Sail effect of disk wheels?
cwhite
On page 42 of the April 18, 1998 edition of
VeloNews, Charles Pelky makes comments regarding
the "sail effect" of disc and deep-dish wheels.
He states that "a sidewind generates a force
vector component in the direction of travel."
My understanding of Physics and sails tells me
that this is not so - there is no vector component
in the direction of travel. I understand that the
force on a sail is perpendicular to the sail
itself. Since the bicycle wheel is colinear with
the direction of travel, the force is then
perpendicular to the direction of travel. Because
of this there can be no beneficial sail effect
with bicycle wheels.
In sailboats, the sail is never colinear with the
direction of travel and therefore a beneficial
force vector component is present. There can be
aerodynamic benefits of deep-dish and disc wheels,
but I don't think these benefits can be attributed
to sail effect.
Does anyone have corrections, clarifications, or
other comments with regard to Pelkey's and my
thoughts?
Tim McNamara
>On page 42 of the April 18, 1998 edition of
>VeloNews, Charles Pelky makes comments regarding
>the "sail effect" of disc and deep-dish wheels.
>He states that "a sidewind generates a force
>vector component in the direction of travel."
>
>My understanding of Physics and sails tells me
>that this is not so - there is no vector component
>in the direction of travel.
<snip>
>Does anyone have corrections, clarifications, or
>other comments with regard to Pelkey's and my
>thoughts?
You are right and Pelkey is wrong, according to all the engineering types
who put their heads together about this when I asked two years ago. Since
the wheel is symmetrical, any lift generated (which is the only way the
wheel could generate a force vector from the wind) is cancelled out, since
it's lifting in both directions (if I understand the previous discussions
correctly).
Now, a good stiff crosswind and a pair of disc wheels ought to make your
life *real* interesting!
--
Tell me all that you know- I'll show you
snow and rain.
-Robert Hunter
THolland63
cwhite says:
> In sailboats, the sail is never colinear with the
> direction of travel and therefore a beneficial
> force vector component is present.
Is this why pulling the sail all the way in (so the boom is parallel with the
centerboard) results in capsizing the boat?
I think you're onto something.
Crosswinds suck,
T. Holland
PS Didn't they also metion that smaller wheels have more drag because the upper
portion of the spokes move faster into the wind? Hah!
Dave Bailey
cwhite <cwh...@cruzio.com> wrote:
>On page 42 of the April 18, 1998 edition of
>VeloNews, Charles Pelky makes comments regarding
>the "sail effect" of disc and deep-dish wheels.
>He states that "a sidewind generates a force
>vector component in the direction of travel."
>
>My understanding of Physics and sails tells me
>that this is not so...
[...]
>Does anyone have corrections, clarifications, or
>other comments with regard to Pelkey's and my
>thoughts?
What was his point? Maybe he meant that the sidewind
generates a force in the direction of travel of the wind, and
that disk and deep-dish wheels are less stable in crosswinds
because of the "sail" effect.
--
Dave Bailey
dbail...@mindspring.com
Stergios Papadakis
cwhite wrote:
> On page 42 of the April 18, 1998 edition of
> VeloNews, Charles Pelky makes comments regarding
> the "sail effect" of disc and deep-dish wheels.
> He states that "a sidewind generates a force
> vector component in the direction of travel."
>
> My understanding of Physics and sails tells me
> that this is not so - there is no vector component
> in the direction of travel. I understand that the
> force on a sail is perpendicular to the sail
> itself. Since the bicycle wheel is colinear with
> the direction of travel, the force is then
> perpendicular to the direction of travel. Because
> of this there can be no beneficial sail effect
> with bicycle wheels.
>
> In sailboats, the sail is never colinear with the
> direction of travel and therefore a beneficial
> force vector component is present. There can be
> aerodynamic benefits of deep-dish and disc wheels,
> but I don't think these benefits can be attributed
> to sail effect.
>
> Does anyone have corrections, clarifications, or
> other comments with regard to Pelkey's and my
> thoughts?
Actually, it is possible to generate some forward component
of force. An airfoil generates lift nearly perpendicular
to the apparent wind, not perpendicular to the foil (a foil with zero
drag would generate a force
exactly perpendicular to the wind, drag tilts the vector back a little
bit).
If the apparent wind is, for example, 20 degrees
off of straight ahead, a disk wheel will generate a component
of force along the direction of travel.
Another poster mentioned that this won't work because the
wheels are symmetrical, but symmetrical foils do generate lift if they
are at a non-zero
angle of attack. Try sticking a flat board out your moving car
window and then tilting it. In the example I gave, the wheels are at
20 degrees angle of attack.
Stergios
Stergios Papadakis
THolland63 wrote:
> cwhite says:
> > In sailboats, the sail is never colinear with the
> > direction of travel and therefore a beneficial
> > force vector component is present.
>
> Is this why pulling the sail all the way in (so the boom is parallel with the
> centerboard) results in capsizing the boat?
> I think you're onto something.
>
>
Actually, I race sailboats, and centerlining the boomis often the correct sail trim
for going upwind. It
depends on the conditions, of course. Also, the sail
always twists off as it goes up. But this doesn't
matter, see my other post in this thread on why
the disk wheels can generate some forward force.
> PS Didn't they also metion that smaller wheels have more drag because the upper
> portion of the spokes move faster into the wind? Hah!
The top of the wheel is always going at twice the speed of
the bike, no matter what the diameter of the wheel.
Stergios
Dan Connelly
I believe these statements are based on data
such as those available at:
http://www.cs.umd.edu/users/humphrie/wheels.html
which references
Greenwell, et al.
Aerodynamic Characteristics of Low-Drag Bicycle Wheels.
Aeronautical Journal. March 1995.
Clearly a direct side wind on a stationary bike, or
a direct "relative" side wind, can't break symmetry
and move the bike forward. However, the question is
whether a net headwind can... the data do give HED disks
a negative Cr at headwind angles.
Dan
Dave Bailey wrote:
>
> cwhite <cwh...@cruzio.com> wrote:
>
> >On page 42 of the April 18, 1998 edition of
> >VeloNews, Charles Pelky makes comments regarding
> >the "sail effect" of disc and deep-dish wheels.
> >He states that "a sidewind generates a force
> >vector component in the direction of travel."
> >
> >My understanding of Physics and sails tells me
> >that this is not so...
> [...]
> >Does anyone have corrections, clarifications, or
> >other comments with regard to Pelkey's and my
> >thoughts?
>
> What was his point? Maybe he meant that the sidewind
> generates a force in the direction of travel of the wind, and
> that disk and deep-dish wheels are less stable in crosswinds
> because of the "sail" effect.
>
> --
> Dave Bailey
> dbail...@mindspring.com
cwhite
Two things:
1) it was April 13, issue of VeloNews
2) and it was an Article by Lennard Zinn
Doug Milliken
On Mon, 13 Apr 1998, Stergios Papadakis wrote:
> cwhite wrote:
>
> > On page 42 of the April 18, 1998 edition of
> > VeloNews, Charles Pelky makes comments regarding
> > the "sail effect" of disc and deep-dish wheels.
> > He states that "a sidewind generates a force
> > vector component in the direction of travel."
> > <snip>
> Actually, it is possible to generate some forward component
> of force. An airfoil generates lift nearly perpendicular
> to the apparent wind, not perpendicular to the foil (a foil with zero
> drag would generate a force
> exactly perpendicular to the wind, drag tilts the vector back a little
> bit).
> If the apparent wind is, for example, 20 degrees
> off of straight ahead, a disk wheel will generate a component
> of force along the direction of travel.
>
> Another poster mentioned that this won't work because the
> wheels are symmetrical, but symmetrical foils do generate lift if they
> are at a non-zero
> angle of attack. Try sticking a flat board out your moving car
> window and then tilting it. In the example I gave, the wheels are at
> 20 degrees angle of attack.
>
> Stergios
Very well put. The place where it gets confusing is that in the wind
tunnel, lift (actually sideforce for a bike) and drag forces are usually
reported on wind axes, but what counts when you are riding is the
"x-force", aka, drag-along-the-bike-axis and the "y-force" that is trying
to push you sideways off the road. "Lift" on wind axes can translate to a
positive "x-force" on the bike. Do the transform (draw the vectors) to see
what is happening.
From memory of various wind tunnel tests (I'm too lazy to look up the
data), the positive x-force that can be realized from one or two disk
wheels is not sufficient to overcome the air drag of a normal bike and
rider. So, the best that can be hoped for is probably an apparent
reduction of air drag, when the bike is in an appropriate crosswind.
To take a more radical case, I've built full fairings for my hpv's that
sail quite nicely. These are shaped like a symmetric airfoil when seen in
top view. On a level road, in a good "reach" (wind at about right-angles
to my _path_over_the_ground_) this bike sails along nicely at 15-20 mph
with no pedaling at all.
Warning! "Small boat warnings apply" and these big fairings get very scary
in gusty winds and/or near big fast trucks! They are really only good for
closed course hpv races -- see www.ihpva.org for lots of pictures. My
fairings are based on Alex Moulton bikes so they are ridden in the normal
riding position, but most fairings seem to wind up on recumbents...
Happy land sailing,
-- Doug
Doug Milliken <bd427@@freenet.buffalo.edu>
Daniel Connelly
cwhite wrote:
>
>...it was an Article by Lennard Zinn
Please, leave the ad-hominum attacks out of it.
Address the points being made.
The windtunnel data show negative drag.... I have
already cited a reference for this, and it has already
been pointed out that symmetry arguments don't forbid
negative drag from occuring in a headwind. Whether
this is a simple application of Bernouilli or a complex
issue of turbulent flow I cannot say (I recommend
http://cycling.org/lists/hardcore-bicycle-science/ for this
sort of thing), but the effect is potentially quite real.
Dan
Gregory White
Daniel Connelly wrote:
> cwhite wrote:
> >
> >...it was an Article by Lennard Zinn
>
> Please, leave the ad-hominum attacks out of it.
> Address the points being made.
I don't know what "ad-hominum" means and I certainly don't know what
"attacks" your talking about. I made the original mistake of
attributing the article to the wrong author and then corrected it. I'll
leave the decision regarding what points I address up to myself - you'll
play no part. If I didn't care about the issue, and its points, I would
not have brought it up in the first place.
> The windtunnel data show negative drag.... I have
> already cited a reference for this, and it has already
> been pointed out that symmetry arguments don't forbid
> negative drag from occuring in a headwind.
That doesn't mean that it is a "sail effect" - at least as far as what I
picture as sail effect. It might more appropriately be called a "lift"
or "wing effect" by what I gather from the better educated posters on
this newsgroup. Perhaps the problem is, in part, layperson definitions
for things like sail effect. It may only be a mincing of words.
> ...but the effect is potentially quite real.
At this point, I think that it is real, but "sail effect"....?
Dave Bailey
Gregory White <gwh...@metricom.com> wrote:
>
>
>Daniel Connelly wrote:
>
>> cwhite wrote:
>> >
>> >...it was an Article by Lennard Zinn
>>
>> Please, leave the ad-hominum attacks out of it.
>> Address the points being made.
>
>I don't know what "ad-hominum" means and I certainly don't know what
>"attacks" your talking about. I made the original mistake of
>attributing the article to the wrong author and then corrected it.
It's "ad hominem" - 'to the man'. An ad hominem attack is
essentially a personal insult. Dan's implication that the mere
mention of Lennard Zinn constitutes an ad hominem attack
could be interpreted as a wry comment about Zinn's notoriety
in the bike tech community. You'll recall that one of Zinn's
previous VN articles was a study on aero wheel drag, sponsored
by Spinergy, which surprisingly found Spinergy wheels to have
the lowest drag coefficient of any aero wheel - a finding which
was contradicted by other studies, and which consequently
became the topic of a raging debate on this very newsgroup.
>That doesn't mean that it is a "sail effect" - at least as far as what I
>picture as sail effect. It might more appropriately be called a "lift"
>or "wing effect" by what I gather from the better educated posters on
>this newsgroup. Perhaps the problem is, in part, layperson definitions
>for things like sail effect. It may only be a mincing of words.
Essentially what we have here is a semantic disagreement between
you and Zinn. None of the terms is exactly applicable, so no matter
which is chosen, somebody will be unhappy. All three terms - "sail
effect", "lift effect", "wing effect" describe exactly the same
physical phenomenon - that airflow around a solid object exerts
a force on the object. We're no closer to understanding the physics,
but it's a fair bet that rigid body dynamics, which you invoked at the
start of this thread, are not the tool of choice when attempting to
make statements about aerodynamic flow.
--
Dave Bailey
dbail...@mindspring.com
Cervelo Cycles
Tim McNamara wrote:
>
> In article <35311CFB...@cruzio.com>, cwh...@cruzio.com wrote:
>
> >On page 42 of the April 18, 1998 edition of
> >VeloNews, Charles Pelky makes comments regarding
> >the "sail effect" of disc and deep-dish wheels.
> >He states that "a sidewind generates a force
> >vector component in the direction of travel."
> >
> >in the direction of travel.
> <snip>
> >other comments with regard to Pelkey's and my
> >thoughts?
>
> who put their heads together about this when I asked two years ago. Since
> the wheel is symmetrical, any lift generated (which is the only way the
> wheel could generate a force vector from the wind) is cancelled out, since
> it's lifting in both directions (if I understand the previous discussions
> correctly).
The wheel is symmetrical, but in a crosswind the set-up isn't. The fact
that the wheel is symmetrical only means there is no lift in a pure
headwind, but in a crosswind there can be. In this case the lift (which
is perpendicular to the air flow) can have a component in the direction
of travel of the bike, which lowers the apparent drag of the wheel. But
to get a net thrust is quite hard, and it only happens in very specific
conditions.
Sincerely,
Gerard Vroomen, Cervelo Cycles
e-mail: intern...@cervelo.com
website: http://www.cervelo.com
jver...@lucent.com
In article <timmcn-1204...@dialup-pm1-23.minn.net>,
tim...@minn.net (Tim McNamara) wrote:
>
> In article <35311CFB...@cruzio.com>, cwh...@cruzio.com wrote:
>
> >On page 42 of the April 18, 1998 edition of
> >VeloNews, Charles Pelky makes comments regarding
> >the "sail effect" of disc and deep-dish wheels.
