Convex or flat wheel covers ?

[MetroMPG]

Originally Posted by 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 ?

From:

http://ecomodder.com/forum/showthrea...-7-a-2684.html

1963, Walter Korff ( Lockheed Aircraft ) claims "A smooth,slightly crowned disc out to the sidewall ( MOON EQUIPMENT CO. ),on each side of the wheel, provides a big improvement."

"out to the sidewall" may be the key bit there.

[Christ]

In defense of "out to the sidewall", I think this is because the "bulge" of the tire represents the point of highest frontal area, but doesn't normally stick out from the fender well. I think (THINK, no evidence) that the slightly convex wheel cover allows a larger area to stick into flow, keeping it steady, not allowing it to dip into the wheel house as much.

[aerohead]

I have no good science to contribute here.
I do recall reading at least one specific treatise on the difference but it didn't stick,I didn't flag the article.
One practical element of the MOON type disc,which borrows heavily from aeronautical technology,is that at the time it was introduced in hot-rodding,it would clear the dust caps on the spindles,something a flat cover would not.
With modern cars and different wheel offsets it may not be an issue for some vehicles.
A flat cover will not work with my trailer without hole-cutting the center.
Another aspect is strength.The convex shape adds tremendous rigidity to the cover which allows thinner material for same strength,and consequently a lower polar moment of inertia,something extremely important in racing.
Ryan Aeronautical calculated that the fabric wheel covers added 7-miles to the range of the Spirit of St. Louis and helped keep Charles Lindbergh out of the English Channel.
I believe convex is better aerodynamically,the proof is out there.I think modern aerospace technology speaks to the issue.

[donee]

Hi All,

This aerodynmic problem has been solved - kinda. Think frisbee and the successors to it.

So, do not dimple the flush hub caps, but put a little lip around the outer edge.

See: Aerobie® Flying Rings: Scientific Paper by Alan Adler, Inventor

Even Frisbee's have a rough set of ridges (also for grip) at the seperation points of the air-foil. But no dimples.

[Frank Lee]

Originally Posted by 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.

Solved!

CapAds not rotating wheel cover - No non not rotating advertising media hubcap

...and you can put big phat EM logos on there too.

[aerohead]

I dug into my mess.I'd read an explicit argument somewhere about the merits of the convex vs flat covers,however did not,or could not copy the article,so I don't have it.
So all I can do is default to Shinella's R&D of the Trans Am at Lockheed,Marrietta.They got numbers for an open steel wheel,a vented flat wheel cover,the same cover taped closed,and Shinella's
secret weapon discs which shaved another 0.003 off the Cd.
The production covers are convex and also vented and no data are presented for them,nor are the secret covers shown.
Since Ultralite sports a shallow convex,minimally perforated cover,and at 100-mpg,being fully optimized for low Cd,I would be comfortable with the notion that the crown of the convex cover has a distinct advantage over the flat.
We're splitting hairs though I think.

[cfg83]

aerohead -

Would it be safe to say that all the salt-flat racers with the slightly convex racing disks are the standard, or are they just following tradition?

CarloSW2

[gone-ot]

...not aerodynamic, but a perfectly "flat" disc will then to bend and flex under loads, whereas a "dished" disc (either concave or convex, doesn't matter) will tend to resist bending and flexing.

...I believe this is the reason they're convex.

[aerohead]

Originally Posted by cfg83

aerohead -

Would it be safe to say that all the salt-flat racers with the slightly convex racing disks are the standard, or are they just following tradition?

CarloSW2

I don't think it's tradition.I believe the MOON cover to be superior.
The crown adds hen'egg geodesic strength allowing a thinner/lighter gauge.
The crown also better respects Whitcomb's "area-rule" or the ballistician's "sectional density.
" The MOON cover doesn't add frontal area,as it is hiding behind the sidewall of the tire.
MOON's cover is actually thickest at the center,where strength is needed and polar moment the minimum.
MOON's are not inexpensive,and a flat home made unit might look attractive from price standpoint.
MOON's offer no brake cooling,but that's not an issue with lakebed racing as parachutes do the lion's share of deceleration.
The MOON 3-Dzus quick-fastening system makes the cover popular with racers,as they're the easiest to get on and off the car.
Southwest Airlines employs a MOON style cover for it's fleet of Boeing 737 airliners.Their bean-counters may have seen a cost-saving business advantage for takeoff operations,intra-gear-up ops,and landings.( landing gear can double the drag of a plane)( wasted expensive JET-A ).
Danika Patrick runs MOONs on the "inside" of her Indycar wheels.Multi-million dollar racing enterprises with access to the best wind tunnels wouldn't "guess" with their use.

[cfg83]

aerohead -

Originally Posted by aerohead

I don't think it's tradition.I believe the MOON cover to be superior.
The crown adds hen'egg geodesic strength allowing a thinner/lighter gauge.
The crown also better respects Whitcomb's "area-rule" or the ballistician's "sectional density.
" The MOON cover doesn't add frontal area,as it is hiding behind the sidewall of the tire.
...

Just a quick distinction. The snap-in racing disks do add frontal area. I know because mine was damaged going through a car wash that grabs the wheel. I think the snap-ins are compromising on the possibility that there won't be enough interior clearance, so they bow out even more.

The snap-ins are NOT the MOONs, they just look like them.

CarloSW2