Dum Wheel Aerodynamics Q

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?

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 here:

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

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

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?

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

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?

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

<[email hidden]> wrote in message
news:[email hidden]...

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?

carlfogel 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?

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 said:

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

Quoted message said:

Mind you I am talking about longitudinal drag (i.e. headwind etc.), not the
drag of the wheel spinning itself.

Quoted message said:

Are there any wind tunnel tests on this?

Yes.

Andy Birko said:

<[email hidden]> wrote in message
news:[email hidden]...

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

"Benjamin Weiner" <[email hidden]> wrote in message
news:40d24556$1@darkstar...

Quoted message said:

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

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?
-Andy B.

Just some quick followup. I've found from a reliable source that aero
advantage ends at about 24spokes. i.e. there's little difference in aero
advantage in a tri spoke over a 24spoke. Some of the wheel data at
analyticcycling.com bears this out.

Next question, would anyone trust a 24hole for a racing and training
wheel (zipp 505 or HED Jet 60 rim)

-Andy B.

--

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?

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?

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&c2coff=1&threadm=3ms5pk%241pa%40senator-bedfellow.MIT.EDU&rnum=16&prev=/groups%3Fq%3Dsail%2Beffect%2Bof%2Bdisk%2Bwheels%2Bgroup:rec.bicycles.tech%26hl%3Den%26lr%3D%26ie%3DUTF-8%26group%3Drec.bicycles.tech%26c2coff%3D1%26start%3D10%26sa%3DN

"Any symmetrical airfoil-like object with a blunt leading edge
can experience a forward force when subjected to a cross wind
component. The origin of this is called "leading edge suction",
and is confirmed by mountains of theoretical and experimental data.
Whether this leading edge suction can overcome the other surface
friction and pressure drag forces acting rearward very strongly
depends on the body's shape and surface quality. Apparently, for
some disk wheels the leading edge suction just barely wins."

Google rbt for

sail effect of disk wheels

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&c2coff=1&threadm=3536482B.6DC08D42%40princeton.edu&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26c2coff%3D1%26q%3Dsail%2Beffect%2Bof%2Bdisk%2Bwheels%26btnG%3DSearch%26meta%3Dgroup%253Drec.bicycles.tech

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&c2coff=1&threadm=6hjbbg%24en7%40hplntx.hpl.hp.com&rnum=3&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26c2coff%3D1%26q%3Dsail%2Beffect%2Bof%2Bdisk%2Bwheels%26btnG%3DSearch%26meta%3Dgroup%253Drec.bicycles.tech

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&c2coff=1&threadm=6hom1g%24a5c%40senator-bedfellow.MIT.EDU&rnum=7&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26c2coff%3D1%26q%3Dsail%2Beffect%2Bof%2Bdisk%2Bwheels%26btnG%3DSearch%26meta%3Dgroup%253Drec.bicycles.tech

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&c2coff=1&threadm=ErGwHC.vs%40freenet.buffalo.edu&rnum=5&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26c2coff%3D1%26q%3Dsail%2Beffect%2Bof%2Bdisk%2Bwheels%26btnG%3DSearch%26meta%3Dgroup%253Drec.bicycles.tech

"beerco" <[email hidden]> wrote in message
news:[email hidden]...

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?
-Andy B.

Just some quick followup. I've found from a reliable source that aero
advantage ends at about 24spokes. i.e. there's little difference in aero
advantage in a tri spoke over a 24spoke. Some of the wheel data at
analyticcycling.com bears this out.

Next question, would anyone trust a 24hole for a racing and training
wheel (zipp 505 or HED Jet 60 rim)

-Andy B.

For deep section rims (Zipp, Reynolds, et al) 24 spoke front wheels are
considered overkill, but are "just right" for rear wheels. I have two pair of
Zipps: 1 track pair with 24/24 and a road pair with 18/28. I bought the second
pair used, so didn't have a chance to pick the spokes.

Of course, this all depends on the rider NOT being 200#+

M

Weisse Luft <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

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.

