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Old 04-22-2020, 01:03 PM   #21 (permalink)
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Originally Posted by aerohead View Post
How does that conclusion square with counterfactual evidence achieved from direct wind tunnel measurement? Why is Julian authorized to overturn nearly a century of observations and measurements?
Um, because I went and looked at what the latest tech papers actually say about current cars, rather than just quoting outdated tests that were often performed in technologically simplistic wind tunnels? The Tesla paper, quoted in the book, was highly illuminating.

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Old 04-22-2020, 01:25 PM   #22 (permalink)
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I went back to the:
Index of Phil Knox Aerodynamic Seminars & Mod-data lists,under, Mod-data Lists: #7.
General Motors investigators found that the John Shinella aero wheels (think MOON wheels) for the Pontiac Trans Am,returned 0.027 lower drag than the open,standard steel wheels,as measured at the Lockheed,Marietta,Georgia full-scale wind tunnel.
Historic data is always interesting, but quoting wind tunnel test results for wheel drag when (1) the wheels weren’t turning, and (2) the undercar airflow was completely different, is not likely to be helpful when discussing wheel drag on current cars.
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Old 04-22-2020, 01:42 PM   #23 (permalink)
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I want to offer this study into the mix. Ventilation drag seems to be a closely-related issue.

Alexey Vdovin, "Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars," (Gothenburg, Sweden: Department of Applied Mechanics, CHALMERS UNIVERSITY OF TECHNOLOGY, 2013).
http://publications.lib.chalmers.se/...302/176302.pdf

See also:
http://publications.lib.chalmers.se/...ext/133661.pdf
"Blocking the crossflow through the rims result in increased static pressure in the front wheel house and thereby an increase in lift, whereas blocking the crossflow in the rear result in increased downforce due to an increased flow rate through the diffuser. It was however shown that the resulting flow around the rear wheel was highly dependent of the rim design in the front."
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Old 04-22-2020, 02:40 PM   #24 (permalink)
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turning wheels/undercar

Quote:
Originally Posted by JulianEdgar View Post
Historic data is always interesting, but quoting wind tunnel test results for wheel drag when (1) the wheels weren’t turning, and (2) the undercar airflow was completely different, is not likely to be helpful when discussing wheel drag on current cars.
General Motors explored moving-floor wind tunnels in the early 1950s,and reported that the variability of drag data,as a function of the moving wheels was statistically insignificant,and the difference could easily be compensated for during static wheel testing,deeming the extra trouble not worth the effort as far as cost/benefit/accuracy.
As to the underbody flow,I'm unsure where the disconnect would be,compared to contemporary facilities.They were suctioning the tunnel floor,to prevent boundary layer buildup,which appeared to be the sole concern.'Windage' effects of rotating tires to wheelhouse and underbody flow had been explored.
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Old 04-22-2020, 06:12 PM   #25 (permalink)
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Originally Posted by aerohead View Post
General Motors explored moving-floor wind tunnels in the early 1950s,and reported that the variability of drag data,as a function of the moving wheels was statistically insignificant,and the difference could easily be compensated for during static wheel testing,deeming the extra trouble not worth the effort as far as cost/benefit/accuracy.
As to the underbody flow,I'm unsure where the disconnect would be,compared to contemporary facilities.They were suctioning the tunnel floor,to prevent boundary layer buildup,which appeared to be the sole concern.'Windage' effects of rotating tires to wheelhouse and underbody flow had been explored.
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.

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Old 04-22-2020, 06:21 PM   #26 (permalink)
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For anyone who wants to read the latest research data on wheel drag for themselves, here are the relevant papers I referenced in my book:


Vdovin, A., “Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars” Chalmers University of Technology, Gothenburg, Sweden, 2013

Mayer, W. and Wiedemann, J., "The Influence of Rotating Wheels on Total Road Load," SAE Technical Paper 2007-01-1047, 2007

Landström, C., Walker, T., Christoffersen, L., and Löfdahl, L., "Influences of Different Front and Rear Wheel Designs on Aerodynamic Drag of a Sedan Type Passenger Car," SAE Technical Paper 2011-01-0165, 2011

D'Hooge, A., Palin, R., Johnson, S., Duncan, B. et al., "The Aerodynamic Development of the Tesla Model S - Part 2: Wheel Design Optimization," SAE Technical Paper 2012-01-0178, 2012

...and too late for this edition of my book, but relevant is:

Wolf, T., "The aerodynamic development of the new Porsche Cayenne", Journal Automotive Engineering, I-19, 2019
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Old 04-22-2020, 06:33 PM   #27 (permalink)
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To make it clear, based on the research I have cited in the book, I am not saying that fully covered wheels are bad. What I am saying is that fully-covered wheels aren't always best.

Do full wheel covers always reduce drag (from my YouTube channel):

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Old 04-22-2020, 06:38 PM   #28 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
...Vdovin, A., “Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars” Chalmers University of Technology, Gothenburg, Sweden, 2013
...
This is the same study that I cited in post 23 above. It supports the idea that a flat disc surface is excellent in reducing drag. It finds a small added benefit inone other design. A graphic from the piece:

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Old 04-22-2020, 06:52 PM   #29 (permalink)
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This is the same study that I cited in post 23 above. It supports the idea that a flat disc surface is excellent in reducing drag. It finds a small added benefit inone other design. A graphic from the piece:

That shows that the wheel with the "thick outer radius cover" is in fact better than a fully covered rim. (The thick outer radius cover wheel has openings in it.)

It's one of the reasons that I state in the book that one should not always assume that full wheel covers are best for drag.

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Old 04-22-2020, 09:43 PM   #30 (permalink)
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That shows that the wheel with the "thick outer radius cover" is in fact better than a fully covered rim.
Right, but it is a quite modest improvement from the solid cover and without a wind tunnel for testing, guys could be forgiven if they went with the rule of thumb and used a solid cover. I think a nuance is getting lost here. The smooth wheel covers are still REALLY GOOD compared to baseline. Although COCyclist may have found a design that seems by his (necessarily imprecise) testing to work better than a smooth disc, it seems highly unlikely to me that smooth discs are infact worse than baseline.

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.



In my case I made rims I knew would not conform to the smooth cover standard, but I liked them better because they are lighter and don't scream HYPERMILER. 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")

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