Rim Aerodynamics

[trikkonceptz]

After reading a couple of threads, I need to ask the question ....

How to we know what impact our rims are having on our vehicles? We seem to think ... that flat dish rims are the best for mpg's, assuming highway driving, but is there a way to know if your current wheel set up is detrimental? I doubt turf testing would apply in this circumstance .. LOL

Please let me know ...

Oh to relate it specifically to my case, I have the 17" oem upgraded 5 spoke alloy wheel that came with Vibes. I think stock Vibes/Matrix come with 16" wheels ...

[MetroMPG]

I don't think there's any easy way to tell you what the drag of your wheels are vs smoothies.

I didn't crack open Hucho to see what he had to say on the matter (I'm sure he's got stuff about it), but from the PDF Contribution of different devices to the total drag (Road Vehicle Aerodynamic Design, Nédélec-Berroche, Neiter, Rousseau, Suard)...

Effect of rim type:

There are two main types of rims. First, the rims which are entirely full, i.e. without any discontinuity or gap in the surface (see below the example of Citroën Bx rims) are the best choice for reducing the drag coefficient. Indeed, if the rims are full, there is no possibility for the air to go threw the wheels and so, it doesn’t disturb the flow under the body of the vehicle and on its side. But this type of rims has an important disadvantage: it would not enable an effective brake cooling which becomes very important when dealing with performance-oriented cars. That is one reason why the car manufacturers have introduced some rims with spokes which enable a certain air rate to flow threw the wheels and hence to cool the brakes. Of course, this type of rims has also often a good impact on styling but there is no denying that it is drag-consuming. Indeed by completely sealing all four rims, Cd can be reduced by 0.08 to 0.012 (ref. 6). The maximum drag difference between a good and a poor set of rims is probably even larger.

(I think the .08 is a typo and should be .008.)

Unfortunately it doesn't give examples of the rim types they're comparing to smooth covers, but .008 to .012 improvement is nothing to sneeze at.

[MetroMPG]

Can you post a pic of your rim style? We can at least have fun guessing whether it's good, bad or ugly! (Aerodynamically speaking )

[Cd]

I notice that even with huge open spoked wheels, cars such as the Lexus LS430 still manage to get an Insight matching .25 Cd.
Hot Rod magazine tested a '79 Camaro and managed to get the car down to point twenty .Cd just by blocking the grille and adding a small spoiler extention.
They took the mirrors off, but made no other changes to the body besides the grill block.
They did however test the car with 'moon' disks and ended up running the car with them at the tracks, citing a reduction of two to four counts when they tested the car in the wind tunnel.

I swear I never in a hundred years would have thought that a '79 Camaro with a flat piece of metal screwed to the bumper would be able to almost match the EV1s' sleekness.
I'm still in disbelief.

But anyways ... yah post the image of your rim style and we can all guess at how 'sleek' that they are.

[PaleMelanesian]

Very interesting here - Lexus LS600h (hybrid). You know they examined rim aerodynamics very carefully. It's low-hanging fruit.

[Daox]

Its also a luxury/performance vehicle which needs brake cooling if driven aggressively. Reviewers would crush it if it had bad brake fade, and you know they beat those cars when they take them out.

[MetroMPG]

Good point, Daox.

Even the Prius alloys are fairly chunky, as is the style these days. Though the compromise is the inset trim ring, which makes sense since the outer (and top) portions of the rim create the most turbulence.

PM: I found a larger picture with more detail of the Lexus rims. They have an odd spoke shape, and it wouldn't surprise me to learn they're optimized to minimize drag despite the open design.

Pic: (2000+ pixels W): http://caraphoto.caradisiac.com/r/e/...H/HR_71271.jpg

[PaleMelanesian]

That is indeed interesting Metro. I had to look - the rims are the same on the other side, meaning the raised part is trailing on the right side and leading on the left side. I'm really confused now.

[MetroMPG]

Good find. So I retract my "optimized" guess. I'd think they'd need to be unidirectional if the asymmetric spoke design was useful.

[jim-frank]

I was driving over on the interstate in a rainstorm, and was musing over the spray ejected from the sides of cars by the wheels. I could see fairly clearly that the spoked type of wheels had a larger volume of spray/mist coming from around the wheels than the smoother wheels that just had holes around the rim.

The spokers also seemed to have more spray coming from higher on the wheelwells than the smoother rims.

Have you ever looked at the 'airbrakes' that are used on some stationary bicycles to provide a load for the rider to work against? The amount of drag is adjusted by varying the amount of air that is admitted to the spinning flat-bladed fan.

Consider that a spinning wheel is a centrifugal pump as far as air is concerned, and you can visualize that the spoke structure gives more room for air to enter at the hub and be accelerated out to the rim.

The plumes of air ejected to the sides will cool brakes, but will also extract energy from the vehicle.

These are some of the things that come to mind. The longitudinal airflow past the wheels might be of lesser concern than this kind of transverse flow.