If you are right about disk wheels versus 20-30mm height rims, you
should be advising Lance Armstrong and all those others who are using
disk wheels in flat time trials who are apparently ignorant of the
data you are citing.

"SDMike" <[email hidden]> wrote in message
news:hhGAc.10755$ey.734@fed1read06...

Quoted message said:

For deep section rims (Zipp, Reynolds, et al) 24 spoke front wheels are
considered overkill, but are "just right" for rear wheels. I have two

pair of

Quoted message said:

Zipps: 1 track pair with 24/24 and a road pair with 18/28. I bought the

second

Quoted message said:

pair used, so didn't have a chance to pick the spokes.

Of course, this all depends on the rider NOT being 200#+

M

Hmmm... Still sounds kind of scary to me. This would be built on a powertap
hub. I own a 32hole hub (built on an OP rim), but if I were to splurge
(which I can't right now) I'd build the 24 hole zip 505 for racing in 2x and
hang on to the 32 hole for training.

As it is, I may be stuck with 32 hole with whatever deep section rim I can
find for cheap.

-Andy B.

skuke <[email hidden]> wrote in message news:<[email hidden]>...

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?

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.

I think you know a lot. I agree with what you have said except the
part about being a dumb ass machinist, which sounds like an oxymoron.
I would like to get on my soapbox and add to what you have said,
please correct me if you think I'm wrong.

Lift and drag are very similar, both are propelling forces, both are
resistance forces, they even share the same directions (infinite). The
lift that the wheel creates in a crosswind can just as easily be
opposite the direction of the bikes forward motion as in it, depending
on its (cross wind)direction. The drag from a crosswind tends to
propel the bike in the direction of the wind. If this wind had a
forward component so will the drag caused by it. When a sailboats
sails directly down wind it is propelled entirely by aerodynamic drag.
Nasa sponsored web pages tell kids that drag is that force that makes
it harder to peddle when peddling against the wind. They never tell
kids that drag is that force that makes peddling easier or even
unnecessary when riding with the wind.

The wind is the only airflow that is influencing the bike but
aerodynamic force does not require an airflow or the motion of an
object through the air. Aerodynamic force only requires motion between
an object and air (relative air flow). As you most probably know the
relative airflow that is influencing the bike aerodynamically is
caused by a combination of the wind and the motion of the bike through
and while in (linear and circular) the air.

spock said:

skuke <[email hidden]> wrote in message news:<[email hidden]>...

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?

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.

I think you know a lot. I agree with what you have said except the
part about being a dumb ass machinist, which sounds like an oxymoron.
I would like to get on my soapbox and add to what you have said,
please correct me if you think I'm wrong.

Lift and drag are very similar, both are propelling forces, both are
resistance forces, they even share the same directions (infinite). The
lift that the wheel creates in a crosswind can just as easily be
opposite the direction of the bikes forward motion as in it, depending
on its (cross wind)direction. The drag from a crosswind tends to
propel the bike in the direction of the wind. If this wind had a
forward component so will the drag caused by it. When a sailboats
sails directly down wind it is propelled entirely by aerodynamic drag.
Nasa sponsored web pages tell kids that drag is that force that makes
it harder to peddle when peddling against the wind. They never tell
kids that drag is that force that makes peddling easier or even
unnecessary when riding with the wind.

The wind is the only airflow that is influencing the bike but
aerodynamic force does not require an airflow or the motion of an
object through the air. Aerodynamic force only requires motion between
an object and air (relative air flow). As you most probably know the
relative airflow that is influencing the bike aerodynamically is
caused by a combination of the wind and the motion of the bike through
and while in (linear and circular) the air.

Thanks for the kind words Spock.

I don't think your additional comments are wrong, but that's mostly cuz I'm
not sure what you said!

I think you basically said if the wheel is capable of creating lift
(propulsion), it doesn't neccessarily have to be in the forward direction.
Correct?