> >He states that "a sidewind generates a force
> >vector component in the direction of travel."
> >
> >My understanding of Physics and sails tells me
> >that this is not so - there is no vector component
> >in the direction of travel.
> <snip>
> >Does anyone have corrections, clarifications, or
> >other comments with regard to Pelkey's and my
> >thoughts?
>
> You are right and Pelkey is wrong, according to all the engineering types
> who put their heads together about this when I asked two years ago. Since
> the wheel is symmetrical, any lift generated (which is the only way the
> wheel could generate a force vector from the wind) is cancelled out, since
> it's lifting in both directions (if I understand the previous discussions
> correctly).
I'm not arguing with either Pelkey or the "engineering types", but most
lens-shaped rear discs I've ever seen (Hed, Campy) are not symmetrical. The
non-drive side on a rear disc is typically more convex than the drive side,
same as dishing on a spoked wheel.
> Now, a good stiff crosswind and a pair of disc wheels ought to make your
> life *real* interesting!
A front disc in a crosswind will make your life interesting, I've never
noticed much problem using my Hed rear disc though. I've been using it in all
weather conditions since 1990, never had an "interesting" moment as a result
of crosswinds. Think about the cross-section your body presents to a
side-wind, and how large and high it is in relation to a disc wheel.
Something else I noticed: I've ridden both a flat (Zipp) and lens-shaped
(Hed) rear disc in a crosswind. While I can't notice a difference between the
Hed and a spoked wheel (in terms of side force), the flat disk does seem to
exert a bit more side force on the bike. I've also ridden my road bike short
distances while carrying each of these discs (one at a time). Even at low
speed (~15mph) it's nealy impossible to keep the flat disc straight - it
"wants" to sail in either direction. This is much less an issue with the lens
disc.
I've got theories on all this (my degree is in Math and I work in the
computer industry - am I an "engineering type"?), but I'm just reporting my
personal experience here - having used disc wheels fairly regularly for the
last 8 years.
John
> --
> Tell me all that you know- I'll show you snow and rain.
>
> -Robert Hunter
>
"When ignorance reigns life is lost...lost...LOST!"
-Zach de la Rocha
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
Keith Klein
Gregory White wrote:
> > The windtunnel data show negative drag.... I have
> > already cited a reference for this, and it has already
> > been pointed out that symmetry arguments don't forbid
> > negative drag from occuring in a headwind.
>
> That doesn't mean that it is a "sail effect" - at least as far as what I
> picture as sail effect. It might more appropriately be called a "lift"
> or "wing effect" by what I gather from the better educated posters on
> this newsgroup. Perhaps the problem is, in part, layperson definitions
> for things like sail effect. It may only be a mincing of words.
>
> > ...but the effect is potentially quite real.
>
> At this point, I think that it is real, but "sail effect"....?
If I might weigh in here: A sail generates "lift" to propel the boat
forward. In fact most sail boats rigged with fore-and-aft sails go faster
into the wind than away from it for this very reason. They can't point
directly into the wind because the filling of the sail generates the airfoil
shape which in turn provides the lift. Square riggers are less efficient at
generating this shape, and so must point "off the wind" to a greater degree,
altough they are better at going down wind. So I guess that "sail
effect"would be proper.Keith
Old, slow, and in the way
Jobst Brandt
Gerard Vroomen writes:
> The wheel is symmetrical, but in a crosswind the set-up isn't. The
> fact that the wheel is symmetrical only means there is no lift in a
> pure headwind, but in a crosswind there can be. In this case the
> lift (which is perpendicular to the air flow) can have a component
> in the direction of travel of the bike, which lowers the apparent
> drag of the wheel. But to get a net thrust is quite hard, and it
> only happens in very specific conditions.
This claim has been made often in this newsgroup with no evidence to
back it up. In response, I took a Specialized Trispoke wheel that was
said to have these crosswind qualities, and mounted it in a bicycle
fork attached to the end of a broomstick. This wheel, held out of the
window of a moving car produced no rotation nor did it sustained any
initial rotation in any orientation to the wind. The wheel was held
so that it was not in the air stream of tha car.
I performed this experiment in response to all the wind claimes made by
the various expensive wheels offered because they are so preposterous.
Nothing has changed.
Jobst Brandt <jbr...@hpl.hp.com>
Doug Milliken
On Tue, 14 Apr 1998, Daniel Connelly wrote:
> The windtunnel data show negative drag.... I have
Just to keep the terminology straight -- "drag" is normally measured on
wind axes, parallel to the airflow -- it won't be negative for bike parts.
The component that can be "negative" (==thrust) is the "x-force" (for lack
of a better term) which is aligned with the centerline of the bike.
To address the question of "sailing", my understanding is that boats made
with symmetric vertical airfoils (like the vertical tail on an airplane)
can sail in certain winds, even if the airfoil is fixed with its chord-line
(centerline) aligned with the centerline of the boat. The ability to
"steer" the sail around to different headings increases the range of
relative winds and headings that the boat can sail on.
The key to sailing is that the vehicle is at the interface between two
media traveling at different speeds -- air (moving wind) and water (near
stationary); for bikes with vertical surfaces in the wind, the tires supply
the keel effect much more efficiently than a keel in water.
> http://cycling.org/lists/hardcore-bicycle-science/ for this
A similar discussion should be in the HBS archives somewhere... HBS is
on hiatus at the moment but I suspect that Jim P will start up again
soon.
-- Doug Milliken
Damon Rinard
jver...@lucent.com wrote:
> I'm not arguing with either Pelkey or the "engineering types", but most
> lens-shaped rear discs I've ever seen (Hed, Campy) are not symmetrical. The
> non-drive side on a rear disc is typically more convex than the drive side,
> same as dishing on a spoked wheel.
The new Mavic sandwich disks appear to be highly dished.
The old Masi disks (who made those, anyway?) are dished.
But my HED lens disk wheel is symmetrical.
The Accel tensioned Kevlar(r) track disk I have is symmetrical.
The Russian road and track tension disks appear to be symmetrical.
Damon Rinard
Damon Rinard's Bicycle Tech Site:
http://home.earthlink.net/~rinard/
Stergios Papadakis
Jobst Brandt wrote:
I think you are confused about the original question.Picture this:
a disk bicycle wheel is touching the road and the wind
is blowing such that the angle of attack of the wheel is 15
degrees.
What are the forces on the wheel?
One is aerodynamic lift: the airflow over the wheel produces some force
perpendicular to the apparent wind.
Another is aerodynamic drag: The airflow produces some force
parallel to the apparent wind.
The vector sum of these two forces is the total aerodynamic force
on the wheel. That vector probably points around 80 degrees
off of the wheel's direction of travel.
Now, the other forces on the wheel are rolling resistance, which
is negligible, and the static friction that keeps the wheel from
being swept sideways (parallel to the axle) by the wind.
It is this static friction that is NECESSARY for you to detect
the thrust.
So far, we have the total aerodynamic forces, which are
trying to pull the wheel in a direction 80 degrees off of its
direction of travel, and we have the force generated by the rubber
against the ground, which points perpendicular to the
direction of travel, nearly opposite the aerodynamic force. Add the
vectors and you get a small vector left over, pointing along
the direction of travel. When I say small, I mean about 1/5
of the total aerodynamic force.
The bike frame counteracts that force, it prevents the
wheel from accelerating away.
The experiment you performed, with the wheel off of the ground,
will of course show nothing. The side force from the tire on
the ground is necessary.
On a regular bicycle, where the rider is a big blob that
creates a lot of drag and no lift, you will never have the
wind accelerate you, but if you look at the forces on a disk
wheel alone, the sum of the aerodynamic forces and the road
forces are pushing the wheel forward while the drag of the
frame and rider are holding it back.
If you want an example of a vehicle that can easily accelerate
into the wind, look at an iceboat. It operates on this same
principle. Start with the boat stationary and the wind
45 degrees off the starboard bow. Pull the sail all the way in and let
the boat go. The boat will accelerate until the apparent
wind is nearly dead ahead. Now the sail is generating a huge
force to the left, and the runner blades are generating a huge force
to the right, and there is about 1 pound of aerodynamic force that
points in the direction of travel that counteracts the 1 pound
of sliding force from the skates.
Sailboats (the kind that float) do this also also, but
the keel is far less efficient than either a tire on the road or
a skate on ice at preventing sideways motion while allowing
forward motion.
Stergios
Doug Milliken
On 15 Apr 1998, Jobst Brandt wrote:
> Gerard Vroomen correctly writes:
>
> > The wheel is symmetrical, but in a crosswind the set-up isn't. The
> > fact that the wheel is symmetrical only means there is no lift in a
> > pure headwind, but in a crosswind there can be. In this case the
> > lift (which is perpendicular to the air flow) can have a component
> > in the direction of travel of the bike, which lowers the apparent
> > drag of the wheel. But to get a net thrust is quite hard, and it
> > only happens in very specific conditions.
>
> This claim has been made often in this newsgroup with no evidence to
> back it up. In response, I took a Specialized Trispoke wheel that was
> said to have these crosswind qualities, and mounted it in a bicycle
> fork attached to the end of a broomstick. This wheel, held out of the
> window of a moving car produced no rotation nor did it sustained any
> initial rotation in any orientation to the wind. The wheel was held
> so that it was not in the air stream of tha car.
>
> I performed this experiment in response to all the wind claimes made by
> the various expensive wheels offered because they are so preposterous.
> Nothing has changed.
I've tried to explain this to you before, Jobst, but you haven't
gotten it yet... you are not doing the right experiment and you are
not looking at the proper resolution of the aero forces.
In the wind tunnel (and in aerodynamics in general) drag, lift, sideforce,
etc are defined on wind axes. When a disk wheel is yawed in the wind
tunnel it shows some increase in drag (along the direction of the wind)
and also a large sideforce, at _right_angles_to_the_wind.
When these forces are viewed from the point of view of the bike, then
the vector force (from the wheel alone) can be in the + x-force
direction. On a bike, this will reduce the drag in certain wind
conditions.
If you take this to an extreme, hpv fairings (which can be shaped like
symmetrical airfoils in top view) will allow a bike to sail quite nicely,
I've done it for miles at a time, -- reacting the wind forces against the
ground in a fashion just like a sail boat reacts the wind against the keel
in the water.
Stergios Papadakis
Stergios Papadakis wrote:
> The experiment you performed, with the wheel off of the ground,
> will of course show nothing. The side force from the tire on
> the ground is necessary.
>
>
> Stergios
Actually, this isn't precisely true. You could do the experiment
like you performed, just putting the wheel in an airflow,
but you would need a device to measure the
total aerodynamic force vector on the wheel. Whether
or not the wheel spins doesn't matter, just the
direction of that vector.
Stergios
Jobst Brandt
Doug Milliken writes:
>> This claim has been made often in this newsgroup with no evidence
>> to back it up. In response, I took a Specialized Trispoke wheel
>> that was said to have these crosswind qualities, and mounted it in
>> a bicycle fork attached to the end of a broomstick. This wheel,
>> held out of the window of a moving car produced no rotation nor did
>> it sustained any initial rotation in any orientation to the wind.
>> The wheel was held so that it was not in the air stream of the car.
>> I performed this experiment in response to all the wind claims
>> made by the various expensive wheels offered because they are so
>> preposterous. Nothing has changed.
> I've tried to explain this to you before, Jobst, but you haven't
> gotten it yet... you are not doing the right experiment and you are
> not looking at the proper resolution of the aero forces.
> In the wind tunnel (and in aerodynamics in general) drag, lift,
> sideforce, etc are defined on wind axes. When a disk wheel is yawed
> in the wind tunnel it shows some increase in drag (along the
> direction of the wind) and also a large sideforce, at
> _right_angles_to_the_wind.
> When these forces are viewed from the point of view of the bike,
> then the vector force (from the wheel alone) can be in the + x-force
> direction. On a bike, this will reduce the drag in certain wind
> conditions.
The claim was that wind imparts rotational energy to the Trispoke and
that is what I measured. Now you are proposing that this is a sail
effect in which the rotation would have no effect. I don't believe
that anyone has measured any propulsive force caused by these wheels.
That some wheels have less drag than others has never been disputed,
only that net propulsive force is generated by any of these wheels.
> If you take this to an extreme, HPV fairings (which can be shaped
> like symmetrical airfoils in top view) will allow a bike to sail
> quite nicely, I've done it for miles at a time, -- reacting the wind
> forces against the ground in a fashion just like a sail boat reacts
> the wind against the keel in the water.
Your example is not analogous to a wheel that is not airfoil (tear
drop) shaped but has fore and aft symmetry. I'm sure your vehicle did
not have a symmetrical canoe shape as a bicycle disk essentially has,
or it would not have had a preferred direction of travel in a pure
side wind and especially not in a headwind. That the vanes of the
Trispoke are air foil shaped is self canceling because they average to
zero due to radial symmetry. I think my experiment, that was designed
to disprove the claim, tested that effect.
In your HPV example you didn't mention from where the wind came.
Jobst Brandt <jbr...@hpl.hp.com>
Contractor - Eric Cross
Jobst Brandt (jbr...@hpl.hp.com) wrote:
: Gerard Vroomen writes:
: > The wheel is symmetrical, but in a crosswind the set-up isn't. The
: > fact that the wheel is symmetrical only means there is no lift in a
: > pure headwind, but in a crosswind there can be. In this case the
: > lift (which is perpendicular to the air flow) can have a component
: > in the direction of travel of the bike, which lowers the apparent
: > drag of the wheel. But to get a net thrust is quite hard, and it
: > only happens in very specific conditions.
: This claim has been made often in this newsgroup with no evidence to
: back it up. In response, I took a Specialized Trispoke wheel that was
: said to have these crosswind qualities, and mounted it in a bicycle
: fork attached to the end of a broomstick. This wheel, held out of the
: window of a moving car produced no rotation nor did it sustained any
: initial rotation in any orientation to the wind. The wheel was held
: so that it was not in the air stream of tha car.
: I performed this experiment in response to all the wind claimes made by
: the various expensive wheels offered because they are so preposterous.
: Nothing has changed.
