Heresy! Smooth Wheel Covers- Myth Busted!!!

[Vman455]

Note that the XL1 uses wheels vented at the perimeter as well:

[California98Civic]

And i love the look of the XL1 wheels, too. But that design is quite a lot different than the Tesla or Insight design. Smaller openings, recessed into the disc... It is much closer to a fully-closed surface than the Tesla design. Don't you think? I'd love to see VW testing comparisons.

[Vman455]

It strikes me as the opposite--quite similar to the Tesla and Insight designs but with shortened arc length. The perspective might be throwing things off as far as seeing the size of the openings--here's another picture from a larger angle:



The inside edge of the vents is at the secondary inscribed circle.

Tesla for comparison:

[aerohead]

Originally Posted by JulianEdgar

I am sorry, but are you seriously suggesting that wind tunnel testing of wheel drag, performed on a 1982 car without its wheels turning, is somehow relevant to modern cars?

Wind tunnels now test cars with a moving floor and spinning wheels. They do this for good reasons. I think this is probably the major reason that the research results have changed so much over the last decade or so.

The airflow patterns under a rough underside car like a 1982 Trans Am Firebird would be completely different to any modern car. The latest tech papers show that car drag can be influenced by how the wheels and under-car airflow interact.

I also note that "Index of Phil Knox Aerodynamic Seminars & Mod-data lists,under, Mod-data Lists: #7", as far as I can see, largely references material from the 1960s. As I said, old info is always interesting, but to apply it to current cars, when in fact current tech papers disagree, is in my opinion, not wise.

*Perhaps we might establish a cut-off date for 'current' cars,before which,we'd discount published drag coefficients.
* Additionally,so as not to disrespect the best efforts of the top investigators of the day,we might re-test a sampling of specific vehicles,in rolling-road wind tunnels,and publish back-to-back results,highlighting the actual difference in drag measurements,on a case-specific basis.
*Since 'current' vehicles might include drag coefficients upwards of Cd 0.70,it would be very interesting to see drag variability associated with rotating,versus stationary wheels.
*Contemporary aerodynamicists painting with broad-brush commentaries with respect to the inferiority of non-rotating wheel drag measurements have an obligation to provide specificity.Context. It's a slap in the face otherwise.
*I reviewed an online article about the significance of the aero wheels on,I believe,the Tesla Model 3.The conclusion of the investigation would be a 'fail',historically,by SAE metrics,unless considered within a very narrow interpretation of language used by the reporter.
*Wheel drag contributions have been part of the automotive literature since 1922.Advancements and refinements in testing are a natural progression,and welcome,however, contemporary investigators may not want to inadvertently disrespect the very shoulders they stand on.It's matter of simple courtesy.

[California98Civic]

As I know you know, aerohead, cut-off dates are neither good scholarship or good science. A more nuanced and careful critical reasoning and theorizing is always necessary. And though this is not my field, it seems to me that the two studies I have been sharing here, which seem high grade, do not support a conclusion that the "Tesla wheel" is always good, that a "thick outter radius" is neccesarily superior, or that the smooth cover is necessarily inferior. We are back to the old problem, discussed here often over the 9 years I have been here: each vehicle is a different context and different procedures and testing tools can yield different results. YMMV. Be skeptical of claims. And that skepticism is also where this thread began...

[aerohead]

Originally Posted by California98Civic

As I know you know, aerohead, cut-off dates are neither good scholarship or good science. A more nuanced and careful critical reasoning and theorizing is always necessary. And though this is not my field, it seems to me that the two studies I have been sharing here, which seem high grade, do not support a conclusion that the "Tesla wheel" is always good, that a "thick outter radius" is neccesarily superior, or that the smooth cover is necessarily inferior. We are back to the old problem, discussed here often over the 9 years I have been here: each vehicle is a different context and different procedures and testing tools can yield different results. YMMV. Be skeptical of claims. And that skepticism is also where this thread began...

Absolutely! Hucho addresses a particular vehicle in a particular wind tunnel.A case-specific scenario.No generalities.
My personal finances preclude the indulgence in $30 SAE Papers,Journals,& Transactions.And I do appreciate Julian's access and willingness to share contemporary research.
General Motors past researches left an indelible impression upon me that, they'd left no stone un-turned,and if they didn't test with rotating wheels,they had a good understanding and defense for that decision.
And while I applaud Tesla,they are not the low-drag leader in automobile manufacture,and in particular reference the the 'aero' wheel research shared online,they took particular liberties with how they presented their data,which would inflate the significance of wheel drag when compared in light of their 'liberties.' No single messenger gets to re-write the rules of engagement,and that include Tesla Motors.