If so, then I'd like to modify it. Let's say the top half of the wheel is
creating forward lift. Since the shape of the sail/wing/spoke didn't change
and neither did the wind direction, then it would reason that the bottom
half of the wheel is the converse. It is also creating lift in the opposite
direction (backward). The two forces being equal because all the other
factors are equal, would negate each other and zero lift is the net result.

Personally, when I read the Specialized ad many years ago, I didn't
understand how it was possible for the reason I state above. But being that
I'm a machinist and not a fluid dynamics engineer, I took their word at
marketing face value. But I also further reasoned that if they were able to
get forward lift, they would continue to develop that wheel. Well, as we
are all aware, that wheel is dead and nobody else (that I'm aware) is
working on any "wind propulsion" wheel design. All the work is on weight
savings and clean air flow.

What I know about aerodynamics is tiny. I learned from people much more
knowledgable in that discipline than I. But in a nutshell, what I basically
learned is that the trailing edge is more important than the frontal area.
Take for example all the work going into Armstrong's TT bike and position.
Yes, the wizards are trying to decrease frontal area, but look at the
trailing edges of everything. They are all designed to reassemble the air
after the object in question passes through it. I guess it's probably
against the rules , but why haven't they tried adding fairings to the back
of Armstrong's triceps, calves, shoes and butt? They basically did that
with his head, in the name of "protective" equipment. There are lots of
rules governing how the bike must look, but are there rules on what the
rider can/can't wear (in regards to aerodynamic efficiency)?

....That's why I thought bladed spokes where more efficient. Not because the
leading knife edge allowed the spoke to better "cut" into the wind, but
because the trailing knife edge better reassembled the air molecules
creating less drag behind each spoke. Yes, the smaller leading edge
(frontal area) helped, but more benefit came from the back side.

I do disagree(?) with one statement above that you made:
"When a sailboats sails directly down wind it is propelled entirely by
aerodynamic drag."
I thought directly down wind (wind coming directly from the back of the
craft) was the only time a boat was being PUSHED by the wind. All other
wind angles (to a greater or lesser degree) involved the the boat being
PULLED by the sail. That is to say, there is some degree of lower pressure
on the front of the sail doing some amount of pulling versus whatever amount
in the back (concave side) being pushed.

I'm sure those more knowledgeable will correct me. Please do.
--
Skuke
Reverse the domain name to send email

skuke said:

... I guess it's probably
against the rules , but why haven't they tried adding fairings to the back
of Armstrong's triceps, calves, shoes and butt? They basically did that
with his head, in the name of "protective" equipment. There are lots of
rules governing how the bike must look, but are there rules on what the
rider can/can't wear (in regards to aerodynamic efficiency)?...

Here is a picture of a rider fairing used by Oscar Egg:
<http://www.velorizontal.com/images/mochet/oeufdeberthet.jpg>.

--
Tom Sherman – Quad City Area

Tom Sherman said:

Here is a picture of a rider fairing used by Oscar Egg:
<http://www.velorizontal.com/images/mochet/oeufdeberthet.jpg>.

Please tell me he doesn't have a "trap door" in his shorts!

Bill "some things better left unknown" S.

Quoted message said:

Thanks for the kind words Spock.

I don't think your additional comments are wrong, but that's mostly cuz I'm
not sure what you said!

I think you basically said if the wheel is capable of creating lift
(propulsion), it doesn't neccessarily have to be in the forward direction.
Correct?

If so, then I'd like to modify it. Let's say the top half of the wheel is
creating forward lift. Since the shape of the sail/wing/spoke didn't change
and neither did the wind direction, then it would reason that the bottom
half of the wheel is the converse. It is also creating lift in the opposite
direction (backward). The two forces being equal because all the other
factors are equal, would negate each other and zero lift is the net result.