: Jobst Brandt <jbr...@hpl.hp.com>
Not that I am quick to believe the wheel builders' claims, but your
experiment above does not include what effects wind has on a spinning
wheel, but only what on a wheel which is not moving. Could wind affect
a spinning wheel differently than it affects a wheel which is not
spinning? I have no idea, but I thought you may have some input on
this.
Eric
John Getsoian
On Thu, 16 Apr 1998 04:04:44 GMT, Doug Milliken wrote:
>If you take this to an extreme, hpv fairings (which can be shaped like
>symmetrical airfoils in top view) will allow a bike to sail quite nicely,
>I've done it for miles at a time,
Doug;
Just to clarify, when you say 'symmtrical airfoils in top view' do
you mean the fairings are symmetrical in both the x _and_ y
directions, like a wheel viewed from the top, or just in y, i.e.,
when reflected along the long axis, like Dennis Conner's rigid
catamaran sail?
regards,
john getsoian
jget...@nojunk.csi.com
delete 'nojunk'
Jobst Brandt
Contractor - Eric Cross writes:
>> This claim has been made often in this newsgroup with no evidence
>> to back it up. In response, I took a Specialized Trispoke wheel
>> that was said to have these crosswind qualities, and mounted it in
>> a bicycle fork attached to the end of a broomstick. This wheel,
>> held out of the window of a moving car produced no rotation nor did
>> it sustained any initial rotation in any orientation to the wind.
>> The wheel was held so that it was not in the air stream of the car.
> Not that I am quick to believe the wheel builders' claims, but your
> experiment above does not include what effects wind has on a
> spinning wheel, but only what on a wheel which is not moving. Could
> wind affect a spinning wheel differently than it affects a wheel
> which is not spinning? I have no idea, but I thought you may have
> some input on this.
Do you or don't you believe that the test was performed as described
or not? If so, then you must have noticed in the above paragraph that
the wheel was both rotating and not rotating in the various attempts
to produce the effects advertised.
Jobst Brandt <jbr...@hpl.hp.com>
Tim McNamara
>The wheel is symmetrical, but in a crosswind the set-up isn't. The fact
>that the wheel is symmetrical only means there is no lift in a pure
>headwind, but in a crosswind there can be. In this case the lift (which
>is perpendicular to the air flow) can have a component in the direction
>of travel of the bike, which lowers the apparent drag of the wheel. But
>to get a net thrust is quite hard, and it only happens in very specific
>conditions.
What conditions might those be? What angle crosswind, what wind velocity
combined with what bike speed?
One person (in rec.bikes.tech) mounted a trispoke wheel on a broomstick
and stuck it out the window of a moving car. No matter what angle the
wheel presented to the wind, it didn't rotate. If there was a "sail
effect," the wheel would have started spinning.
The only net gain that would be seen would be with a tailwind.
--
Ripple in still water
when there is no pebble tossed
or wind to blow.
-Robert Hunter
Stergios Papadakis
Jobst Brandt wrote:
> The claim was that wind imparts rotational energy to the Trispoke and
> that is what I measured. Now you are proposing that this is a sail
> effect in which the rotation would have no effect. I don't believe
> that anyone has measured any propulsive force caused by these wheels.
> That some wheels have less drag than others has never been disputed,
> only that net propulsive force is generated by any of these wheels.
>
First, the original question by the original poster was whetheror not the
aerodynamic forces on a disk wheel generate thrust. It
had nothing to do with any claims made by any manufacturer.
Here is the quote from cwhite:
On page 42 of the April 18, 1998 edition of
VeloNews, Charles Pelky makes comments regarding
the "sail effect" of disc and deep-dish wheels.
He states that "a sidewind generates a force
vector component in the direction of travel." <big snip>
Does anyone have corrections, clarifications, or
other comments with regard to Pelkey's and my
thoughts?
We answered that question.
> >Doug Milliken said:
> > If you take this to an extreme, HPV fairings (which can be shaped
> > like symmetrical airfoils in top view) will allow a bike to sail
> > quite nicely, I've done it for miles at a time, -- reacting the wind
> > forces against the ground in a fashion just like a sail boat reacts
> > the wind against the keel in the water.
>
> Your example is not analogous to a wheel that is not airfoil (tear
> drop) shaped but has fore and aft symmetry. I'm sure your vehicle did
> not have a symmetrical canoe shape as a bicycle disk essentially has,
> or it would not have had a preferred direction of travel in a pure
> side wind and especially not in a headwind. That the vanes of the
> Trispoke are air foil shaped is self canceling because they average to
> zero due to radial symmetry. I think my experiment, that was designed
> to disprove the claim, tested that effect.
>
> In your HPV example you didn't mention from where the wind came.
>
> Jobst Brandt <jbr...@hpl.hp.com>
Yes, it is analogous to a wheel. you don't need a teardrop shape
to generate lift. A piece of plywood will generate lift.
A disk wheel will generate lift, even if it is "canoe shaped".
The fact that the wind is blowing from an angle breaks the
symmetry you are trying to give the problem!
He clearly, in the context of the problem, meant that the
wind was coming at some angle (<90 degrees)
from straight ahead. You are now arguing for the sake of argument, why
don't you just concede that the same aerodynamics that apply to
sailboats, iceboats, land-yachts, and faired HPVs also
apply to disk bicycle wheels. It is far-fetched to
say that disk wheels are somehow excepted.
Stergios
Mark Drela
In article <6h5c01$5...@hplntx.hpl.hp.com>, jbr...@hpl.hp.com (Jobst Brandt) writes:
>
> The claim was that wind imparts rotational energy to the Trispoke and
> that is what I measured. Now you are proposing that this is a sail
> effect in which the rotation would have no effect. I don't believe
> that anyone has measured any propulsive force caused by these wheels.
> That some wheels have less drag than others has never been disputed,
> only that net propulsive force is generated by any of these wheels.
Actually, I think the original claim was that the wind imparts
a driving force, whereas you measured only a driving torque.
The U.Bath experiments did measure the driving force, and it
was definitely slightly positive for some wheels in a narrow
crosswind angle range.
Just to throw more gasoline on the fire...
I believe that the Trispoke would indeed have a driving torque
if the spokes had better aifoils. Wells turbines have precisely
the same flat-pitch geometry as the Trispoke, and they work
quite well. They are used in ocean-wave power stations
on some of the small British isles. They turn in one direction
as the wave-driven flow oscillates back and forth through the disk,
so they must have left/right symmetry, and hence perfectly flat
blades like the Trispoke. Why this works is very well understood.
Mark Drela
_______________________________
o/LO .'
O .' Gravity-Powered Technologies Lab
.' MIT Aero-Astro Department 37-475
'
Dan Connelly
> One person (in rec.bikes.tech) mounted a trispoke wheel on a broomstick
> and stuck it out the window of a moving car. No matter what angle the
> wheel presented to the wind, it didn't rotate. If there was a "sail
> effect," the wheel would have started spinning.
This is incorrect. Spinning moments are not required to
provide propulsive force. I could simply switch the sign and
argue from these test results that since it didn't spin "backwards"
there was no drag force due to the wind, either.
Dan
Jobst Brandt
Stergios Papadakis writes:
>> Your example is not analogous to a wheel that is not airfoil (tear
>> drop) shaped but has fore and aft symmetry. I'm sure your vehicle did
>> not have a symmetrical canoe shape as a bicycle disk essentially has,
>> or it would not have had a preferred direction of travel in a pure
>> side wind and especially not in a headwind. That the vanes of the
>> Trispoke are air foil shaped is self canceling because they average to
>> zero due to radial symmetry. I think my experiment, that was designed
>> to disprove the claim, tested that effect.
>> In your HPV example you didn't mention from where the wind came.
> Yes, it is analogous to a wheel. you don't need a teardrop shape to
> generate lift. A piece of plywood will generate lift. A disk wheel
> will generate lift, even if it is "canoe shaped". The fact that the
> wind is blowing from an angle breaks the symmetry you are trying to
> give the problem!
You get no lift whatsoever if the plane panel is parallel to the
direction of travel, regardless of wind direction. Lift from such a
"wing" requires an angle of attack, something a bicycle wheel, by
definition, does not have.
> He clearly, in the context of the problem, meant that the wind was
> coming at some angle (<90 degrees) from straight ahead. You are now
> arguing for the sake of argument, why don't you just concede that
> the same aerodynamics that apply to sailboats, iceboats,
> land-yachts, and faired HPVs also apply to disk bicycle wheels. It
> is far-fetched to say that disk wheels are somehow excepted.
As I pointed out, the analogy does not follow because the wheel,
unlike a sail, has no angle of attack and has no aerodynamic
preference front to rear. All this talk is wishful thinking about as
valid as people who drive around in cars with pseudo airfoils on the
trunk lid.
Jobst Brandt <jbr...@hpl.hp.com>
Martin Trautmann
Mark Drela wrote:
> I believe that the Trispoke would indeed have a driving torque
> if the spokes had better aifoils. Wells turbines have precisely
> the same flat-pitch geometry as the Trispoke, and they work
> quite well. They are used in ocean-wave power stations
> on some of the small British isles. They turn in one direction
> as the wave-driven flow oscillates back and forth through the disk,
> so they must have left/right symmetry, and hence perfectly flat
> blades like the Trispoke. Why this works is very well understood.
Turbines have a rather well defined flow direction. So you turn round
your wheels, as soon as the angle of wind turns from e.g. + 15 degrees
(from right) to -15 degrees (from left)? No, this will make you sail
backwards, apart from the rear wheel trouble. Thus you have a second
counterpart set of wheels with you? I suggest to attach some wind
surfing sail to the stem instead.
;-)
Martin
Tim McNamara
In article <3536482B...@princeton.edu>, Stergios Papadakis
<papa...@princeton.edu> wrote:
>He clearly, in the context of the problem, meant that the
>wind was coming at some angle (<90 degrees)
>from straight ahead. You are now arguing for the sake of argument, why
>don't you just concede that the same aerodynamics that apply to
>sailboats, iceboats, land-yachts, and faired HPVs also
>apply to disk bicycle wheels. It is far-fetched to
>say that disk wheels are somehow excepted.
It's not clear to me that the same aerodynamic effects are present. I'm
struggling to understand the dynamics here, so bear with my reasoning.
Fluid dynamics is not my specialty.
There is no evidence presented thus far that a crosswind or partial
quarter headwind generates a *propulsive* force on a faired HPV (or a car
or any other solid shape moving through an atmosphere). A *tailwind* may
produce a propulsive effect, as long as the vehicle is traveling under the
wind speed. When the vehicle exceeds the wind speed, the wind is no
longer providing a *propulsive* force, it is reducing the total wind
resistance faced by the vehicle. At best, a fairing or more aerodynamic
shape may be more efficiently moved through the air, but that is
distinctly different than adding a *propulsive* force.
A sailboat, ice boat or land-yacht is a different situation than a bicycle
wheel. For one thing, the sail is not always in the same plane as the
axis of the vehicle, whereas the rear wheel of a bicycle is constrained to
that plane and the front wheel is generally constrained to that plane with
some minor variation of a few degrees (at cycling speeds above about 5
mph). The sail can be at a bias to the vehicle and it's direction of
travel, which alters the aerodynamics significantly, it would seem to me.
A sail is also not a rotating object with its own angular velocity
separate from the velocity of the vehicle as a unit; it is also not a
rigid structure as is an aero wheel, whether a disc wheel or of the
"trispoke" variety. Finally, a bicycle wheel has it's own turbulence and
wind resistance within its rotation through the atmosphere.
If a sailboat is moving at 25 mph, the sail is moving at 25 mph. If a
bicycle with a trispoke wheel is moving at 25 mph, the velocity of each
individual airfoil within that wheel is constantly varying with respect to
the movement of the bicycle as a unit. At a cycling speed of 25 mph, by
my calculations, the airfoil is moving at 22.22 mph in angular velocity
(at a radius of 12" from the hub, whereas the angular velocity of the
circumference of the tire will be moving at an angular velocity that
matches that of the bicycle's forward velocity). So, it seems to me that
sometimes the velocity of the airfoil spoke, with respect to forward
motion, is sometimes greater and sometimes slower, producing varying
aerodynamic effects... or at least it is if my conceptualization is
accurate.
As far as I can tell, the only aerodynamic benefit provided by disc or
"trispoke" type wheels is reduced turbulence. But reducing turbulance
only lessens the wind resistance that must be overcome by the rider; it
does not provide a *propulsive* effect. And, from practical experience, a
crosswind with disc or trispoke wheels creates problems due to
instability, and more than nullifies the aerodynamic gains. Whether or
not there's "lift" or a "net propulsive vector," struggling to control the
bike in a crosswind slows the rider down. In a cross windy TT I'd much
rather ride standard wheels.
--
All I leave behind me is only what I've found.
-Robert Hunter
Stergios Papadakis
Jobst Brandt wrote:
>
>
> > Yes, it is analogous to a wheel. you don't need a teardrop shape to
> > generate lift. A piece of plywood will generate lift. A disk wheel
> > will generate lift, even if it is "canoe shaped". The fact that the
> > wind is blowing from an angle breaks the symmetry you are trying to
> > give the problem!
>
> You get no lift whatsoever if the plane panel is parallel to the
> direction of travel, regardless of wind direction. Lift from such a
> "wing" requires an angle of attack, something a bicycle wheel, by
> definition, does not have.
>
If you are travelling northward at 15 mph and the wind is blowing
from the east at 5 mph (relative to the ground), then your
bicycle wheels have an angle of attack of arctan(5/15) = 18.4 degrees.
Think before you type.
Stergios
> As I pointed out, the analogy does not follow because the wheel,
> unlike a sail, has no angle of attack and has no aerodynamic
> preference front to rear. All this talk is wishful thinking about as
> valid as people who drive around in cars with pseudo airfoils on the
> trunk lid.
>
> Jobst Brandt <jbr...@hpl.hp.com>
Well, I think it is pretty well established that if the wind isblowing, you
the wheels can be at an angle of attack.Are you now going to argue that those
airfoils on the trunk liddon't deflect the flow upward and therefore provide
some
downforce?
