Heresy! Smooth Wheel Covers- Myth Busted!!!
At least on the VW Golf
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 race cars 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. |
This is the most aerodynamic option
https://i.imgur.com/2XOyDbX.jpg
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 |
Which option worked best?
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When I put the mirrored moon discs on my Insight I already had excellent LRR Bridgestone Potenzas tires on it, I did it mostly for the look - the moon discs did make the car look way cooler and they got random strangers to stop and ask about it, which in turn got me talking to people about ecomodding In reality they also got me to work even harder at hypermiling - the car looked like it should do better with them on it so I drove it better if that makes sense |
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I’m curious. Did you use the same rim and tire combination with and without a flat covering ? Because if you didn’t, the difference noticed could have been from the overall weight of the combination used. Not the wheel covering. Lighter wheels make a BIG difference. Lighter wheels and a good LRR tires will make a even BIGGER difference. > |
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More to come. Edit- If this were a newspaper I would be printing a retraction. I posted the quick reply to the question above before looking at my notes. As I was entering the test data last night I realized that the coroplast wheel covers edged out the bare alloys by 1 mpg. |
Test Data Day One
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. This testing was done over 2 days. I chose a long straight road with very little traffic and open fields on both sides. The runs were almost 2 miles in each direction at about 62 mph or 100 kph. Day one the temperature was 44 degrees with a 7 mph wind from the west. The road goes east west. I averaged the east and west runs together. I used cruise control and left the car running while I added or removed wheel covers and used the same start and stop points for all runs. I used the ScanGuage set to current MPG and reset it at the start point with the car up to speed and cruise resumed.
My A runs were with factory steel rims and the factory wheel covers with worn snow tires. The B runs were the same wheel and tire combination but with the wheel covers off. A1 east/west average mpg- 54.7 B1 east/west average mpg- 57.85 (this was just the black steel rims) B2 east/west average mpg- 53.6 A2 east/west average mpg- 55.85 I averaged all the A runs and all the B runs and got 55 mpg and a few decimal points for both. There seemed to be more variation with changing wind perhaps, than the variation of wheel covers on or off. :mad: |
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20 pounds less of rotating mass on both tires total :eek: |
Test Data Day Two
My Day One results were inconclusive so I decided to do another round of testing. Day 2 was 59 degrees with the wind averaging 7 mph but occasionally gusting to 10-15 mph (as per weather radio from the airport just north of the test road). I increased the speed for all runs to 65 mph to try to highlight aerodynamic differences.
A1 runs Day 2 east/west average mpg- 48.45 B1 runs Day 2 east/west average mpg- 48.6 A2 runs Day 2 east/west average mpg- 50.65 The testing was still somewhat inconclusive. The differences could have been from wind changes or other traffic on the road. I went home and changed to my summer tires and rims. My summer tires are replacements of the stock Goodyears which are getting thin. They are mounted on VW Mk III GTI 9-spoke replica Minilite rims. I also have coroplast wheel covers held onto the spokes with zip ties. Again I left the car running while I removed or installed the wheel covers to keep the same cruise control setting for all these runs. The C runs are the naked alloy rims and the D runs have the smooth wheel covers on over them. C1 runs east/west average mpg- 56.45 D1 runs east/west average mpg- 54.1 (coroplast covers on) D2 runs east/west average mpg- 56.1 C2 runs east/west average mpg- 51.75 On my last run of the day my west bound data may have been corrupted. I encountered some slower traffic and had to brake before the end of the run. I got only 47 mpg for that leg. I averaged the C runs with the spinning alloy spokes open to the air and got 54.1 mpg. I averaged the D runs with smooth wheel covers and got 55.1 mpg. There is not a huge difference but the advantage goes to the alloy rims with the coroplast cover by 1 mpg. I have some thoughts on this but will update more tomorrow. The biggest difference was the change from snow tires on steel rims, covered or not, to lighter rims and summer tires- almost 5 mpg or roughly a 10% improvement! |
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 |
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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) |
I also found no improvement with wheel covers: https://ecomodder.com/forum/showthre...yon-32761.html.
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More thoughts
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.
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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
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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. |
busted
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How many,out of the total population of 'modern' cars are represented in these remarkable findings? |
road trips
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smooth wheels
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. |
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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." |
turning wheels/undercar
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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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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. |
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 |
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): https://www.youtube.com/watch?v=pTunD32hO44 |
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https://ecomodder.com/forum/attachme...1&d=1531604758 |
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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. https://i.postimg.cc/9fpztRwt/wheel-designs.png |
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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. https://ecomodder.com/forum/attachme...1&d=1587605142 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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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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You can nuance that any way you like, but that is radically different from the advice so widely given, including on this discussion group. |
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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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. https://ecomodder.com/forum/attachme...1&d=1587611928 |
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https://electrek.co/wp-content/uploa...0-07-24-am.png Honda Insight Gen 1 wheel: https://www.hubcaphaven.com/mm5/grap...1/aly63821.jpg |
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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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I think I'll leave this thread there - the evidence is available for those who wish to look. |
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https://i.postimg.cc/9fpztRwt/wheel-designs.png https://ecomodder.com/forum/attachme...1&d=1531604758 Tesla low drag wheels: https://electrek.co/wp-content/uploa...0-07-24-am.png Honda Insight Gen 1 wheel: https://www.hubcaphaven.com/mm5/grap...1/aly63821.jpg |
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