More data on smooth wheel covers effectiveness
 

A couple key findings on smooth wheel covers of interest to this forum, from:

Sofie Koitrand, Adrian Gaylard, and Gianluca Orso Fiet, "An Investigation of Wheel Aerodynamic Effects for a Saloon Car" - Conference Paper - September 2015
Conference: Progress in Vehicle Aerodynamics and Thermal Management: Proceedings of the 10th FKFS-Conference, At Stuttgart, Germany

The PDF is currently available online for free. Thanks to the authors and their professional org for that. I will share just a small portion of their findings, but there is more. Go get their paper and read it if you want more.

In another thread, a few forum members have been discussing the effectiveness of several aeromods for wheels. This study uses a wind tunnel (with 5 belts) and CFD to study these different wheel designs:

https://ecomodder.com/forum/attachme...1&d=1587618780

The conclusion was that the fully blanked, smooth cover wheel saw the greatest reduction in drag:

https://ecomodder.com/forum/attachme...1&d=1587618819

That graph presents a combination of the wind tunnel & CFD data. In the wind tunnel alone, the fully blanked wheel saw a delta of -0.013 Cd, the 85mm annular blanked wheel -0.009, and the 430mm center-blanked ("Tesla" style) wheel saw a -0.003 Cd delta.

Their conclusion is that in this test fully smooth covered or blanked wheels produced the greatest Cd benefit over baseline, whether measured in a wind tunnel or using CFD. This is a different conclusion than some other recent studies.

Good find. Note that the graph you have shown simply displays the difference in CFD prediction vs wind tunnel results (ie the error) - not the actual changes in drag.

Versus the standard rim (wind tunnel results):

• fully blanked - Cd change of -0.013
• 85mm annular outer ring - Cd change of -0.009

So the Jaguar results are different to the Tesla results (where fully covered wheels increased drag), which in turn are different to the SAE 2011-01-0165 results (where ventilated front and fully covered rear wheels gave the best result).

As I wrote in the other thread:

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.

I am guessing it's highly dependent on the car to which the wheels are fitted.

I already noted these data points in the OP. But you get the number wrong here. The wind tunnel results for the 85mm ring were -0.009. You are mistakenly quoting the CFD number of -0.012. It's on p.248, table 18.3.

On a car, at 65mph that could be up to a 1hp reduction in power requirement to maintain speed.
Not bad.

Right?! Their testing speed was 140 kph, I think. But still. I'd like to think my wheel covers gimme back an HP at 65 mph cruising. :thumbup:

At autobahn speed you are definitely saving at least 1hp.

You are right! I have edited my post.

Of course, absent from this discussion is the need for brake cooling, which from a manufacturer's perspective is probably at least as important as aerodynamic efficiency. (And I think it is pretty important too!)

Citation needed?

www.researchgate.net: An Investigation of Wheel Aerodynamic Effects
for a Saloon Car

I remember the name Adrian Gaylard from discussions a few years back.
LinkedIn wants an email address. No thanks. U of Georgia and Coventry University were also notable. A lot of the materials disappeared, I had captured a few Powerpoint slides like this (added to my album in 2013(!)):

https://ecomodder.com/forum/member-f...31-1-26-08.png

I did a search and surfaced a few other interesting articles at ReasearchGate:

www.researchgate.net/publication: Automotive Aerodynamics Special Issue Surface contamination of cars: a review

https://www.researchgate.net: The importance of unsteady aerodynamics to road vehicle dynamics

There are links to other articles at the bottom of some of those. Examples:
I [think I] remember this one:
www.researchgate.net: The Effect of Base Bleed and Rear Cavities on the Drag of an SUV

I wonder whether they really went away and are back, or what?

Yes, Adrian is a lovely guy.

He gave extraordinarily detailed feedback on each of the book chapters I sent him.

Through him, I also got the Jaguar XE for a week to aero test, and then I was able to compare the on-the-road test results with his 'official' Jaguar CFD - and have him comment on the differences!

Really down-to-earth and quite frank in his feedback.

Only recently have brake rotors been exposed by alloy wheels. Up through the 1990s brake rotors were covered buy big steel wheels and it was never a problem.

Steel wheels that invariably had brake ventilation openings in them.

Typical wheel found on US cars trucks and SUVs through the 1990s. Now resides on my trailer.
https://ecomodder.com/forum/attachme...1&d=1587758444

Auto makers put vents on wheels because they look cool and make the wheel lighter, they don't really start to make any functional difference until you are on a race track.

Brake cooling has been part of road car wheel design for at least 50 years. For example, a textbook I have on brake design that was published in 1967 mentions it.

In a current context, a recent Porsche paper I was reading on the development of an aerodynamically slippery wheel spends over half that section on the difficulties of developing an aero wheel that doesn't too adversely impact brake cooling. In fact, you can see exactly how much extra drag they accepted in order to achieve adequate brake cooling.

I think every reference (text or paper) that I read when writing the section in my book on wheel drag mentions the importance of brake cooling.

Alternatively, just make brakes that can take the heat:

https://i.ytimg.com/vi/X3zxgSTGN_k/hqdefault.jpg
youtu.be: Crazy Red Hot Brakes Compilation! *HOT*

Don't drive like that and you won't need major brake cooling.
Those brake rotors probably aren't steel or cast iron.

https://www.youtube.com/watch?v=G5ZdyG1HHKU

I actually did this video a few weeks ago but only just released now.