Stergios
Stergios Papadakis
Tim McNamara wrote:
> In article <3536482B...@princeton.edu>, Stergios Papadakis
> <papa...@princeton.edu> wrote:
>
> >He clearly, in the context of the problem, meant that the
> >wind was coming at some angle (<90 degrees)
> >from straight ahead. You are now arguing for the sake of argument, why
> >don't you just concede that the same aerodynamics that apply to
> >sailboats, iceboats, land-yachts, and faired HPVs also
> >apply to disk bicycle wheels. It is far-fetched to
> >say that disk wheels are somehow excepted.
>
> It's not clear to me that the same aerodynamic effects are present. I'm
> struggling to understand the dynamics here, so bear with my reasoning.
> Fluid dynamics is not my specialty.
>
> There is no evidence presented thus far that a crosswind or partial
> quarter headwind generates a *propulsive* force on a faired HPV (or a car
> or any other solid shape moving through an atmosphere).
I don't know how familiar you are with sailboats, but
they do make progress against the wind. If the wind is
blowing from the North, a sailboat can travel northeast or
northwest. Iceboats, because their blades are better at
preventing sideslipping and because they provide significantly
less drag than a keel in water, can point much closer to the wind
and go much faster. These are examples of solid objects that
generate a propulsive force from a quarter-headwind. A sailboat
in water travelling upwind often has the wind at about 20 degrees
off the bow. Iceboats have it nearly dead-ahead, because they
will be propelled to 70 mph by 20 mph winds! (those numbers
are correct, that is not a typo)
Another
poster said that he has a faired HPV which has been propelled
by the wind.
> A *tailwind* may
> produce a propulsive effect, as long as the vehicle is traveling under the
> wind speed. When the vehicle exceeds the wind speed, the wind is no
> longer providing a *propulsive* force, it is reducing the total wind
> resistance faced by the vehicle. At best, a fairing or more aerodynamic
> shape may be more efficiently moved through the air, but that is
> distinctly different than adding a *propulsive* force.
>
> A sailboat, ice boat or land-yacht is a different situation than a bicycle
> wheel. For one thing, the sail is not always in the same plane as the
> axis of the vehicle, whereas the rear wheel of a bicycle is constrained to
> that plane and the front wheel is generally constrained to that plane with
> some minor variation of a few degrees (at cycling speeds above about 5
> mph). The sail can be at a bias to the vehicle and it's direction of
> travel, which alters the aerodynamics significantly, it would seem to me.
You will find that sails on boats sailing upwind are only a few
degrees off centerline. Iceboats have their sails on-centerline because
they go so fast and their apparent wind is so close to dead-ahead.
The lift of a foil is proportional to the angle of attack, so if
the angle of the wind relative to the centerline of
the vehicle is 15 degrees, it doesn't completely change the
picture if your foil is on-centerline or 5 degrees off-centerline. There
is still lift.
> A sail is also not a rotating object with its own angular velocity
> separate from the velocity of the vehicle as a unit; it is also not a
> rigid structure as is an aero wheel, whether a disc wheel or of the
> "trispoke" variety. Finally, a bicycle wheel has it's own turbulence and
> wind resistance within its rotation through the atmosphere.
>
The rotation of the disk wheel does not significantly change themacroscopic
flow over it. It changes the boundary layer a little
bit, but it is not that big an effect.
> If a sailboat is moving at 25 mph, the sail is moving at 25 mph. If a
> bicycle with a trispoke wheel is moving at 25 mph, the velocity of each
> individual airfoil within that wheel is constantly varying with respect to
> the movement of the bicycle as a unit. At a cycling speed of 25 mph, by
> my calculations, the airfoil is moving at 22.22 mph in angular velocity
> (at a radius of 12" from the hub, whereas the angular velocity of the
> circumference of the tire will be moving at an angular velocity that
> matches that of the bicycle's forward velocity). So, it seems to me that
> sometimes the velocity of the airfoil spoke, with respect to forward
> motion, is sometimes greater and sometimes slower, producing varying
> aerodynamic effects... or at least it is if my conceptualization is
> accurate.
>
Yes, tri-spoke wheels are more complicated, which is whyI restricted myself to
disks in my examples, and you
are right that the forces on each blade are constantly changing
as the wheel rotates:
Imagine the a bicycle travelling in a crosswind. The bicycle
is travelling North at 15 mph. The wind is blowing from the
east at 5 mph.
Consider the moment when one bladed spoke is vertical, above the
hub. The hub end of the spoke is travelling the same speed
as the bike, the rim end is travelling twice the speed of the
bike. It acts as
an airfoil. It can generate lift in the way we have been describing
through some of the upper half of the cycle.
Now consider a bladed spoke that is vertical, below the hub. It
is nearly stationary (the outer end is stationary, the inner end is
movind the same speed as the bike). So it produces much less lift,
probably zero, it sees only a 5mph sidewind at the rim end, and
that obviously produces only a sideforce.
So, you are right that the aerodynamic forces are constantly changing
as the wheel rotates. Tri-spokes don't lend themselves to a
simple analysis.
You have described angular velocity measured in mph. This is
confusing, angular velocity around a hub is always measured
in (unit of angle)/(unit of time) i.e. radians/second.
Your calculation of 22.22 mph
means that that point on the spoke is moving linearly at 22.22mph
relative to the hub of the bike.
> As far as I can tell, the only aerodynamic benefit provided by disc or
> "trispoke" type wheels is reduced turbulence. But reducing turbulance
> only lessens the wind resistance that must be overcome by the rider; it
> does not provide a *propulsive* effect. And, from practical experience, a
> crosswind with disc or trispoke wheels creates problems due to
> instability, and more than nullifies the aerodynamic gains. Whether or
> not there's "lift" or a "net propulsive vector," struggling to control the
> bike in a crosswind slows the rider down. In a cross windy TT I'd much
> rather ride standard wheels.
>
> --
> All I leave behind me is only what I've found.
>
> -Robert Hunter
I agree, ride what you think makes you the fastest. I am
sure that in gusty winds, the instability is a problem. In
steady winds, however, it might not be too bad. However,
another poster metioned that the propulsive effect has
been demonstrated in a wind tunnel. Here is how to
do the experiment: put the wheel in a wind tunnel,
measure its lift to drag ratio at various angles of
attack. Whenever arctan(drag/lift)
is less than the angle of attack, you get a propulsive
force when the wheel is on the road.
Stergios
Stergios Papadakis
Jobst Brandt wrote:
Why are you so hostile when everyone around you is having
a polite discussion? Eric Cross asked politely for a little
bit more information. You should read what you wrote
again, it says nothing about "both rotating and not rotating".
Anyway, in response to Eric. The issue is less whether
or not the wheel is rotating and more whether or not
it is on the road. I have posted, in this thread,
more detailed descriptions
of the forces involved.
Stergios
Stergios Papadakis
Stergios Papadakis wrote:
> Tim McNamara wrote:
>
> > There is no evidence presented thus far that a crosswind or partial
> > quarter headwind generates a *propulsive* force on a faired HPV (or a car
> > or any other solid shape moving through an atmosphere).
>
> Another
> poster said that he has a faired HPV which has been propelled
> by the wind.
I don't have pictures of an HPV (meaning bicycle)
but for something analogous, check out skate-sailing
at www.bahnhof.se/~ansar/s.html
Stergios
Cervelo Cycles
Tim McNamara wrote:
>
> One person (in rec.bikes.tech) mounted a trispoke wheel on a broomstick
> and stuck it out the window of a moving car. No matter what angle the
> wheel presented to the wind, it didn't rotate. If there was a "sail
> effect," the wheel would have started spinning.
This thread seems to re-appear from time to time. I believe it was Mark
Drela who pointed out some years ago that this experiment only proves
the Trispoke is a bad windmill, not that it would be a bad sail (it
isn't a very efficient sail either, and a disc is better). Other posters
have already indicated how the sail principle works as oposed to the
windmill principle. Look at Doug Milliken's comments in this respect.
Jobst Brandt
Stergios Papadakis writes:
>>> Yes, it is analogous to a wheel. you don't need a teardrop shape
>>> to generate lift. A piece of plywood will generate lift. A disk
>>> wheel will generate lift, even if it is "canoe shaped". The fact
>>> that the wind is blowing from an angle breaks the symmetry you are
>>> trying to give the problem!
>> You get no lift whatsoever if the plane panel is parallel to the
>> direction of travel, regardless of wind direction. Lift from such a
>> "wing" requires an angle of attack, something a bicycle wheel, by
>> definition, does not have.
> If you are traveling northward at 15 mph and the wind is blowing
> from the east at 5 mph (relative to the ground), then your bicycle
> wheels have an angle of attack of arctan(5/15) = 18.4 degrees.
> Think before you type.
The implication is obvious. You needn't get rude to support your
point of view.
The wind may have a an angle to a neutral flat panel, but it has no
net surface fore and aft to propel with. It should be noted that a
direct side wind has a retarding force on a moving bluff body
(bicyclist and bicycle). Drag increases with wind even for winds from
more than 10 degrees from behind, assuming the rider's speed is
approximately as fast as the wind.
A flat panel (or a disk bicycle wheel), mounted lengthwise on a
frictionless rail mounted car, will not propel that car forward except
by drag caused by tailwinds. What is being proposed here is that a
symmetrical disk wheel will generate forward thrust for quartering
headwinds as a sailboat does when its sail is at an angle to its long
axis. The analogy is wishful thinking. It has not been measured nor
is there any reasonable explanation for it. Every proof presented has
been of a non parallel analogy.
Proof that a Trispoke propels, is offered through a bi-directional
water propeller that turns in incoming and outgoing sea water. The
Trispoke bicycle wheel neither created nor sustained rotation at any
angle when put to such a test. The claim that a symmetrical disk
wheel produces forward thrust is even weaker, there being no parallel
except that if a wing shape is tilted in a fluid flow, there is lift
at an angle to the flow. No one has said that there are no side
forces on a bicycle in a cross wind. However, there are no forward
propelling forces except by tailwinds and these are not due to lift
but rather drag.
>> As I pointed out, the analogy does not follow because the wheel,
>> unlike a sail, has no angle of attack and has no aerodynamic
>> preference front to rear. All this talk is wishful thinking about
>> as valid as people who drive around in cars with pseudo airfoils on
>> the trunk lid.
> Well, I think it is pretty well established that if the wind is
> blowing, the wheels can be at an angle of attack.
There is no angle with respect to the direction of travel and if the
pressure is higher on one surface of a disk wheel than the other, then
there is a pure side force, but no propulsion. For propulsion there
needs to be a higher pressure from rear to front. Can you explain how
that could be.
> Are you now going to argue that those airfoils on the trunk lid
> don't deflect the flow upward and therefore provide some downforce?
No doubt these devices adversely affect fuel consumption, but they
have no beneficial effects for the vehicle's handling, especially at
speeds that these cars travel on highways. Maybe you can explain what
benefit they have other than for the driver's aesthetic appreciation.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
Stergios Papadakis writes:
>>>> This claim has been made often in this newsgroup with no evidence
>>>> to back it up. In response, I took a Specialized Trispoke wheel
>>>> that was said to have these crosswind qualities, and mounted it in
>>>> a bicycle fork attached to the end of a broomstick. This wheel,
>>>> held out of the window of a moving car produced no rotation nor did
>>>> it sustained any initial rotation in any orientation to the wind.
>>>> The wheel was held so that it was not in the air stream of the car.
>>> Not that I am quick to believe the wheel builders' claims, but your
>>> experiment above does not include what effects wind has on a
>>> spinning wheel, but only what on a wheel which is not moving. Could
>>> wind affect a spinning wheel differently than it affects a wheel
>>> which is not spinning? I have no idea, but I thought you may have
>>> some input on this.
>> Do you or don't you believe that the test was performed as described
>> or not? If so, then you must have noticed in the above paragraph that
>> the wheel was both rotating and not rotating in the various attempts
>> to produce the effects advertised.
> Why are you so hostile when everyone around you is having a polite
> discussion? Eric Cross asked politely for a little bit more
> information. You should read what you wrote again, it says nothing
> about "both rotating and not rotating".
Lest it not be obvious, the following quote from the above paragraph
clearly states that the wheel did not begin turning or remain turning
at any angle in the test. To repeatedly question that concept is
tiring to repeatedly explain and leads me to suspect willful
misinterpretation for argument's sake.
"This wheel, held out of the window of a moving car produced no
rotation nor did it sustained any initial rotation in any orientation
to the wind. The wheel was held so that it was not in the air stream
of the car."
> Anyway, in response to Eric. The issue is less whether or not the
> wheel is rotating and more whether or not it is on the road. I have
> posted, in this thread, more detailed descriptions of the forces
> involved.
If you would dissect the problem into which forces act where, I think
you will see that whether the ground constrains the position of the
wheel or a fixture, it should react to wind in the same way.
Jobst Brandt <jbr...@hpl.hp.com>
John Many Jars
In article <6h8joa$6...@hplntx.hpl.hp.com>,
Jobst Brandt <jbr...@hpl.hp.com> wrote:
>The implication is obvious. You needn't get rude to support your
>point of view.
somebody check for pods!
jmj
Jeffrey J. Potoff
> Stergios Papadakis writes:
> > Are you now going to argue that those airfoils on the trunk lid
> > don't deflect the flow upward and therefore provide some downforce?
The 'airfoils' found on the deck lid of many sporty passenger cars
serve no purpose other than for looks. In fact, they aren't even
airfoils.
You won't find any test that shows the wing does anything to improve
handling.
Jeff
todd marks bauer
Stuck a wheel and fork on a broomstick and out the window of a car?!?!
HAAAAAAHHAAAAAAaaaHHAAaaahahAHAHHahahaaaa!!!!!!
Wait till Jobst Brandt reads this! He's going to flame this guy to his
grave!
Wait a second...
On 17 Apr 1998, Jobst Brandt wrote:
> Stergios Papadakis writes:
>
> >>>> This claim has been made often in this newsgroup with no evidence
> >>>> to back it up. In response, I took a Specialized Trispoke wheel
> >>>> that was said to have these crosswind qualities, and mounted it in
> >>>> a bicycle fork attached to the end of a broomstick. This wheel,
> >>>> held out of the window of a moving car produced no rotation nor did
> >>>> it sustained any initial rotation in any orientation to the wind.
> >>>> The wheel was held so that it was not in the air stream of the car.