Lets say the cross wind is coming at you at a 45 degree angle from the
rear. The lift generated by the wheel is 90 degrees or perpendicular
to the relative airflow that caused it. The direction of the lift
generated by the wheel has a rearward component. Hold your hand out in
front of you to represent the bike then hold the other hand out to
represent the wind. Lift is 90 degrees to the hand representing the
wind. If the wind were coming at you at a 90 degree angle there would
be no net lift just as you said. If the wind were coming at you from
45 degree angle from the front the lift generated by the wheel will
have a forward component. I have over simplified, because just because
the wind is coming at you at a particular angle does not mean that is
the angle of the relative airflow, sailors call it apparent wind. I
think the amount of lift generated by the wheel is negligible.

Quoted message said:

Personally, when I read the Specialized ad many years ago, I didn't
understand how it was possible for the reason I state above. But being that
I'm a machinist and not a fluid dynamics engineer, I took their word at
marketing face value. But I also further reasoned that if they were able to
get forward lift, they would continue to develop that wheel. Well, as we
are all aware, that wheel is dead and nobody else (that I'm aware) is
working on any "wind propulsion" wheel design. All the work is on weight
savings and clean air flow.

What I know about aerodynamics is tiny. I learned from people much more
knowledgable in that discipline than I. But in a nutshell, what I basically
learned is that the trailing edge is more important than the frontal area.
Take for example all the work going into Armstrong's TT bike and position.
Yes, the wizards are trying to decrease frontal area, but look at the
trailing edges of everything. They are all designed to reassemble the air
after the object in question passes through it. I guess it's probably
against the rules , but why haven't they tried adding fairings to the back
of Armstrong's triceps, calves, shoes and butt?

Making the surface rough will tend to have the same effect. It will
cause the flow to be more turbulent and stay attached longer around
Armstrong's triceps, calves, shoes and butt causing less wake and
dynamic drag.

They basically did that

Quoted message said:

with his head, in the name of "protective" equipment. There are lots of
rules governing how the bike must look, but are there rules on what the
rider can/can't wear (in regards to aerodynamic efficiency)?

...That's why I thought bladed spokes where more efficient. Not because the
leading knife edge allowed the spoke to better "cut" into the wind, but
because the trailing knife edge better reassembled the air molecules
creating less drag behind each spoke. Yes, the smaller leading edge
(frontal area) helped, but more benefit came from the back side.

The drag generated by a rotating bike tire is represented in a torque
force that opposes rotation. Each spoke pushes and pulls the air
around with it. This is characterized by areas of high pressure in
front of the spoke and low pressure behind it. This is called dynamic
drag. Dynamic drag can be totally eliminated by a disk wheel. The only
drag the rim generates is surface drag or friction drag or viscous
drag witch ever one you want to subscribe to.

Quoted message said:

I do disagree(?) with one statement above that you made:
"When a sailboats sails directly down wind it is propelled entirely by
aerodynamic drag."
I thought directly down wind (wind coming directly from the back of the
craft) was the only time a boat was being PUSHED by the wind. All other
wind angles (to a greater or lesser degree) involved the the boat being
PULLED by the sail. That is to say, there is some degree of lower pressure
on the front of the sail doing some amount of pulling versus whatever amount
in the back (concave side) being pushed.

I'm sure those more knowledgeable will correct me. Please do.

It is not only pushed by the wind it is pulled by the wind but that is
not what makes it drag. Pressure differentials are a characterization
of lift and dynamic drag. Any and all motion or tendency to move of a
solid object as a result of a relative airflow is an aerodynamic
force. There are only two aerodynamic forces (lift and drag). Lift is
perpendicular to the flow that caused it and drag is parallel or more
specifically in the same direction as the relative airflow that caused
it. Determine the direction of the sailboat and the relative airflow
that caused it and you can determine whether it is lift or drag. I
really appreciate the fact that you are not gullible. Always apply
what you are told with actual occurrence and question it if it
contradicts . Aircraft have circumnavigated the earth using drag
exclusively for horizontal acceleration yet texts define drag as a
resistance force. The major use of lift in aeronautics is to resist
gravity yet Texts define drag as a resistance force. Being a
resistance force is only a fraction of what lift and drag are all
about. I could write a book. If you think drag is a force that you
always have to overcome while ridding your bike carry a spinnaker with
you for a free ride down wind.