That's right. I hardly use my brakes even in "stop-n-go" traffic because of engine breaking and EOC. Certainly not heavily. My brake pads last forever.

i thought they had asbestos brake pads back then
I saw a set of pads that said "now without asbestos". that my grandpa had in the garage that was sitting there for ages..

Interesting that the wheel with radial slots ended up with the second-largest delta-Cd. After looking at the pictures of the XL1, Insight, and Tesla Model S wheels in the other thread with their circumferential openings, I was reminded that most manufacturers today have shifted to radial slots on their economy cars and lower-drag cars. For example, the Tesla Model 3:

https://www.teslarati.com/wp-content...3-Red-side.jpg

The 2016+ Civic:

https://www.autoguide.com/blog/wp-co...ivic-Wheel.jpg

Hyundai Ioniq (the base Blue trim has even smaller openings than these!):

https://car-data.com/clients/car-data/Ioniqrear3.jpg

Hyundai Sonata Hybrid (until 2020, the new one has more openings):

https://static.cargurus.com/images/s...0-640x480.jpeg

Honda Accord Hybrid:

https://images.caricos.com/h/honda/2..._2560x1440.jpg

...and many others.

Great pictures. Thx. Noteworthy, too, that these designs are beyond what Porshe deemed safe for brake cooling. It's almost as if maybe Hyundai does not expect its cars to get driven like a Porshe. ;)

With my leaf normally I can stop, get out and put my finger on the brake rotor with out it being uncomfortably hot.
If you have some brake regen, and can keep it under 140mph no need for massive brake cooling.

Horses for courses, of course. When in my Gen 1 Insight I brake from 125 mph to 60 mph at the end of a local straight, I don't want brake fade.

As I have said, I've never read any formal reference that doesn't mention the importance of brake cooling when developing aero wheels for road cars. They all mention it!

OP- Thank you for posting this. I especially liked the color representations in the full paper. There is one thing I would like to point out. In post #1 the graph you posted is from the averaged wind tunnel vs CFD results. I was shocked by the amount that front lift went up in the graph. As I read the whole article I found there was a discrepancy between wind tunnel and CFD results. The wind tunnel did not show such drastic increase in lift with the fully blanked wheel. I copied the following from the conclusions.

"Overall, CFD predicts the trends from the wind tunnel results well; however, in the case with the fully blanked wheels there is a noticeable over-shoot in the drag decrease and the front lift increase. CFD also fails to capture the drag trend for the smallest centre blanking (350mm), though it predicts the rear lift trend well. Based on these two cases, it is thought that CFD over-predicts the effects of the interactions between the onset flow and the flow through the front wheels."

The CFD was done without modeling turning wheels. Great article. Thanks.

As I said in Post #2, the graph displays the difference in CFD prediction vs wind tunnel results (ie the error) - not the actual changes in drag. The heading explains that - change of CD from CFD minus change in CD from wind tunnel.

Also, it should be noted that they did model wheel rotation. Under 18.2 Methodology:

And in the conclusion, they point out the error in the modeling of tire rotation specifically:

functional difference
 

From a safety standpoint,wheel porosity/aspiration as a function of brake heat flux dissipation,has been an issue for a century.
Worse-case scenarios would be factored into design, in order to prevent brake fade,especially descending extended mountain grades while pulling at gross vehicle weight load.Engineers must anticipate motorists operating right at the edge of the envelope.Fatalities are bad for marketing.

importance
 

No kidding! There's a whole train of product-liability attorneys who'd love to take on class-action lawsuits against any auto maker who inadvertently overlooked brake compromise/failure during R&D.
In the 1970s,heading eastbound into Chattanooga,Tennessee,one might notice the 'orange landscaping' at the bottom of mountain valleys,from all the ROADWAY tractor-trailer rigs who's brakes burned up,while failing to find a runaway truck ramp in time to save itself.:(

asbestos
 

The medical injury,class-action lawsuit against John-Mansville Corporation, for asbestosis and mesothelioma drove clutch disc and brake shoe/pad technology away from bonded asbestos friction surfaces.A friend's husband died from his job remanufacturing clutch discs.Dead worker.Dead husband.Dead father.Not good.

Porsche
 

Bear in mind that,there remain sections of the Autobahn and Autostrada, which are posted for unlimited speed.And since Porsche cars are capable of significant velocity,in a 'panic' situation,the cars must be capable of dissipating tremendous quantities of heat energy to avoid collisions,something Hyundai might not have to design for.
Hyundai's track version of the Veloster may have high-porosity wheels,with enhanced load-shedding capacity.Same for Honda's Type-R.A few others.

importance
 

ROAD and TRACK recently reported on the Porsche Taycan Turbo S,which is capable of 160-mph (257 km/h).It's a 5,246-lb car.Lifting off the potentiometer in a Cd 0.25 vehicle doesn't offer an abundance of braking drag.In an emergency braking situation,that's a lot of kinetic energy converted to thermal energy in a matter of seconds.The extra drag of the porous wheels would be most welcome.:)