>
>
> Jobst Brandt <jbr...@hpl.hp.com>
>
>
Doug Milliken
On Thu, 16 Apr 1998, John Getsoian wrote:
> On Thu, 16 Apr 1998 04:04:44 GMT, Doug Milliken wrote:
> >If you take this to an extreme, hpv fairings (which can be shaped like
> >symmetrical airfoils in top view) will allow a bike to sail quite nicely,
> >I've done it for miles at a time,
> Just to clarify, when you say 'symmtrical airfoils in top view' do
> you mean the fairings are symmetrical in both the x _and_ y
> directions, like a wheel viewed from the top, or just in y, i.e.,
> when reflected along the long axis, like Dennis Conner's rigid
> catamaran sail?
The latter. The shape I wound up with has similar proportions to the
conning tower on a submarine.
But -- disk wheels also generate lift (sideforce) if they are at an angle
of attack (yawed to the relative wind). Because disk wheels are not very
good airfoils, they stall at a lower angle of attack than a proper airfoil
would.
Cervelo Cycles
Jobst wrote:
> Lest it not be obvious, the following quote from the above paragraph
> clearly states that the wheel did not begin turning or remain turning
> at any angle in the test. To repeatedly question that concept is
> tiring to repeatedly explain and leads me to suspect willful
> misinterpretation for argument's sake.
But why would it have to start turning if it acted like a sail. I don't
know the kind of sail boat you have, but mine translates over the water,
it doesn't tumble, except for very high seas :-)
I think everybody on this thread thinks the other side is "willfully
misinterpreting", so this problem cannot be solved in this manner.
All I can say is that if you had a miniature sailboat on your broom and
the broom was attached at the same height as the cp of the boat, the
boat would not start turning, no matter how good the sail was. If the
broom was not attached at the same height as cp, the boat would turn
until it was (presuming gravity was small relative to the wind forces).
Gerard
Tim McNamara
In article <3537EC12...@rhea.umd.edu>, "Jeffrey J. Potoff"
<jpo...@rhea.umd.edu> wrote:
>> Stergios Papadakis writes:
>
>> > Are you now going to argue that those airfoils on the trunk lid
>> > don't deflect the flow upward and therefore provide some downforce?
>
>The 'airfoils' found on the deck lid of many sporty passenger cars
>serve no purpose other than for looks. In fact, they aren't even
>airfoils.
Looking at recently published (in the general news) information about new
wing designs, it seems possible that a "spoiler" on the truck deck of a
car could create vortices that generate *lift* rather than downforce,
since there is no airflow under the spoiler to create a low pressure area.
--
Going where the wind don't blow so strange,
maybe off on some high cold mountain range.
-Robert Hunter
Stergios Papadakis
Jobst Brandt wrote:
> If you would dissect the problem into which forces act where, I think
> you will see that whether the ground constrains the position of the
> wheel or a fixture, it should react to wind in the same way.
>
> Jobst Brandt <jbr...@hpl.hp.com>
That's true, my apologies for being imprecise.Here is how to do the
experiment:
put the wheel in a wind tunnel, or out the window
of your car, and
Mike Bundy
I haven't read all of this thread (gosh it's long...) but has anyone
mentioned the part about a moving wheel (a tri-spoke is a good case in
point) having a differential in drag between the top and bottom ??
Relative to the air, the bottom of a moving wheel is traveling slower
than the top ('cos it's going "backwards") hence the drag co-efficient
will be lower at the bottom than the top. I'm no engineer, so I haven't
got the faintest idea of this implication, but am just tossing it up for
consideration.
-- MB.
Cervelo Cycles wrote in message <35386D...@cervelo.com>...
Cervelo Cycles
Mike Bundy wrote:
>
> I haven't read all of this thread (gosh it's long...) but has anyone
> mentioned the part about a moving wheel (a tri-spoke is a good case in
> point) having a differential in drag between the top and bottom ??
> Relative to the air, the bottom of a moving wheel is traveling slower
> than the top ('cos it's going "backwards") hence the drag co-efficient
> will be lower at the bottom than the top. I'm no engineer, so I haven't
> got the faintest idea of this implication, but am just tossing it up for
> consideration.
This has also been covered, but it has more to do with being a windmill
than with being a sail. The original post was on disc wheels, which
obvious is symmetric in this respect. Then somebody switched to
Trispokes (and later flamed people for changing back to discs, but
that's another story). Even the rotation of a disc wheel does not change
the symmetry by much, as the rotation has little effect on the air flow
around a disc wheel. Of course this doesn't hold for other types of
wheels.
M.Scully
that was basically a large diameter tube which was mounted vertically
and turned round slowly this was claimed to be very efficient and worked
in very slight winds.
Sorry about the lack of technicall details but it was some time ago.
Jobst Brandt
Stergios Papadakis writes:
>> If you would dissect the problem into which forces act where, I think
>> you will see that whether the ground constrains the position of the
>> wheel or a fixture, it should react to wind in the same way.
> That's true, my apologies for being imprecise.Here is how to do the
> experiment: put the wheel in a wind tunnel, or out the window of
> your car, and measure its lift to drag ratio at various angles of
> attack. Whenever arctan(drag/lift) is less than the angle of
> attack, you get a propulsive force when the wheel is on the road.
I think we are going around in circles. There is no angle between the
bicycle course and the wheel. The wheel is symmetrical fore and aft.
Hence, there can be no lift in the direction of motion, this requiring
there be a difference in pressure fore and aft, not left to right
since that cannot translate into forward thrust.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
Cervelo Cycles writes anonymously:
>> Lest it not be obvious, the following quote from the above
>> paragraph clearly states that the wheel did not begin turning or
>> remain turning at any angle in the test. To repeatedly question
>> that concept is tiring to repeatedly explain and leads me to
>> suspect willful misinterpretation for argument's sake.
> But why would it have to start turning if it acted like a sail. I
> don't know the kind of sail boat you have, but mine translates over
> the water, it doesn't tumble, except for very high seas :-)
The advertising claim that instigated my experiment was that wind
imparts rotation to the Trispoke wheel. It doesn't.
> I think everybody on this thread thinks the other side is "willfully
> misinterpreting", so this problem cannot be solved in this manner.
Either you boarded this train recently and don't know what was being
discussed or you are misinterpreting what is being said.
> All I can say is that if you had a miniature sailboat on your broom and
> the broom was attached at the same height as the cp of the boat, the
> boat would not start turning, no matter how good the sail was. If the
> broom was not attached at the same height as cp, the boat would turn
> until it was (presuming gravity was small relative to the wind forces).
If your sail is symmetrical front to rear and is in-line with the
vehicle, it cannot add a forward propulsive force because it doesn't
know which way you want to travel, there being no other information
about direction of motion but your desire. A bicycle wheel as a sail,
is symmetrical front to rear by its rotational nature and in-line with
its course. If the spokes do not produce rotation or sustain it, then
there can be no resultant except retarding drag.
Jobst Brandt <jbr...@hpl.hp.com>
Kevin Collins
Cervelo Cycles wrote:
>
> But why would it have to start turning if it acted like a sail. I
> don't know the kind of sail boat you have, but mine translates over
> the water, it doesn't tumble, except for very high seas :-)
>
You seem to be implying that a wheel can provide some net propulsive
force while not turning. If it could, how would it move the bicycle in
any direction other than by turning? And, as Jobst Brandt has pointed
out, the manufacturer is implying a net forward propulsive force (why
would I buy a wheel that would generate a backward propulsive force?);
being symmetrical from front to back, how would the wheel "know" in
which direction to move to be of any benefit to the rider?
Though I am a novice at sailing, the last time that I was on a racing
yacht it seemed that the crew was continually trying to change the shape
and position of the sails (which were clearly not symmetrical) relative
to the wind in order to provide a net propulsive force to move the boat
forward (and I was happy that despite the high seas we didn't start to
tumble :-]). I have never seen a sailboat with rigid sails that were
symmetrical from bow to stern move forward, so I don't see how the
analogy applies.
Kevin Collins
kev...@mdsoftware.com
Cervelo Cycles
Jobst Brandt wrote:
> > But why would it have to start turning if it acted like a sail. I
> > don't know the kind of sail boat you have, but mine translates over
> > the water, it doesn't tumble, except for very high seas :-)
>
> The advertising claim that instigated my experiment was that wind
> imparts rotation to the Trispoke wheel. It doesn't.
But for somebody flaming others for changing the subject, why do you
keep bringing up your Trispoke experiment when even the title of this
thread reads DISK wheel. And why do you bring up the windmill effect of
this trispoke when the title says sail effect?
>
> > I think everybody on this thread thinks the other side is "willfully
> > misinterpreting", so this problem cannot be solved in this manner.
>
> Either you boarded this train recently and don't know what was being
> discussed or you are misinterpreting what is being said.
We'll let others decide if I misrepresent something, all I noted was
that everybody claims to be misrepresented, and with the above statement
you are basically agreeing.
> If your sail is symmetrical front to rear and is in-line with the
> vehicle, it cannot add a forward propulsive force because it doesn't
> know which way you want to travel, there being no other information
> about direction of motion but your desire.
Proof? It cannot add a forward propulsive force because it doesn't know
in which way you want to travel? What does this have to do with it. To
me proof would be either:
1) theoretical analysis which shows that a symmetric body in an airflow
under an angle of attack cannot produce lift. This I have not seen yet
2) experiments. Here I will agree that data is sparse, and that the
actual thrust of a disc wheel is very rare if existant. But there is no
physical reason why it would be impossible, and some have cited
experiments that show thrust. While these may be inaccurate, I have not
seen any experiment that proves the oposite, and your
trispoke-on-a-broom experiment investigates the windmill effect, not the
sail effect.
Preferably a combination of the two would be great. I have seen little
proof of thrust being possible (many thanks to Doug Milliken and Mark
Drela), and none of the oposite. I have seen some proof of the
Specialized being a bad windmill (many thanks to Jobst Brandt and Doug
again).
David Suryan
In article <Pine.A41.3.96.980417...@auriga.unm.edu>,
todd marks bauer <ba...@unm.edu> wrote:
>
>Stuck a wheel and fork on a broomstick and out the window of a car?!?!
>
>HAAAAAAHHAAAAAAaaaHHAAaaahahAHAHHahahaaaa!!!!!!
I fail to see why you find this so humorous. This appears to be a simple
way to determine how a tri-spoke wheel is affected by wind as opposed to
the speculation many have offered here. Perhaps you could offer a better
experiment.
Cervelo Cycles
Kevin Collins wrote:
>
> Cervelo Cycles wrote:
> >
> > But why would it have to start turning if it acted like a sail. I
> > don't know the kind of sail boat you have, but mine translates over
> > the water, it doesn't tumble, except for very high seas :-)
> >
>
> force while not turning.
The rotation of a disk wheel hardly changes the flow over it. That's why
I made the statement. And the sail effect by definition is a force that
makes the wheel (or something else) translate, it's not a torque that
makes the wheel turn (this would be a windmill effect as Doug Milliken
pointed out).
> If it could, how would it move the bicycle in
> any direction other than by turning? And, as Jobst Brandt has pointed
> out, the manufacturer is implying a net forward propulsive force (why
> would I buy a wheel that would generate a backward propulsive force?);
> being symmetrical from front to back, how would the wheel "know" in
> which direction to move to be of any benefit to the rider?
I am sceptical about getting much thrust out of a wheel, and the tests
also indicate it rarely happens. but remember that we are getting this
thrust from the flow around the wheel, not from the wheel itself. My
wording is so cripple in order to demonstrate that while the wheel may
be symmetric, there is no reason why the flow around it would be. In
fact, it obviously isn't, per definition. So with the air flow not being
symmetric, can there be a resulting force? Yes. Will this force render a
net thrust? According to the tests, rarely. How accurate are the tests
that show a net thrust? I don't know, that's why I am hesitant, but
there is no physical law broken should there be net thrust.
> I have never seen a sailboat with rigid sails that were
> symmetrical from bow to stern move forward, so I don't see how the
> analogy applies.
This analogy was only meant to indicate we are talking about a sail
effect, as opposed to the windmill effect. I did not want to imply that
a disk wheel is an efficient sail, and of course if it was only used as
a sail and didn't have to roll, it would look differently. sorry for
that confusion.
James Neale
Jobst Brandt wrote in message <6hg339$c...@hplntx.hpl.hp.com>...
[snip]
>The advertising claim that instigated my experiment was that wind
>imparts rotation to the Trispoke wheel. It doesn't.
This probably won't clear things up, but here's the first paragraph
of the first post in this thread, dated April 12, 1998 which clearly
seemed to refer to disc wheels:
"On page 42 of the April 18, 1998 edition of
VeloNews, Charles Pelky makes comments regarding
the "sail effect" of disc and deep-dish wheels.
He states that "a sidewind generates a force
vector component in the direction of travel.""
J. Neale
Dan Connelly
Jobst Brandt wrote:
> If a wheel is symmetrical in its long axis, then it has no preferred
> direction when struck by wind. Therefore, how can it propel the
> bicycle "forward" when there is no indication where forward is? That
> is why the point is germane.
Because the wind is from the front. Symmetry is broken.
Leading edge and trailing edge don't cancel. Work is extracted.
Sanity is maintained.
> (tire scrubbing argument omitted)
reference :
http://www.cs.umd.edu/users/humphrie/wheels.html
John Getsoian
On Sat, 18 Apr 1998 05:07:52 GMT, Doug Milliken wrote:
>Because disk wheels are not very
>good airfoils, they stall at a lower angle of attack than a proper airfoil
>would.
Doug;
No problem. This just means you have to go fast enough into those
cross-winds to get the apparent angle of attack down to the
effective range. So if you work hard enough it will get easier!
<g>
regards,
john getsoian
jget...@nojunk.csi.com
delete 'nojunk'
Jobst Brandt
Cervelo Cycles writes anonymously:
>>> But why would it have to start turning if it acted like a sail. I
>>> don't know the kind of sail boat you have, but mine translates over
>>> the water, it doesn't tumble, except for very high seas :-)
>> The advertising claim that instigated my experiment was that wind
>> imparts rotation to the Trispoke wheel. It doesn't.
> But for somebody flaming others for changing the subject, why do you
> keep bringing up your Trispoke experiment when even the title of this
> thread reads DISK wheel. And why do you bring up the windmill effect of
> this Trispoke when the title says sail effect?
