Convex or flat wheel covers ?

[Cd]

I had seen it mentioned in previous threads that a convex wheel disk is more aerodynamically efficient than a flat one.
Since the convex one sticks out into the wind and increases frontal area, how can it be better than a flat one ?
Thanks

After seeing the awesome disk that Orange4Boy is doing on his Prius, I wanted to ask the question again, but didn't want to 'jack the thread.

[Piwoslaw]

Actually, Orange4Boy couldn't find a flat pizza pan, so he installed a slightly concave wok

The only explanation I can think of (if this really works) is something about the sticking out hubcap not actually sitting in the airflow, but helping to reattach it (big '?'). This may work in the front, but the rear should be flat. Subscribed

[orange4boy]

Actually, Orange4Boy couldn't find a flat pizza pan, so he installed a slightly concave wok

Will wok for wheels. Who brought the soy sauce?

I have been thinking about this too. I have a set of the pop in style of moonies on the van and was never quite satisfied by the way the wheel looks in section. The tire has a bulge that returns to the rim, then the cap bulges again so you get three "curves" the air has to travel over.

When I tried my 18" platters, it made more sense that the curve continue where the tire leaves off

I think of it like the sugar cube-egg analogy. A tire is a blunt sugar cube rectangle in section. If you round out the sides, you are theoretically reducing detached flow which should reduce the wake. You can go from the cube up to as big as the egg and still have lower drag in that example.

I would love to be able to see the air flow pattern around my wheels to see the angles.

[orange4boy]

CFD images of a simulation of a stationary and rotating wheel: http://www.fluent.com/solutions/auto...bile_wheel.pdf

Check out the low pressure at the back of the rotating wheel.(lower image) Wheel boattailing anyone?

[AlaricD]

How would a golf-ball dimpled hubcap work compared to a flat hubcap?

[Christ]

Originally Posted by AlaricD

How would a golf-ball dimpled hubcap work compared to a flat hubcap?

It would create turbulent vortices sufficient to keep the air attached at the cap's surface, and force it to wrap around the shape of the wheel at the rear.

The problem comes when the rear of the moving wheel directs the flow into the fender pocket.

How ofter are your wheels truly straight?

[thatguitarguy]

Separating the forces of the wheel from the vehicle is not very helpful. I think the aerodynamic qualities of the shape of the hubcap, and it's effect on the overall aerodynamics of the vehicle will always be affected by the areas of the car fore and aft of the rotating wheel. A "one shape fits all" approach to flat or convex hubcaps probably won't work, and different shapes and sizes needed for different vehicles will be the case.

[gone-ot]

Originally Posted by Christ

How ofter are your wheels truly straight?

...two times:

1) at highway speeds on an uncrowned roadway somewhere in either northern AZ, NM, or Texas...or,

2) when parked.

[Piwoslaw]

Originally Posted by orange4boy

I think of it like the sugar cube-egg analogy. A tire is a blunt sugar cube rectangle in section. If you round out the sides, you are theoretically reducing detached flow which should reduce the wake. You can go from the cube up to as big as the egg and still have lower drag in that example.

I read in an aerodynamics book about research on lowering the Cd of busses that one of the options was to add a bulge in the front, where the airflow separates behind the windshield. From what I can remember, if the bulge is the correct size and shape, the airflow may stay attached, slightly reducing Cd. But the gain is less than what is lost to the increase in frontal area.
That is for busses. I've no idea if it's the same for a wheel that is rotating in a turbulent wheelwell.
As Thatguitarman said, it's all comes down to the individual car's aerodynamics. How close to attached is the airflow before and aft of front wheel?
I wonder if tuft testing a wheel while driving is possible (I don't mean the spare )?

[Christ]

The smoother the wheel is, the less vortex it will create. I was incorrect in my earlier post, the dimples on the wheel pan would only serve to create a thicker boundary layer, no vortices.

You ideally want as small a vortex as possible on the wheels, since the vortex is created by the wheels acting like an inefficient fan, sucking/pushing air during rotation. The smoother the wheel's surface is, the less the rotation matters, and the more it acts like a smooth stagnant surface, which is desirable.