Dum Wheel Aerodynamics Q

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?

Before you blurt out an answer, consider this: A disc wheel has infinite
spokes (or one spoke depending on how you look at it) and is always
considered the most aerodynamic of wheels.

Obviously air can flow between the spokes of a spoked wheel and can't in a
disk wheel. But, when rotating, wouldn't a high spoke wheel still be more
like a disc than a low spoke count?

-Andy B.

Andy Birko wrote:

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?

Before you blurt out an answer, consider this: A disc wheel has infinite
spokes (or one spoke depending on how you look at it) and is always
considered the most aerodynamic of wheels.

A disk wheel is like a flat plate, a spoked wheel is a collection of
tiny cylinders. Compare the drag characteristics of cylinders and
flat plates he

http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

--
terry morse Palo Alto, CA http://bike.terrymorse.com/

On Thu, 17 Jun 2004 19:22:08 -0400, Andy Birko wrote:

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?

Before you blurt out an answer, consider this: A disc wheel has infinite
spokes (or one spoke depending on how you look at it) and is always
considered the most aerodynamic of wheels.

Obviously air can flow between the spokes of a spoked wheel and can't in a
disk wheel. But, when rotating, wouldn't a high spoke wheel still be more
like a disc than a low spoke count?

-Andy B.



Well, if you want to use your own arguments, you already won! You see a
disc wheel as having only one spoke, and you say in your first paragraph
that low spoke wheels are more aerodynamic....you do the math.

32, 36, 40 whatever number of spokes has that many low pressure zones behind
each spoke and nipple as it rotates. A disc wheel only has one (or zero),
itself. The 40 spoke also has that many frontal areas.

IIRC, the first generation Specialized tri spoke carbon wheels actually
create lift in certain crosswind conditions, like a sail on a boat. That
would seem to me to have propulsion, not drag.

But hey, what do I know? I'm just a dumb ass machinist.
--
Skuke
Reverse the domain name to send email

On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko"
wrote:

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?

Before you blurt out an answer, consider this: A disc wheel has infinite
spokes (or one spoke depending on how you look at it) and is always
considered the most aerodynamic of wheels.

Obviously air can flow between the spokes of a spoked wheel and can't in a
disk wheel. But, when rotating, wouldn't a high spoke wheel still be more
like a disc than a low spoke count?

-Andy B.


Dear Andy,

The smooth disk [he blurted out] is a different kettle of
fish than wire rods whipping through the air like an egg
whisk.

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.

Another way to think of it is by comparing wheels to fans.
More blades (spokes) will whip up more air, but a smooth
disk attached to a fan motor is almost useless for stirring
up the air.

It's the interruption of the ideal surface that causes the
trouble. Another example would be circular saw blades. Until
you interrupt the rim with teeth or carbide grit, they
won't bite into the wood.

Carl Fogel

wrote in message
...
On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko"
wrote:

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?

carlfogel wrote:
On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko"
wrote:
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?

Before you blurt out an answer, consider this: A disc wheel has
infinite spokes (or one spoke depending on how you look at it) and is
always considered the most aerodynamic of wheels.

Obviously air can flow between the spokes of a spoked wheel and can't
in a disk wheel. But, when rotating, wouldn't a high spoke wheel still
be more like a disc than a low spoke count?

Carl Fogel



Not quite the same. There are two components to drag, skin friction and
frontal area. In the disk wheel, there is many times more skin friction
than a spoked wheel while the spoke wheel's drag is from the frontal
area of the spokes.

When you ask which one gives less drag, you have to further specify in
what conditions. While a disk wheel would be the best for high speed,
this high speed on a bike is close enough to prevailing winds and the
resultant apparant wind, if not in line with the bicycle's motion, can
induce far more drag than the disk wheel affords.

The best compromise is a moderate height rim (20-30mm), 16-24 bladed
spokes (butted and bladed are best like CX-Rays) and a smooth tire to
rim profile (advantage to clinchers). Disk wheels only offer advantages
in absolute still conditions, like indoor track.



--

Andy Birko wrote:

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?

There is no "disk" of turbulent air created by the spokes.
There is a lot of airspace between each spoke and the next,
so each generates a little turbulence behind it and gets its
own independent drag.
A long thin streamlined shape like a full disk or the spoke
of a Trispoke induces an airflow with a very different behavior.

IOW, as long as the airspace between spokes is larger than
the spoke diameter, you are in a different regime than the
disk and thinking of the disk as the limit of a large number
of spokes is not correct.

The dimples have something to do with delaying boundary layer
separation. I think "less friction between laminar and turbulent
than laminar and solid" is oversimplifying, see e.g.

http://www.physlink.com/Education/AskExperts/ae423.cfm

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?

Yes.

On Thu, 17 Jun 2004 20:51:30 -0400, "Andy Birko"
wrote:


wrote in message
.. .
On Thu, 17 Jun 2004 19:22:08 -0400, "Andy Birko"
wrote:

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

"Benjamin Weiner" wrote in message
news:40d24556$1@darkstar...

The dimples have something to do with delaying boundary layer
separation. I think "less friction between laminar and turbulent
than laminar and solid" is oversimplifying, see e.g.

When dimpled spokes appear on the market, I will know to whom the blame shuld be
assigned!
--
Mark South: World Citizen, Net Denizen

Also, would tread on a road tire break up the laminar flow and "dimpleize"
the air flow around the tire?

--
Phil, Squid-in-Training