Because the claim that it is a sail is just as empty as the one that
preceded it, the claim that it preferred to rotate forward in a side
wind, even one from the front.
>>> I think everybody on this thread thinks the other side is "willfully
>>> misinterpreting", so this problem cannot be solved in this manner.
>> Either you boarded this train recently and don't know what was being
>> discussed or you are misinterpreting what is being said.
> We'll let others decide if I misrepresent something, all I noted was
> that everybody claims to be misrepresented, and with the above
> statement you are basically agreeing.
You keep arguing things that have long ago been resolved, for instance,
why the Trispoke is mentioned and that the wheel is not a sail.
>> If your sail is symmetrical front to rear and is in-line with the
>> vehicle, it cannot add a forward propulsive force because it doesn't
>> know which way you want to travel, there being no other information
>> about direction of motion but your desire.
> Proof? It cannot add a forward propulsive force because it doesn't
> know in which way you want to travel? What does this have to do
> with it.
If a wheel is symmetrical in its long axis, then it has no preferred
direction when struck by wind. Therefore, how can it propel the
bicycle "forward" when there is no indication where forward is? That
is why the point is germane.
> To me proof would be either:
> 1) theoretical analysis which shows that a symmetric body in an
> airflow under an angle of attack cannot produce lift. This I have
> not seen yet
How about a practical analysis? One that makes sense in all its
simplicity. The burden lies with those who believe unbelievable
physical effects, such as $1000 wheels propel the rider and bicycle.
I think I have given you irrefutable reasons why this cannot happen.
In response people are citing all sorts of sailing analogies that
are in no way parallel to the bicycle wheel.
> 2) experiments. Here I will agree that data is sparse, and that the
> actual thrust of a disc wheel is very rare if existent. But there is
> no physical reason why it would be impossible, and some have cited
> experiments that show thrust.
The only people to claim that were advertisers. The same kind of
people who bring us so many inventions at the trade show annually, ones
that are never seen again.
> While these may be inaccurate, I have not seen any experiment that
> proves the opposite, and your Trispoke-on-a-broom experiment
> investigates the windmill effect, not the sail effect.
As I said, you came aboard late. This has all been hashed out...
tediously as protesters, probably owners of Trispokes, argued that the
experiment was not done accurately enough.
> Preferably a combination of the two would be great. I have seen
> little proof of thrust being possible (many thanks to Doug Milliken
> and Mark Drela), and none of the opposite. I have seen some proof of
> the Specialized being a bad windmill (many thanks to Jobst Brandt
> and Doug again).
I have come to the point where I won;t instrument experiments to
disprove such claims or I would be perpetually busy chasing literal
windmills. As one of these I built a tricycle with three front
wheels, the two wheel forward and steerable. It was claimed that
sprinters, as seen in telephoto shots of stage finishes of the TdF
were scrubbing tires as their wheels were visibly not straight ahead.
I protested that they were riding a wavy course and that if they were
scrubbing they would all come to a halt.
To prove that the drag is roughly W x cos(T), assuming a friction
coefficient of one. The towed tricycle loaded with 180lbs was steered
with toe-out (toe-in being unstable) and correlated about as
accurately as one can measure DRAG = W x cos(T). Bicycling magazine,
after writing a smart reply, refused to print my experiment and text
and left their claim stand that sprinters wear out tires because they
scrub so hard in sprints.
Jobst Brandt <jbr...@hpl.hp.com>
Dave Blake
djco...@flash.net carefully typed:
>Jobst Brandt wrote:
>
>> If a wheel is symmetrical in its long axis, then it has no preferred
>> direction when struck by wind. Therefore, how can it propel the
>> bicycle "forward" when there is no indication where forward is? That
>> is why the point is germane.
>
>Leading edge and trailing edge don't cancel. Work is extracted.
>Sanity is maintained.
So you are proposing that although a side wind has a measured
drag on a spoked wheel, it might have a thrust on a solid
disk wheel.
Lets consider salient points.
1) if the disk wheel is not moving there is no forward thrust, just
like a spoked wheel.
2) For a spoked wheel the drag increases with increasing side
wind.
3) But somehow you propose it is possible that a side wind on a
disk wheel will not only not increase drag, but will result in a
net thrust.
--
Dave Blake
dbl...@phy.ucsf.edu
Dave Bailey
Have you ever taken a watermelon seed and squeezed it between
your thumb and index finger, only to watch it squirt across the
room and hit someone on the cheek? In a similar way, the airflow
pattern around a lenticular disk wheel sitting behind a bicycle
seat tube and a cyclist's body may provide a forward thrust to
the wheel. It does not, however, work for a flat disk or spoked
wheel - just like you can't make a wet penny do the watermelon
thing.
--
Dave Bailey
dbail...@mindspring.com
Jeffrey J. Potoff
Cervelo Cycles wrote:
>
> Jobst Brandt wrote:
>
> > If your sail is symmetrical front to rear and is in-line with the
> > vehicle, it cannot add a forward propulsive force because it doesn't
> > know which way you want to travel, there being no other information
> > about direction of motion but your desire.
>
> Proof? It cannot add a forward propulsive force because it doesn't know
> me proof would be either:
>
> 1) theoretical analysis which shows that a symmetric body in an airflow
> under an angle of attack cannot produce lift. This I have not seen yet
Probably because you, along with nearly everyone else participating
in this thread, haven't taken a course in fluid mechanics ? If they
had, one could be assured that this nonsense about a disk wheel acting
like a sail would have ended a long time ago.
Jeff
Jeffrey J. Potoff
Stergios Papadakis wrote:
>
> Jobst Brandt wrote:
>
> > you will see that whether the ground constrains the position of the
> > wheel or a fixture, it should react to wind in the same way.
> >
>
> That's true, my apologies for being imprecise.Here is how to do the
> experiment:
> put the wheel in a wind tunnel, or out the window
> of your car, and
> measure its lift to drag ratio at various angles of
> attack. Whenever arctan(drag/lift)
> is less than the angle of attack, you get a propulsive
> force when the wheel is on the road.
So you've obviously worked the equations out. Why don't you post
them here along with your results so we can critique them ? Let's
stop talking in generalities and get down to business.
Jeff
Stergios Papadakis
Jobst Brandt wrote:
> Stergios Papadakis writes:
>
>
> > That's true, my apologies for being imprecise.Here is how to do the
> > experiment: put the wheel in a wind tunnel, or out the window of
> > your car, and measure its lift to drag ratio at various angles of
> > attack. Whenever arctan(drag/lift) is less than the angle of
> > attack, you get a propulsive force when the wheel is on the road.
>
> I think we are going around in circles. There is no angle between the
> bicycle course and the wheel. The wheel is symmetrical fore and aft.
> Hence, there can be no lift in the direction of motion, this requiring
> there be a difference in pressure fore and aft, not left to right
> since that cannot translate into forward thrust.
>
> Jobst Brandt <jbr...@hpl.hp.com>
OK Jobst, draw the picture: draw a bike with
its wheels parallel to its direction of motion. Draw
the wind at an angle of attack to the bike (say 15 degrees). Draw the
Lift vector. Draw the Drag vector. (Remember, this is for
the case where arctan(drag/lift) < angle of attack, so say
arctan(drag/lift) = 10 degrees). Arctan(drag/lift) is the angle between the
Lift vector, which is perpendicular to the apparent wind,
and the (Lift + Drag) vector.
Draw the (Lift+Drag) vector (at 10 degrees behind
the Lift vector). You will find that it
has a component along the direction of motion.
The tires on the road cancel the component perpendicular
to the motion, but the part parellel, and forward, remains.
There can be a pressure difference fore and aft. The
wheels have finite thickness. With
the apparent wind coming from a little bit to the
right of your direction of travel, you can see that it is possible
that there might be higher pressure at the rear-right
of the wheel and lower pressure at the front-left.
Hence it is possible that arctan(drag/lift)<a-o-a.
Now I don't know exactly for what range of
angles-of-attack this equation (arctan (drag/lift)<a-o-a)
holds true, because I haven't measured it or looked
at the data, but I do know that because of the above arguments,
it is not physically impossible
to get thrust from a wheel. I believe others when they say
they have seen it because my experience with lifting bodies
tells me that it is reasonable that something shaped like a disk wheel
might have a decent lift-drag ratio. Airplanes have been flown
with diamond-shaped airfoils! The reason is that they make
good supersonic airfoils, so therefore a fair amount
of research has been done on them. But of course, to get supersonic,
first you have to take off and fly subsonically, and they do.
Stergios
Jobst Brandt
Dan Connelly writes:
>> If a wheel is symmetrical in its long axis, then it has no
>> preferred direction when struck by wind. Therefore, how can it
>> propel the bicycle "forward" when there is no indication where
>> forward is? That is why the point is germane.
> Because the wind is from the front. Symmetry is broken. Leading
> edge and trailing edge don't cancel. Work is extracted. Sanity is
> maintained.
By this do you mean that such a device always produces a resultant
force opposite to the wind direction? Could you expand on that. How
does the wind know the object is moving and in what direction, in
order to decide in which direction the resultant force should act.
What do you mean by "symmetry is broken"?
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
Stergios Papadakis writes:
>>> That's true, my apologies for being imprecise.Here is how to do
>>> the experiment: put the wheel in a wind tunnel, or out the window
>>> of your car, and measure its lift to drag ratio at various angles
>>> of attack. Whenever arctan(drag/lift) is less than the angle of
>>> attack, you get a propulsive force when the wheel is on the road.
>> I think we are going around in circles. There is no angle between
>> the bicycle course and the wheel. The wheel is symmetrical fore
>> and aft. Hence, there can be no lift in the direction of motion,
>> this requiring there be a difference in pressure fore and aft, not
>> left to right since that cannot translate into forward thrust.
> OK Jobst, draw the picture: draw a bike with its wheels parallel to
> its direction of motion. Draw the wind at an angle of attack to the
> bike (say 15 degrees). Draw the Lift vector. Draw the Drag vector.
> (Remember, this is for the case where arctan(drag/lift) < angle of
> attack, so say arctan(drag/lift) = 10 degrees). Arctan(drag/lift)
> is the angle between the
I think you are ignoring the basic concept of lift and the resultant
force. If wind is blowing on the front of a fore and aft symmetrical
object, then air pressure is higher on the front than the rear.
If the claim of forward thrust worked, then an essentially
frictionless test vehicle on rails with such a disk wheel mounted (as
a sail) with its long axis parallel to the rail would make the vehicle
move opposite to the direction of the X-component of the wind. Since
the disk wheel in question has complete symmetry, its rotation has no
effect. It can be positioned as fixed a vane on the vehicle.
You propose that this vehicle would travel opposite to the X-component
of a wind. What is it that would make such a device cause a reversal
of thrust? If the wind came from the side, how would it prefer to
propel the vehicle in whichever direction it already was traveling
rather than retard its motion?
Jobst Brandt <jbr...@hpl.hp.com>
mwe...@mother.com
this is what's wrong with some engineering types. in my opinion, if you
haven't actually performed the experiment under realistic conditions, all you
can do is hypothesize. Nothing is "irrefutable." "making sense" is not
proof. Argue and debate if you want, but trying to sound so certain just
makes you appear like a pompous asshole.
-mark weaver
In article <6hgpgb$k...@hplntx.hpl.hp.com>,
jbr...@hpl.hp.com (Jobst Brandt) arrogantly and foolishly wrote:
> How about a practical analysis? One that makes sense in all its
> simplicity. The burden lies with those who believe unbelievable
> physical effects, such as $1000 wheels propel the rider and bicycle.
> I think I have given you irrefutable reasons why this cannot happen.
> In response people are citing all sorts of sailing analogies that
> are in no way parallel to the bicycle wheel.
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
dlk...@eudoramail.com
mwe...@mother.com wrote:
>
> this is what's wrong with some engineering types. in my opinion, if you
> haven't actually performed the experiment under realistic conditions, all you
> can do is hypothesize. Nothing is "irrefutable." "making sense" is not
> proof. Argue and debate if you want, but trying to sound so certain just
> makes you appear like a pompous asshole.
> -mark weaver
>
I agree with you; mainly because you wrote "some engineering types".
For a nanosecond I thought your comments were (are) directed towards a
frequent poster on these newsgroups.
Too many times designers/inventers/marketers (is that a real word?) take
the face value of something on paper and go to their graves defending
that product because IT WAS PERFECT ACCORDING TO OUR CALCULATIONS.
Then we get to sit back and watch the aforementioned persons go crazy
when their product fails miserably in the real world; both riding and
sales.
I liken many engineers and their testing to those Spinning classes.
Many of those Spinners wonder why they are no better off on the
trails/roads even after a month or two at the YMCA spinning to the
oldies with Richard Simmons. Many of those engineers wonder why their
gizmo won't "hold that line at that speed with that certain amount of
blah blah blah".
Reading reference #12, I believe Mr. Brandt wrote " The same kind of
people who bring us so many inventions at the trade show annually, ones
that are never seen again." All I can say is: POT, KETTLE, BLACK.
John Getsoian
On 21 Apr 1998 00:33:15 GMT, Jobst Brandt wrote:
>If a wheel is symmetrical in its long axis, then it has no preferred
>direction when struck by wind. Therefore, how can it propel the
>bicycle "forward" when there is no indication where forward is? That
>is why the point is germane.
Jobst;
While I agree with your contention that as a practical matter it is
unlikely that a real disk or aero-spoke type wheel generates any
useful forward force, and that it is up to those who claim this to
provide quantitative proof, I think you are wrong in your contention
that on symmetry grounds the effect is not possible _in theory_ .
A body in a streamline flow does not have to be asymmetrical to
develop an asymmetrical flow pattern around it. Either rotation, for
cylinders or spheres (golf balls off the tee and curve balls), or
angle of attack for thin plates (do you remember the old _flat_
winged balsa wood toy gliders common <1970), generate lift
perpendicular to the wind direction (not the section axis!). In fact
the flow pattern around a thin plate at any acute AOA is very
non-symmetrical. Now any stable asymmetry in the streamline flow past
an object will result in some kind of unbalanced force component
perpendicular to the incident streamlines. You may not achieve
anywhere near the efficiency of a NACA airfoil but there will still
be some kind of force acting. But forget the boundary layer theory
for a minute and just consider the momentum balance: Any section at
low AOA that is acting as a vane to produce a net deflection of
airflow into a direction parallel with its trailing edge must
experience a reaction force along the line that bisects the angle
between the incident and discharge flow direction. If the angle of
attack is acute, the angle between incidence and discharge is obtuse
and thus the reaction vector must have a component parallel to the
surface in the windward direction.
