air velocity must decrease as you measure closer and closer to the floor.. There is a boundary layer at the floor where the air velocity is not as fast as the rest of the tunnel. At the face of the floor itself, the air velocity is zero.
The question then is how thick that boundary layer is, and is it important when testing cars that have air dams and skirts which get very close to the floor and thus (presumably) into that boundary layer at the floor.

[ttoyoda]

http://www.aerodynwindtunnel.com/a2.html
Budget wind tunnel for the common man.
The fellow that runs the place worked on the EV1, so he knows what he is talking about.


And I see on this page
http://www.aerodynwindtunnel.com/capabilities.html
they talk about

"The integrated 2D groundplane pressure measurement system is a significant part of this data package.
Two critical systems in the test section are the contoured walls and groundplane boundary layer control. These systems have been developed to perform at or above the level of any current full-scale wind tunnel. "

[donee]

Hmm,

Seems if you had the wind speed profile versus elevation from the floor up, one could then estimate how far up to lift the car during its test to replicate the moving ground. Of course not ever had Fluid Dynamics, I would not where to begin quantitatively to do this, however.

[digitaldissent]

Originally Posted by aerohead

Its a good question and all I can do is relate what I experienced with the CRX.Michelin was touting their Green-X tire in the mid-nineties.Claims were maybe 4-5 % better mpg.They were not available in 13-inch wheel,so $1,000 later I had a set of 14-inch alloy wheels off a CRX Si and Michelin's new wonder donuts.The whole thing was a bust! I never experienced better mpg,either with the low-rolling-resistance of the tires,nor the reduced inertia of the lighter alloy wheels.The tires did have excellent wet and dry traction,not a bad reason to have them,but never delivered on the mpg promise.So all I can say to you is that the experiment didn't work for me.As to the wheel-covers,I did a post installment on Wheels/tires/wheel-covers.It may have some data for you to think about as to what you might expect from a set "of MOONS".Hope it helps,Phil.

After doing allot of reading it looks like factory alloys are heavier than steel wheels on most cars. For example the Yaris steel wheel is 15lbs while the alloy is 17.5lbs. Wonder what the alloys weighed you got? probably heavy.

I guess I assumed in the first post that alloys would be lighter and they can be but in much more pricey aftermarket selections. Plus the tire itself seems to weight different amounts depending on the brand and model.

As it looks like ChrisD proved, lighter wheels = better

[cfg83]

digitaldissent -

I have alloys but I wish I had steelies for wheel cover/racing disk mods. You can work with allows, but it's harder.

CarloSW2

[digitaldissent]

I wish I had both I would really like to see the difference