Andy Birko said:
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Quoted message said:Andy Birko said:All else being equal, what is it about fluid dynamics
which makes a low spoke wheel more aerodynamic than a
high spoke count wheel? Is this truly the case when
rotating?
There's a thin layer of turbulent air on a continuous
disk surface that to some degree "greases" the surface.
Each spoke, on the other hand, flies through the air with
the delicacy of a tumbling brick, its thinness being its
saving grace, and whips up its own horrifying mess of
turbulence.
I understand what you're getting at, but why doesn't this
turbulant air turn into an "air disk" so to speak? I.e., as
you've mentiones, there is less friction between laminar
air and turbulant air than there is between laminar air and
a solid (hence the dimples on golf balls and some of the
Zipps), so why doesn't this "disk" of turbulent air created
by the spokes act in a similar fashion?
Mind you I am talking about longitudinal drag (i.e.
headwind etc.), not the drag of the wheel spinning itself.
Are there any wind tunnel tests on this?
Dear Andy,
The turbulent horror whipped up by widely spaced spokes is a
swirling, billowing mess, with little to constrain it. The
leading edge of the spoke is directly compressing and
displacing helpless air in its path and also dragging innocent-bystander-
style air on either side into the partial void behind it,
where it gets slammed by the next spoke.
(Of course, the air was all knocked silly by the
comparatively massive tire and rim.)
In contrast, a smooth disk just rubs gently against the air.
It doesn't slam into it, the displacement is much gentler,
and the tiny layer of turbulent air greases things along
nicely. (For all I know, minute dimpling or dolphin-style
corrugations might improve things, but the leading edge of
the tire and rim complicate things.)
Remember, it's fluid dynamics. Think of trying to spin a
flat disk in water (a wet grinding wheel moves easily
through its water trough, stirring up little.) Now think of
of normal bicycle wheel with its spokes splashing through
the same water trough, each spoke slamming into the already
stirred-up water.
Perhaps at an outlandishly high speed, a crude "air disk"
might form, but the speed of the spokes would have to be
higher than the speed at which the air under normal
atmospheric pressure rushes into the low pressure area
behind the spoke and starts swirling madly.
With about 14-15 psi at sea level. a triple cylinder 50 cc
motorcycle engine can happily fill its tiny combustion
chambers at 22,500 rpm, which gives you some idea of how
fast air rushes back in behind moving objects. With a solid
disk, there's no rushing in and billowing out.
Keep in mind that aerodynamics is a beastly tricky business.
The Bernoulli pressure-velocity effect, for example, might
lead you to predict baseballs curving the wrong way, so you
have to reach for the Magnus-Robins drag effect or the
pitchers will snicker at you.
Similarly, the erratic flight of the almost spinless
knuckleball is often mistakenly invoked to explain the
inaccuracy of smooth-bore muskets, which actually are
inaccurate because they're practically guaranteed to put a
wicked (but unpredictable) spin on any lead ball--just not
the special spin of a rifled slug that is the only spin that
allows accuracy. The unrifled musket is just a device for
demonstrating the golfer's slice through 360 degrees.
The only parallel to the almost spinless knuckleball in
firearms is the special case of shotgun pellets, which are
contained in a plastic cup until they leave the barrel and
therefore acquire no spin. The spinless shotgun pellets are
remarkably accurate, as waterfowl and clay pigeons know to
their sorrow, since unlike baseballs with heavy raised
stitching and seams, the pellets are as smooth as the
ammunition makers can manage.
Modern smoothbore tank cannons--
I beg your pardon, We were discussing disk wheels and
spokes, not the outrageous price of Benjamin Robins' "New
Principles of Gunnery." If those used-book dealers think
that I'm going to pay $300 or even more for a lapsed
Quaker's 1742 ballistics experiments . . .
Carl Fogel
