Better wheel covers than 'smooth'?
 

I recently came across this paper: Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars (Alexey Vdovin, 2013) which draws the interesting conclusion that in some cases, "ventilation resistance" (fluid forces due to the rotation of the wheel and the pressure differential between the wheel well and the side of the car, as opposed to fluid forces due to the overall motion of the car) may have a sufficiently large effect on the energy use of a wheel that a flat wheel cover may not actually be the best solution overall, despite having nicer airflow from an 'overall motion of the car' perspective.

The solution which appeared to give the best results (under the particular parameters tested) was a cover over the outer radius of the wheel while leaving small openings near the hub (see pg. 20, figure 17 (f)):
http://ecomodder.com/forum/attachmen...1&d=1412926868

While pondering how this scheme could best be applied to my mags, I happened to park next to a Prius which had these wheels:
http://upload.wikimedia.org/wikipedi...s2004Wheel.JPG - apparently from the 2004 model. The 'trim' on this wheel looks suspiciously similar in general layout to the suggested design from the above paper. I can't find any reference to this being an efficiency feature, but maybe Toyota knew something about this that it wasn't letting on?

I'm thinking there would be two ways to fabricate something like this:
(1) As a ring of material ala the existing style of smooth covers seen here, with appropriate holes cut out, and zip-tied or similar to a mag wheel.
(2) As a set of 'gap fillers' which would fit between the spokes of a mag.
I don't think this is likely to work with covers on pressed steel wheels though, as the existing holes are generally nearer the rim than the above examples...

At the end they admit they're only scratching the surface.
I wrote out this long analysis, but vBulletin ate it. It does that whenever you don't Copy before you Preview.

Anyway, the thick outer radius cover trades places with the full wheel cover in Figures 20 and 21, looking at the combined overall drag.

I wonder how well thought out the original hub caps were on my Sentra.

regards
mech

Wouldn't a perfect smooth surface be best? Just not best sellers?

True, but looking at figs. 19, 20 and 21, thick outer radius wins for the ventilation resistance component, smooth wins for the aerodynamic resistance component, but thick outer radius wins for the combined effect. They may not know exactly why & how yet, but if experimental data shows lower overall resistance for a different desgn, that at least says "worth a look" to me :D

Lest we gloss over the obvious, let me point out that they used a seemingly very effective way to attach wheel covers: two threaded holes in each spoke. Presumably not enough to significantly weaken the wheel, but with stainless screws and loc-tite, quite professional looking and long-holding.

earlier research
 

earlier research singled out the solid,convex,full-coverage,'MOON' type disc as having lowest drag.
For an aspirated cover, GM's Ultralight covers demonstrated low drag while allowing for brake cooling.

Am I missing something ? I read this as meaning that a fully covered wheel is best for low drag :

" Also, as predicted, the Fully covered rim design produced the lowest aerodynamic resistance, closely followed by the Thick outer radius cover."

One thing that occurs to me is that the basic rims that were under the wheel covers in these tests were very well-ventilated 5-spoke rims. In Fig. 21, the "total" curves for a full cover and the optimal cover are quite close together. If the test were done with standard pressed steel rims, which are less well ventilated because they have many small holes, perhaps the full covers would give better comparative performance than in the test.

Sentra
 

They look really efficient.