Thus if the wind is from the forward quarter, there can be some lift
and it will have a forward component (wrt the wheel). This effect
will reduce the net Cd. IF you create a system where this effect is
larger than the rest of the drag forces, THEN you can get a negative
net Cd and sail forward (the sail-boat case).
Of course, what must be added is that if the wind is from the rear
quarter, there will be a similar effect acting to decrease the net
Cd, which in this case is detrimental since drag is now helpful.
However, under the assumption that wind is equally likely from any
direction, then when the forward motion of the bike
is added, the resulting average wind vector is weighted to the
forward quarter, so the fluid mechanics does allow for a
theoretically sound _claim_ . While this does not mean the magnitude
is meaningful for a section which is as inefficient as a disc wheel
at bicycle velocities, neither does it mean that that those trying to
explain the _possibility_ of lift for a symmetric body are incorrect
about the fluid mechanics of lift.
Likewise for an Aerospoke. While a rotation effect on a zero pitch
turbine can be achieved in water practically, the vastly smaller
momentum available from a low velocity gas flow to a cross-section
like an Aero-spoke, even if given an optimized shape, probably puts
any potential usefulness of this effect in the same class as shaving
body hair.
regards,
j. getsoian
David Casseres
In article <353C1A82...@princeton.edu>, Stergios Papadakis
<papa...@princeton.edu> wrote:
>OK Jobst, draw the picture: draw a bike with
>its wheels parallel to its direction of motion. Draw
>the wind at an angle of attack to the bike (say 15 degrees). Draw the
>Lift vector. Draw the Drag vector. (Remember, this is for
>the case where arctan(drag/lift) < angle of attack, so say
>arctan(drag/lift) = 10 degrees). Arctan(drag/lift) is the angle between the
I've never taken a course in aero- or hydrodynamics, but as a pilot and a
sailor I know what pilots and sailors know: the lift of a wing or a sail
never (really, never) gives you a component that is forward, parallel to
the chord of the airfoil. If it did, we would hear aerodynamicists
talking about "lift-to-thrust ratio" instead of "lift-to-drag ratio."
On an airplane, if you could get a forward component of lift it would mean
that the airplane, once set in motion, would be able to fly forward
indefinitely without ever using any power or losing any altitude, i.e.
without requiring any energy. In real life, the wing's total
lift-plus-drag vector points backward, and the airplane needs energy to
keep it moving.
On a sailboat going into the wind, of course you do get a lift component
that is "forward," but it's parallel to the line of motion, not to the
chord of the sail's airfoil. The sail is always (always!) at an angle to
the line of motion, such that the total force of lift and drag on the sail
points forward, and powers the boat. If you sheet the sail in so hard
that its chord is parallel to the centerline, the boat will go backward.
On a bike, the wheel is always parallel to the line of motion, and I will
bet anyone as many beers as they want that the only way you will ever get
a forward-pointing total aerodynamic force on the wheel is if the relative
wind is blowing from behind.
--
David Casseres
Exclaimer: Hey!
Jobst Brandt
John Getsoian writes:
>> If a wheel is symmetrical in its long axis, then it has no
>> preferred direction when struck by wind. Therefore, how can it
>> propel the bicycle "forward" when there is no indication where
>> forward is? That is why the point is germane.
> While I agree with your contention that as a practical matter it is
> unlikely that a real disk or aero-spoke type wheel generates any
> useful forward force, and that it is up to those who claim this to
> provide quantitative proof, I think you are wrong in your contention
> that on symmetry grounds the effect is not possible _in theory_ .
In practice and theory it is impossible.
> A body in a streamline flow does not have to be asymmetrical to
> develop an asymmetrical flow pattern around it.
In fact it is highly unlikely that a symmetrical pattern will develop
around any object unless the airflow approaches on an axis of symmetry.
In this case it is not doing so.
> Either rotation, for cylinders or spheres (golf balls off the tee
> and curve balls), or angle of attack for thin plates (do you
> remember the old _flat_ winged balsa wood toy gliders common <1970),
> generate lift perpendicular to the wind direction (not the section
> axis!). In fact the flow pattern around a thin plate at any acute
> AOA is very non-symmetrical.
Let's not re-introduce the golf ball. That and baseballs, tennis
balls and other small diameter sports balls all get their bias from
spin. That has nothing to do with disk bicycle wheels.
The flat balsa models had a wing that was not in-line with the
direction of motion and is just like putting your hand out the window
of a car, allowing the tilt of the hand raise and lower the hand in
the airstream. The disk wheel is constrained to remain in the line of
motion having no bias to the long axis of the bicycle.
> Now any stable asymmetry in the streamline flow past an object will
> result in some kind of unbalanced force component perpendicular to
> the incident streamlines. You may not achieve anywhere near the
> efficiency of a NACA airfoil but there will still be some kind of
> force acting. But forget the boundary layer theory for a minute and
> just consider the momentum balance: Any section at low AOA that is...
Forget the NACA and boundary layers. Jargon won't pull this out of
the fire. All the models proposed by the proponents of disk
propulsion are invalid. Each citing wings, sails, asymmetric (front
to rear) airfoil shapes, spinning vaned balls and the like. This is a
disk wheel with no vanes or features on it and aligned with the
direction of motion.
> Thus if the wind is from the forward quarter, there can be some lift
> and it will have a forward component (wrt the wheel). This effect
> will reduce the net Cd. IF you create a system where this effect is
> larger than the rest of the drag forces, THEN you can get a negative
> net Cd and sail forward (the sail-boat case).
If this were true, you could blow at this wheel from any angle between
45 degrees to 90 and get "forward motion" in the direction of 0
degrees the direction of the vehicle. So what happens when the wind
comes from 90 to 135 degrees? Does the vehicle go backwards? With a
tailwind?
> Of course, what must be added is that if the wind is from the rear
> quarter, there will be a similar effect acting to decrease the net
> Cd, which in this case is detrimental since drag is now helpful.
How does the wheel know where front is? The relative motion of the
ground with respect to the vehicle cannot have an influence on thrust.
> However, under the assumption that wind is equally likely from any
> direction, then when the forward motion of the bike is added, the
> resulting average wind vector is weighted to the forward quarter, so
> the fluid mechanics does allow for a theoretically sound _claim_ .
I think you have problems with isolating variables. Whether the
vehicle is moving or not has no effect on the "lift" you perceive.
The "forward motion" or even the "motion" of the vehicle has no
influence on the effect of wind on the wheel. The net force in line
with the direction of travel can be modified by the intensity and
direction of the wind, independent of whether the ground is moving
relative to the vehicle.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
Mark Weaver writes:
> jbr...@hpl.hp.com (Jobst Brandt) arrogantly and foolishly wrote:
>> How about a practical analysis? One that makes sense in all its
>> simplicity. The burden lies with those who believe unbelievable
>> physical effects, such as $1000 wheels propel the rider and bicycle.
>> I think I have given you irrefutable reasons why this cannot happen.
>> In response people are citing all sorts of sailing analogies that
>> are in no way parallel to the bicycle wheel.
What in particular is it that you find foolish and arrogant? Do you
believe it is the reader's chore to disprove what advertisers claim,
or is it possibly the advertiser who should convince the reader that
his preposterous claim is true?
Jobst Brandt <jbr...@hpl.hp.com>
Jeffrey J. Potoff
dlk...@eudoramail.com wrote:
>
> mwe...@mother.com wrote:
> >
> > this is what's wrong with some engineering types. in my opinion, if you
> > haven't actually performed the experiment under realistic conditions, all you
> > can do is hypothesize. Nothing is "irrefutable." "making sense" is not
> > proof. Argue and debate if you want, but trying to sound so certain just
> > makes you appear like a pompous asshole.
> > -mark weaver
> >
>
> I agree with you; mainly because you wrote "some engineering types".
> For a nanosecond I thought your comments were (are) directed towards a
> frequent poster on these newsgroups.
>
> Too many times designers/inventers/marketers (is that a real word?) take
> the face value of something on paper and go to their graves defending
> that product because IT WAS PERFECT ACCORDING TO OUR CALCULATIONS.
Really ?! Just who are these people who make such claims ?
'Engineering
types' seem to be pretty successful designing complicated structures
like bridges, high-rise buildings, automobiles and space craft. How
do you think they come up with these ideas ? All of them come out of
calculations. Most 'engineering types' I know don't 'go to their
graves defending a product because it was perfect according to their
calculations'. If the phenomena at work doesn't match the model
they find out why and resolve the difference.
>
> Then we get to sit back and watch the aforementioned persons go crazy
> when their product fails miserably in the real world; both riding and
> sales.
>
> I liken many engineers and their testing to those Spinning classes.
> Many of those Spinners wonder why they are no better off on the
> trails/roads even after a month or two at the YMCA spinning to the
> oldies with Richard Simmons. Many of those engineers wonder why their
> gizmo won't "hold that line at that speed with that certain amount of
> blah blah blah".
What the hell are you talking about ?
>
> Reading reference #12, I believe Mr. Brandt wrote " The same kind of
> people who bring us so many inventions at the trade show annually, ones
> that are never seen again." All I can say is: POT, KETTLE, BLACK.
Again, what are you talking about ?
Jeff
Jeffrey J. Potoff
mwe...@mother.com wrote:
>
> this is what's wrong with some engineering types. in my opinion, if you
> haven't actually performed the experiment under realistic conditions, all you
> can do is hypothesize. Nothing is "irrefutable." "making sense" is not
> proof. Argue and debate if you want, but trying to sound so certain just
> makes you appear like a pompous asshole.
> -mark weaver
>
And this is just what is wrong with many non-engineering types.
They don't understand the phenomena at work and go on and on
hypothesising about this at that, ignoring proof because they
simply don't have the education to understand it. How can someone
without any knowledge of fluid mechanics even begin to judge
the validity of an experiment used to determine if an object has
lift ?
Jeff
> In article <6hgpgb$k...@hplntx.hpl.hp.com>,
> jbr...@hpl.hp.com (Jobst Brandt) arrogantly and foolishly wrote:
>
> > How about a practical analysis? One that makes sense in all its
> > simplicity. The burden lies with those who believe unbelievable
> > physical effects, such as $1000 wheels propel the rider and bicycle.
> > I think I have given you irrefutable reasons why this cannot happen.
> > In response people are citing all sorts of sailing analogies that
> > are in no way parallel to the bicycle wheel.
>
Dave Bailey
On Tue, 21 Apr 1998 13:28:20 -0700, David Casseres <cass...@apple.com> wrote:
>
>On a bike, the wheel is always parallel to the line of motion, and I will
>bet anyone as many beers as they want that the only way you will ever get
>a forward-pointing total aerodynamic force on the wheel is if the relative
>wind is blowing from behind.
If you are talking about a wheel just by itself, then I agree
with what you said here. However, in reality the rear wheel
sits in the wake generated by everything in front of it (the
rider's legs, seat tube, etc.). This is why I think all the
comparisons with sails and wings are a little off.
Because the rear wheel is always in the wake, it may be possible
for the pressure to be greater on the rear half of the rear wheel
than on the front half (I'm imagining streamlines coming together
from being spread apart around the rider's legs). If the rear
wheel is a lenticular disk, this pressure differential may
actually be great enough to give a net force of zero on the
rear wheel. Note that by this argument, a flat disk or spoked
wheel will not exhibit that effect.
The bottom line is, somebody is going to have to sit down and
solve the Euler equations for a geometry similar to that of
the bicycle/rider/rear wheel (but simplified for tractability).
Or, better yet, more experimental data should be collected.
--
Dave Bailey
dbail...@mindspring.com
Dave Bailey
On Tue, 21 Apr 1998 00:26:51 -0400, Jeffrey J. Potoff
<jpo...@rhea.umd.edu> wrote:
>Cervelo Cycles wrote:
>>
>> Jobst Brandt wrote:
>>
Which textbook did you use in your fluid mechanics course? Would
you mind consulting it and posting the equations we need to solve
so I can get to work?
--
Dave Bailey
dbail...@mindspring.com
Jobst Brandt
dlkouba snipes anonymously:
> Too many times designers/inventers/marketers (is that a real word?) take
> the face value of something on paper and go to their graves defending
> that product because IT WAS PERFECT ACCORDING TO OUR CALCULATIONS.
Oh those engineers. They really know little and we shouldn't drive
cars designed by their ilk or fly in aircraft, that is unless some
down to earth mechanic has fixed it so it is reasonably safe. At
least that is the way many non engineers like to see it. According to
calculations, one can predict the trajectory of a space ship from
earth to mars and back. Such calculations are made taking into
account the gravitational forces of many moving planets and their
moons. But that would better be done by people who would do this
experimentally by dlkouba if I'm getting his message.
> Then we get to sit back and watch the aforementioned persons go crazy
> when their product fails miserably in the real world; both riding and
> sales.
How do you do that? I suspect you imagine this as well.
> Reading reference #12, I believe Mr. Brandt wrote " The same kind of
> people who bring us so many inventions at the trade show annually, ones
> that are never seen again." All I can say is: POT, KETTLE, BLACK.
So what have I brought you that doesn't stand up under scrutiny?
Jobst Brandt <jbr...@hpl.hp.com>
Dave Blake
dbail...@mindspring.com carefully typed:
>On Tue, 21 Apr 1998 13:28:20 -0700, David Casseres <cass...@apple.com> wrote:
>>
>>On a bike, the wheel is always parallel to the line of motion, and I will
>>bet anyone as many beers as they want that the only way you will ever get
>>a forward-pointing total aerodynamic force on the wheel is if the relative
>>wind is blowing from behind.
>
>If you are talking about a wheel just by itself, then I agree
>with what you said here. However, in reality the rear wheel
>sits in the wake generated by everything in front of it (the
>rider's legs, seat tube, etc.). This is why I think all the
>comparisons with sails and wings are a little off.