[COcyclist]

Thank you all for responding. I think we have had some good discussion. I would recommend that if you are reading this thread, in the Aerodynamics forum, on Ecomodder, that you buy the book by Julian Edgar and decide for yourself. (I have never met Julian and do not stand to profit in any way from this recommendation) I agree that Moon style wheel covers are best for smooth airflow and low drag, in isolation, as proven in the wind tunnel. In the real world with spinning wheels and tires, a moving road surface, disk brakes inside the wheel and complex airflow around and under the vehicle, it becomes much more difficult to say with certainty.

Several studies are linked in this thread. The Chalmers study is on the web and graphics from it have been posted here. The others are SAE papers. IIRC they cost can $50 each which makes it harder for us to access them. Julian Edgar has purchased and read these and numerous other papers and studies. He worked with an aerodynamicist while writing his book. He has done countless hours of testing himself and shares his test results and test methods in the book. There are lots and lots of photos of aerodynamic vehicles (many shared by Ecomodder members) and color graphics. In the US the book is available on Amazon for $50 or about the cost of one SAE research paper.

In fairness, I did not test full Moon wheel covers. I have run taped factory covers in the winter. Eventually the tape came off and I did not see a measurable difference. (Once I did get snow buildup behind them that melted, refroze and threw the tire balance off enough to make the car almost undrivable). I have a bumper to bumper belly pan with no underbody vents and the upper grill blocked. Perhaps this causes more cooling air to be vented through the front wheels than on some other cars. I feel that based on the research, I may benefit from smooth covers on the rear wheels only. Perhaps that is what I should test next, if it ever quits snowing in Colorado and I can put the summer tires back on.

[COcyclist]

Originally Posted by California98Civic

For what it is worth, COCyclist's rims are a little like the "thick radius" rims, but they are not the same. The openings are larger and longer, extending further out from the center. A maxim to remember: each car is different, YMMV, and be skeptical of how any test applies to your vehicle.



If COCyclist is digging the OEM look and feels confident they give him good results, I don't think it is "heresy" (or a "myth" "busted")

In one of the studies Julian cites, disk brake diameter to wheel inner diameter influences overall drag greatly. The styling trend toward "Dubs" is the worst offender. In the photo you linked, the 15" alloy rim fits very near the disk if you look closely. Could this make the combination perform more like the vented rims from Tesla, Honda and VW???

FWIW I do dig the GTI Mininlite look. Most of my mods are very stealthy and would not even be noticed at a casual glance.

[COcyclist]

Originally Posted by aerohead

Fuji researched wheel cover drag with respect to their Subaru XT.
*The ventilated flat disc,when taped closed,revealed a delta Cd 0.002,for an overall drag reduction of 0.69% (and no brake cooling).
--------------------------------------------------------------------------
John Shinella,of Pontiac styling, researched wheel cover drag in the Lockheed Marietta wind tunnel during development of the Trans Am Firebird of 1982.
*The car,@ Cd 0.32,with no covers
*with ventilated flat discs dropped to Cd 0.296
*TAPING over the vents produced Cd 0.290
*Shinella's ventilated convex covers showed Cd 0.287
--------------------------------------------------------------------------
So,taping was good for delta Cd 0.006,which cut overall drag by 2.02% (a 1% increase in mpg @ 55-60 mph).
--------------------------------------------------------------------------
The difference between no wheel covers,and Shinella's covers is a 10.31% overall drag reduction.(5% MPG @ 55-60 mph).
A set of MOONs would come in at a little lower drag and mpg increase.

I get confused with these two GM designers. John Schinella designed the Trans Am but I thought it was Larry Shinoda that did the aerodynamic studies. I couldn't find it in a quick search.

Interesting reading https://bangshift.com/general-news/c...ohn-schinella/
https://books.google.com/books?id=3t...namics&f=false

*Shinella's ventilated convex covers showed Cd 0.287

I do not have that paper but the way I read this it looks like the ventilated convex covers have lower drag than the TAPED covers. Am I missing something?

[aerohead]

eletrek.com has an article about Tesla's Model 3 Aero Wheels,which includes a link to a road test conducted by nextmove,car rental in Germany.
Three cars were rented and driven over a 94-Km circuit,with different hubcap configurations,at 150 km/h (93.205 mph).
The hubcaps improved range from 312 Km (193.867 miles),to 320 Km (198.838 miles),a 2.564% increase.
Following the velocity-cubed law,at 136.8 km/h (85 mph),(Most allowed in the USA) the advantage would be 1.944%.
At 104.6 km/h (65 mph [where cars may hit their EPA HWY rating]) the benefit would be 0.869%.
At the old 'double-nickel' 55-mph national speed limit,we'd see 0.526%,perhaps a rational for not obsessing over 'rotating wheels' by automakers for at least 20-years.