Experiment: smooth wheel discs tested A-B-A - 4.6% mpg improvement @ 65 mph

[volvompg]

Originally Posted by Thalass

Out of curiosity: Has anyone made transparent wheel covers? That way you'd get to still have nice looking wheels while improving your CoD.

I have not, but i wonder if you could take the clear lids off of the circular veggie trays from walmart. You could cut out a circle to fit your wheels and mount them to the edges of your rims with something like 3m adhesive tape?? I really dont know how durable this would be, but it would sure be cheap if you could just pick up some lids after a church luncheon or something!

[Otto]

Ideally, the wheel cover would be a smooth, slightly convex disk extending to the outer bulges on the tire sidewall, to make the wheel/tire have as smooth a face as possible.

See also Sig. Hoerner's cites and pics in his Fluid Dynamic Drag, which covers this nicely in various aircraft wind tunnel studies.

[Istas]

I would worry when extending any discs out to the sidewall bulge, about possible rubbing/chafing from repeated tire deformation when on the road. But maybe I'm being too paranoid. It certainly would be the best for aero.

As for the original post, congrats on the improvement! And good job getting a good bit of data down to back it up. Really a good argument for at least trying something, if it's that cheap to try.

[Otto]

You're not being too paranoid--the sidewalls have to flex, so extension of the wheel cover to the sidewall bulge is not practical, and the pizza pan would be bent or dislodged at the first pothole.

Best compromise would be a wheel cast into the proper shape which needs no hubcap, such as Porsche aluminum "sewer lid" or "manhole cover" wheels as seen on the 928 cars, although these were never a popular style. My Porsche 944 Turbo has "Design 90" wheels, which are similar but which have slots by the perimeter to allow brake cooling air to escape. Style is what drives wheel choices in the market, rather than aerodynamic or mechanical efficiency.

For an ecomodder standpoint, maybe the best application would be a pizza pan just a tad wider than the wheel rim, such that the pan snugs down against the rim and just barely touches the tire sidewall next to the rim. Attachment could be by conventional hubcap methods.

BTW, the mileage as reported by Concrete is excellent, especially given the size, weight, and basic shape of his vehicle.

[Cd]

Originally Posted by Otto

Ideally, the wheel cover would be a smooth, slightly convex disk extending to the outer bulges on the tire sidewall, to make the wheel/tire have as smooth a face as possible.

See also Sig. Hoerner's cites and pics in his Fluid Dynamic Drag, which covers this nicely in various aircraft wind tunnel studies.

I had seen MetroMPG mention this before as well. I had these wheels covers :

that I bought for cheap. Unfortunately, I misplaced them, and they no longer make this style.
I took them off because they were convex and I thought that they did more harm than good, since they would increase the frontal area slightly.

Please expand on this subject for me. I'm curious how that a convex cover can be better than a flat surface. ( I'm not doubting it - just confused. )

I have been to a few junk yards to look for flat covers, but none are 13 ".
The hub caps that are cheap at places like AutoZone are all the chrome 'pimped' style covers. Replacement hub caps online are a 'friggin' joke. $ 80 - $100 ... for hubcaps ??

[Cd]

- bump -

I'm about to buy some convex hub caps. I'd still like to know how that a convex wheel is better than a flat one. Thanks

[Vekke]

Originally Posted by Istas

I would worry when extending any discs out to the sidewall bulge, about possible rubbing/chafing from repeated tire deformation when on the road. But maybe I'm being too paranoid. It certainly would be the best for aero.

I did that in my rear wheels and they rubbed little pot holes to tires. So dont do that for your own safety!.

[Concrete]

shape is as important as cross section
a normal bicyclist has a Cd of ~1 but a bicyclist in a high end fairing can have a Cd of .1
usually we want that as drag reduction - but you could use it as size multipliers
Thus a good fairing allows you to enclose 10 times more space

or a combination
a fairing that is twice as big but drops your Cd to 1/4 of it's original still drops drag by 50%

same with wheel covers (fairing for your wheels)

In my truck's case the convex almost perfectly aligns with the bulge in the tire
it is a very smooth combo (see pic#2 in first post)

but it all depends on your vehicles aerodynamic configuration
so your milage may vary

[PaleMelanesian]

Cd - I have those same caps on my car. I found them a couple months back at Advance Auto. Do you have one of those nearby? I've also seen the same style on ebay, only in white.

[brickinthewind]

just chiming in to say that Concrete could also get some pimpin' style with that center-only attachment by converting those pizza pans into spinners. Paint some designs on 'em and watch 'em spin. It would just take a skate bearing in the middle in a housing. Maybe a script that says "SAVE GAS" when they spin at a certain speed?

Seriously, though, great job. You got me back in the hunt for disc wheel covers or something custom for my '04 Scion xB. I could always drill and tap the wheels but there has to be some disc cover out there for my steel wheels.