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Old 04-22-2020, 09:52 PM   #31 (permalink)
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Originally Posted by California98Civic View Post
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")
If you want a rule of thumb, better in fact not to go for complete covers. Saying otherwise simply goes against the evidence. And that does in fact seem to me to be a myth busted.

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Old 04-22-2020, 10:18 PM   #32 (permalink)
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Originally Posted by JulianEdgar View Post
If you want a rule of thumb, better in fact not to go for complete covers. Saying otherwise simply goes against the evidence. And that does in fact seem to me to be a myth busted.
Sure, we could go with a rule of thumb like that. But "the evidence" is considerably more nuanced than you are allowing for some reason. In this article, the conclusion was not presented as universal for all vehicles. The authors knew their findings were dependent a little on the vehicle. Their study tried to account for how ventillation drag and air flow drag interacted. All vehicles are different in that respect and use different methods to ventillate the wheel wells. Results will vary and we're not in a good position, with street cars and street testing conditions, to know precisely what works best. So, you go with a reasonable rule of thumb. A solid pan is still a good intervention. Personally, I have been thinking of ventilating my design in a way similar to this study, but I have too many unfinished projects!

EDIT: however, the study we're discussing suggests that all designs (except thick radius) perform more poorly than smooth covers. A rule of thumb for the thick radius seems supported, but that is not quite what COCyclist made. A rule of thumb for anything other than smooth covers or thick radius would not find support in this study, it seems to me.
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Last edited by California98Civic; 04-22-2020 at 10:28 PM..
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Old 04-22-2020, 10:24 PM   #33 (permalink)
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Originally Posted by California98Civic View Post
Sure, we could go with a rule of thumb like that. But "the evidence" is considerably more nuanced than you are allowing for some reason.
My evidence isn't based on just this one paper. See my reference list above that I used when writing that section of the book. Those references showed that full wheel covers on road cars aren't necessarily the best for lowest drag.

You can nuance that any way you like, but that is radically different from the advice so widely given, including on this discussion group.
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Old 04-22-2020, 11:12 PM   #34 (permalink)
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it seems to me like the smooth full cover is still the best
for one simple reason.

It's something anyone can do to almost any wheel
where as the thick outer rim is not a rim I have ever seen
even once.
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Old 04-22-2020, 11:19 PM   #35 (permalink)
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Originally Posted by ASV View Post
it seems to me like the smooth full cover is still the best
for one simple reason.

It's something anyone can do to almost any wheel
where as the thick outer rim is not a rim I have ever seen
even once.
Totally agree about relative difficulty. But the thick outter rim design could be mimicked for experimentation by modifying cheap plastic rim caps from a big box store.

Or use ones like this... if you blanked the outter portions of the rim and left the openings nearer the hum open, then you would have something to play with.

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Old 04-22-2020, 11:32 PM   #36 (permalink)
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Quote:
Originally Posted by ASV View Post
it seems to me like the smooth full cover is still the best
for one simple reason.

It's something anyone can do to almost any wheel
where as the thick outer rim is not a rim I have ever seen
even once.
It doesn't have to be a 'thick outer rim'. Tesla low drag wheels:



Honda Insight Gen 1 wheel:

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Old 04-22-2020, 11:38 PM   #37 (permalink)
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Originally Posted by California98Civic View Post

EDIT: however, the study we're discussing suggests that all designs (except thick radius) perform more poorly than smooth covers.
It's not what I am discussing! I am discussing the results of five papers, not just one. In fact I found the Tesla paper the most interesting.

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
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Old 04-22-2020, 11:50 PM   #38 (permalink)
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Originally Posted by JulianEdgar View Post
It's not what I am discussing! I am discussing the results of five papers, not just one. In fact I found the Tesla paper the most interesting.

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
Did they do comparisons with the smooth cover design? Have any of that data to share here? The comparison is important for this thread, seems to me.
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Old 04-22-2020, 11:57 PM   #39 (permalink)
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Did they do comparisons with the smooth cover design? Have any of that data to share here? The comparison is important for this thread, seems to me.
Yes of course they did comparisons with fully sealed wheels. Otherwise, how could I have drawn the conclusion shown in my book?

I think I'll leave this thread there - the evidence is available for those who wish to look.
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Old 04-23-2020, 12:03 AM   #40 (permalink)
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Yes of course they did comparisons with fully sealed wheels...
And they found better results than this? Because the Tesla wheel you posted looks a lot like the star wheel on this table, and that star wheel did not do as well as the full cover. A company like Tesla, or Honda for that matter, will partly make final design decisions on the basis of looks, not just function.





Tesla low drag wheels:



Honda Insight Gen 1 wheel:


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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.


Last edited by California98Civic; 04-23-2020 at 12:05 AM.. Reason: forgot to repost graph
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