Heresy! Smooth Wheel Covers- Myth Busted!!!

[2000mc]

https://ecomodder.com/forum/showthre...ard-28594.html
I tried a similar test on my Saturn, unfortunately everything on the car warming up more and more skewed my measurements. Looked like bare steel wheels did the worst for me, but smooth vs factory hubcaps were very similar to each other

[JulianEdgar]

Originally Posted by COcyclist

[...] I recently purchased Julian Edgar’s book and he quotes several studies of airflow and drag on car wheels. These studies show that totally smooth wheel covers are not the lowest drag option in modern automobiles. [...]

Yes, I was surprised with what the formal (SAE-level) research papers showed re full wheel covers. I think a key point is that the wheel drag cannot be looked at in isolation eg the Tesla paper that showed how ventilation airflow through the front wheels could benefit overall drag (despite the wheels themselves then having more drag). My gut feeling is that wheel designs should be asymmetric front/back.

Congrats on your testing.

(Incidentally, I have a video coming up on some recent Porsche results on wheel drag and brake cooling. My channel - https://www.youtube.com/channel/UCK9...mNB9XEmuiCjxcQ)

[JRMichler]

I also found no improvement with wheel covers: https://ecomodder.com/forum/showthre...yon-32761.html.

[COcyclist]

I realized last night as I was entering the test data that if you only look at the averages, my results show the covered rims give a slight improvement over the bare alloy rims. As I noted in that post my final run may have been impacted by heavier traffic. Also, those runs were later in the day when winds are typically higher, gusting and variable here in the high desert. This makes it harder to get good data. I have tried various forms of the wheel covers over 12 years of driving VW TDI and my tank mpg averages would always go down when I used them. My best mpg tanks all occurred on summer tires with the bare alloy rims. I was puzzled by this so I assumed my wheel covers were somehow deflecting and trapping oncoming air. I doubled up on the wire ties. No improvement. My first attempt was larger, matching the widest part of the tire, so I tried a smaller set. No improvement. I assumed the ties through the spokes were too close to the center so I moved them closer to the circumference. No improvement. I purchased Julian Edgar’s new book- Modifying the Aerodynamics of Your Road Car, and found some information that could explain why I was not seeing positive results from this widely accepted modification.

[California98Civic]

I don't think it is heresy. The point about smooth discs has been published professionally. In the wheel discs showcase thread I posted a citation to a great informative essay (with pictures!). In short, smooth is very good but not the best. Here is the link: https://ecomodder.com/forum/showthre...tml#post603176

[Frank Lee]

Originally Posted by COcyclist

Kudos to anyone who decides to do A-B-A testing. To do it right takes time and ideal test conditions to get good results. <> There seemed to be more variation with changing wind perhaps, than the variation of wheel covers on or off.

Back in another epoch I did a college paper on aerodynamic devices and road-tested the standard fare: air dam, skirts, shovel-nose grille block, mirror deletes, etc. As much as I wanted my devices to create earth-shattering improvements to be honest they didn't rise above the signal-to-noise ratio of on-road testing and I had to admit as much in the paper. One reason I'm loathe to make mpg claims for mods is I know first-hand how hard it is to get good trustworthy data.

[MeteorGray]

I don't see how data can be trusted unless the effects of wind can be controlled (amongst other things, of course.)

So, testing in variable winds is a deal killer, for me.

[aerohead]

Originally Posted by COcyclist

At least on the VW Golf


https://imgur.com/gallery/7sVGG7k


I have tried various forms of wheel covers for over 10 years on my diesel Golf 5 speed manual with no real improvement in highly mpg. I have had several entire tanks testing them where the mpg seems to go down. Wheel covers are one of the first mods many people try on this forum. There is a historical tradition of using Moon wheel covers on salt flats and intuitively they “look” like they would work much better than the blender blade mess of most alloy rims. I recently purchased Julian Edgar’s book and he quotes several studies of airflow and drag on car wheels. These studies show that totally smooth wheel covers are not the lowest drag option in modern automobiles. This would fit with my personal experience and motivated me to do some further testing in an A-B-A or even A-B-A-C-D options. I will post my test data and observations in a following 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?
How many,out of the total population of 'modern' cars are represented in these remarkable findings?

[aerohead]

Originally Posted by COcyclist

The VW GTI alloy 9 spoke rim gave me the best mpg in 2 days of testing and previous extended road trips. The alloy rims alone were better than with flat coroplast wheel covers.

Edit- It turns out this statement may be slightly misleading. Read on

The likelihood of isolating the drag contribution of wheels on a road trip is highly dubious.The SAE would never say it's even possible.This would be something which could only be confirmed in a wind tunnel.And the same wind tunnel,if done on separate days of testing.

[aerohead]

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.