>
>Because the rear wheel is always in the wake, it may be possible
>for the pressure to be greater on the rear half of the rear wheel
>than on the front half (I'm imagining streamlines coming together
>from being spread apart around the rider's legs).
Keep imagining. Try imagining this. The front wheel creates
a largely turbulent wake that is still largely turbulent
immediately behind the rear wheel. The rear wheel contributes
a fraction of the drag of the front wheel.
The front wheel is essentially fully exposed to the wind.
>The bottom line is, somebody is going to have to sit down and
>solve the Euler equations for a geometry similar to that of
>the bicycle/rider/rear wheel (but simplified for tractability).
>Or, better yet, more experimental data should be collected.
Why don't you just go out into the parking lot with
your disk wheels mounted and do a track stand facing into
the wind and see what direction you begin to roll ?
Does anyone really believe that disc wheels will provide
thrust into the wind ?
--
Dave Blake
dbl...@phy.ucsf.edu
Dave Blake
dbail...@mindspring.com carefully typed:
>Which textbook did you use in your fluid mechanics course? Would
>you mind consulting it and posting the equations we need to solve
>so I can get to work?
Start with the Bernoulli equation.
--
Dave Blake
dbl...@phy.ucsf.edu
Jobst Brandt
Dave Bailey writes:
> Because the rear wheel is always in the wake, it may be possible
> for the pressure to be greater on the rear half of the rear wheel
> than on the front half (I'm imagining streamlines coming together
> from being spread apart around the rider's legs). If the rear
> wheel is a lenticular disk, this pressure differential may
> actually be great enough to give a net force of zero on the
> rear wheel. Note that by this argument, a flat disk or spoked
> wheel will not exhibit that effect.
> The bottom line is, somebody is going to have to sit down and
> solve the Euler equations for a geometry similar to that of
> the bicycle/rider/rear wheel (but simplified for tractability).
> Or, better yet, more experimental data should be collected.
No one is going to do that and they can't, because the turbulence at
the rear wheel defies any systematic analysis within practical
computing limits. It also does not warrant such attention because any
effect there is not limited to any wheel shape. The airflow has been
disturbed by the front wheel, the frame and the rider's legs that are
also changing position continuously. You are grasping at straws in a
whirlwind.
Jobst Brandt <jbr...@hpl.hp.com>
Dave Bailey
On Tue, 21 Apr 1998 00:23:27 -0400, Jeffrey J. Potoff
<jpo...@rhea.umd.edu> wrote:
>
>Let's stop talking in generalities and get down to business.
I agree completely. To that end, let me propose a little contest.
Let's try and solve the incompressible Navier-Stokes equations in 2D
for the following geometry:
A solid cylinder of radius a located at the origin (the seat tube).
Two solid cylinders, each of radius b, located at x = +/- 3b/2,
y = b (the legs).
An ellipsoid with major axis c, minor axis d, centered at x = 0,
y = -c/2 - a (the rear wheel - a lenticular disk). The major axis
is perpendicular to the line connecting the two solid cylinders.
For reference, let us also solve them for the same geometry, with a
rectangle of length c and width e (flat disk wheel) in place of the
ellipsoid.
Let us impose the following boundary condition: Far from the origin,
the pressure is 1 atmosphere and the wind velocity is a constant value.
Also, of course, the cylinders, the ellipsoid, and the rectangle are
all solid bodies.
Let the kinematic viscosity be that of air. Let the velocity have a
magnitude of 15 m/s and a direction nearly parallel to the major axis
of the rear wheel. Let a = 1.5 cm, b = 8 cm, c = 70 cm, d = 10 cm, and
e = 2.5 cm.
I think we can neglect compressibility because 15 m/s is much smaller
than 330 m/s (speed of sound in air at 1 atm), so the Mach number is
small. At first I thought we could neglect viscosity as well, which
would mean we'd get to solve the Euler equations instead. However,
neglect of viscosity means neglect of turbulence, and I don't think
we can do that for this geometry. So I think we are going to have to
pay attention to viscosity.
I'm actually serious, by the way. I think it would be fun for
once to see us stop bitching at each other and try and work together
to solve a problem which is somewhat relevant. If the results aren't
completely ridiculous, maybe we can take a shot at 3D.
--
Dave Bailey
dbail...@mindspring.com
Dave Bailey
On 22 Apr 1998 00:13:39 GMT, Dave Blake <dbl...@phy.ucsf.edu> wrote:
>dbail...@mindspring.com carefully typed:
>>Because the rear wheel is always in the wake, it may be possible
>>for the pressure to be greater on the rear half of the rear wheel
>>than on the front half (I'm imagining streamlines coming together
>>from being spread apart around the rider's legs).
>
>a largely turbulent wake that is still largely turbulent
>immediately behind the rear wheel. The rear wheel contributes
>a fraction of the drag of the front wheel.
I'm not sure. I think pressure forces on the rear wheel (if
it is a lenticular disk) might be as great as viscous drag.
As far as I'm concerned, the jury is out until I solve the
equations. Why is it that you're so sure about what you've
imagined? Have you solved the equations? Have you given
any thought as to what the equations might be? Have you
ever solved a nonlinear differential equation numerically
in 2D? I'm curious because you seem to be much more confident
in your intuition about this than I am willing to be.
>>The bottom line is, somebody is going to have to sit down and
>>solve the Euler equations for a geometry similar to that of
>>the bicycle/rider/rear wheel (but simplified for tractability).
>>Or, better yet, more experimental data should be collected.
>
>Why don't you just go out into the parking lot with
>your disk wheels mounted and do a track stand facing into
>the wind and see what direction you begin to roll ?
How does that give me the drag force on the rear wheel?
I'm just guessing here, but I would think other factors
would dominate the outcome of your little experiment. Back
to the drawing board.
>Does anyone really believe that disc wheels will provide
>thrust into the wind ?
My position is, I don't know, and won't be willing to make
a stronger statement until I've done some calculations. But
I can't rule it out, and I think it might be worth
investigating.
--
Dave Bailey
dbail...@mindspring.com
Robert L. Frazier
On 22 Apr 1998 00:03:24 GMT, Jobst Brandt <jbr...@hpl.hp.com> wrote:
>dlkouba snipes anonymously:
>
>> Too many times designers/inventers/marketers (is that a real word?) take
>> the face value of something on paper and go to their graves defending
>> that product because IT WAS PERFECT ACCORDING TO OUR CALCULATIONS.
>
>Oh those engineers. They really know little and we shouldn't drive
>cars designed by their ilk or fly in aircraft, that is unless some
>down to earth mechanic has fixed it so it is reasonably safe. At
>least that is the way many non engineers like to see it. According to
>calculations, one can predict the trajectory of a space ship from
>earth to mars and back. Such calculations are made taking into
>account the gravitational forces of many moving planets and their
>moons. But that would better be done by people who would do this
>experimentally by dlkouba if I'm getting his message.
>
Actually, there is a somewhat interesting point being expressed here. As I
understand it, much engineering is done with some rather strong assumptions,
i.e., it concerns what happens in rather idealised conditions. The difficult
part is to take a good understanding of what happens in idealised conditions
and apply it to non-idealised conditions. A case in point: Mercedes did some
wonderful engineering on a smaller version of their saloon, but it didn't pass
a real world test: the moose test. This requires one to be able to brake to
avoid a moose in the middle of the road. Instead of handling this situation
well, the Mercedes flipped. If I remember correctly, this caused them no end
of commercial troubles.
Of course, I am not trying to denigrate attempts to come up with a theoretical
understanding of empirical phenomena. Indeed, it would be insane to think
that our practial understanding doesn't improve with our theoretical
understanding. One of the things I most enjoy about this newsgroup is how a
discussion will start off with something quite practical (e.g., `what gear
should I use') and move off to something much more theoretical. Consequently,
I've learned an enormous amount, both theoretical and practical, from
following the postings in this newsgroup. And I am grateful. Rather, it is
an expression of a limited scepticism concerning the relationship between our
(abstract?) models of things and their occurrence in the real world.
Best wishes,
Bob
--
Robert L. Frazier EMAIL: robert....@christ-church.ox.ac.uk
Christ Church TELEPHONE: +44 1865 276493
Oxford OX1 1DP FAX: +44 1865 794199
UK
Dave Bailey
On 22 Apr 1998 00:33:20 GMT, Jobst Brandt <jbr...@hpl.hp.com> wrote:
>Dave Bailey writes:
>> The bottom line is, somebody is going to have to sit down and
>> solve the Euler equations for a geometry similar to that of
>> the bicycle/rider/rear wheel (but simplified for tractability).
>> Or, better yet, more experimental data should be collected.
>
>the rear wheel defies any systematic analysis within practical
>computing limits. It also does not warrant such attention because any
>effect there is not limited to any wheel shape. The airflow has been
>disturbed by the front wheel, the frame and the rider's legs that are
>also changing position continuously. You are grasping at straws in a
>whirlwind.
Could you give me a reference on that? I'm curious because I was
just in the library and there was a large body of literature dealing
with computational fluid dynamics. It seems that people have been
able to compute turbulent flow rather nicely for quite some time now.
While the microscopic details of the flow will be dependent on machine
error, the macroscopic properties (its extent, the range of parameters
for which it exists, the viscous drag generated by it) obtained are
reliable, especially for relatively simple geometries. Therefore,
I have to disagree that the turbulence at the rear wheel "defies any
systematic analysis". Also, what do you mean by your statement that
the effect "is not limited to any wheel shape"?
Jobst, have you ever solved a differential equation? I suspect not.
What on earth do you think gives you the authority to make statements
about the solution of set of nonlinear differential equations when
you don't even know what they are, much less how to go about solving
them? All I've said is that the effect is possible, but we can't
know for sure without more study. Experiments would be the best
thing, but numerical work is a place to start. We've got to stop
all this flapping and get some results.
--
Dave Bailey
dbail...@mindspring.com
Dave Bailey
On 22 Apr 1998 00:14:46 GMT, Dave Blake <dbl...@phy.ucsf.edu> wrote:
>dbail...@mindspring.com carefully typed:
>>you mind consulting it and posting the equations we need to solve
>>so I can get to work?
>
>Start with the Bernoulli equation.
I don't think so. The Bernoulli equation neglects viscosity
and we wouldn't get that turbulent wake we want. I think we need
to solve the incompressible Navier-Stokes equations. At first I
thought "Euler", but you're right, turbulence is probably important.
--
Dave Bailey
dbail...@mindspring.com
Dave Blake
dbail...@mindspring.com carefully typed:
>On 22 Apr 1998 00:13:39 GMT, Dave Blake <dbl...@phy.ucsf.edu> wrote:
>>dbail...@mindspring.com carefully typed:
>>>for the pressure to be greater on the rear half of the rear wheel
>>>than on the front half (I'm imagining streamlines coming together
>>>from being spread apart around the rider's legs).
>>
>>Keep imagining. Try imagining this. The front wheel creates
>>a largely turbulent wake that is still largely turbulent
>>immediately behind the rear wheel. The rear wheel contributes
>>a fraction of the drag of the front wheel.
>
>I'm not sure. I think pressure forces on the rear wheel (if
>it is a lenticular disk) might be as great as viscous drag.
>As far as I'm concerned, the jury is out until I solve the
>equations. Why is it that you're so sure about what you've
>imagined?
Well, I am basing it on a few things.
The first is empiric. You can verify it yourself if you
try riding a TT course first with an aero front wheel
and then a rear. The front makes a much larger difference than
the rear.
Second, I have discussed this issue at length with a friend
of mine who is a professional aerospace engineer and biker
racer.
Third, I have seen wind tunnel testing and know that there
is nothing resembling laminar flow near the rear wheel.
>>Why don't you go out in the parking lot with
>>your disk wheels mounted and do a track stand facing into
>>the wind and see what direction you begin to roll ?
>
>How does that give me the drag force on the rear wheel?
>I'm just guessing here, but I would think other factors
>would dominate the outcome of your little experiment. Back
>to the drawing board.
No, there is purportedly a fairly wide range of angles for
which the net force on a disk wheel is into the wind -
at least 65-90 degrees. I can easily hold a track stand long
enough to see if I begin to roll into the wind. I don't have
the disk wheels though, and even if I did I would not
waste the time.
>>Does anyone really believe that disc wheels will provide
>>thrust into the wind ?
>
>My position is, I don't know, and won't be willing to make
>a stronger statement until I've done some calculations. But
>I can't rule it out, and I think it might be worth
>investigating.
Have fun.
--
Dave Blake
dbl...@phy.ucsf.edu
Pierre Ermes
Jobst Brandt wrote:
>
> Dave Bailey writes:
>
> > The bottom line is, somebody is going to have to sit down and
> > solve the Euler equations for a geometry similar to that of
> > the bicycle/rider/rear wheel (but simplified for tractability).
> > Or, better yet, more experimental data should be collected.
>
> No one is going to do that and they can't, because the turbulence at
> the rear wheel defies any systematic analysis within practical
> computing limits. It also does not warrant such attention because any
> effect there is not limited to any wheel shape. The airflow has been
> disturbed by the front wheel, the frame and the rider's legs that are
> also changing position continuously. You are grasping at straws in a
> whirlwind.
>
The sail effect certainly exists, in some people minds at least.
The question is; how did it get there?
I think someone tried to explain the unusual 'boost' he/she felt
during cycling with a disk wheel. Of course this isn't a boost,
it only indicates a reduction of air resistance, or drag.
My theory is that this is caused by the effect of the disk on
the air flow around the disk. As pointed out above the air flow
around the disk is no way laminar. Turbulence is coming from
the frame and the moving(!) legs. With a laced wheel all these
turbulences start interfering which causes larger turbulences
and increases drag. A disk wheel is 'ordening' these turbulences
and seperates the whirls from the left and right leg and thus
causing a smaller drag. Or 'boost' as some call it.
Pierre Ermes
Cervelo Cycles
Pierre Ermes wrote:
> The sail effect certainly exists, in some people minds at least.
> The question is; how did it get there?
how it got here this time I don't know, but a previous reason (in 1995)
for a thread on this was the posting of (British??) windtunnel results
that showed a very small thrust at a very specific angle. The conclusion
definitely wasn't that as long as you have disk wheels, you might as
well take off your pedals, but it was a small thrust. How accurate was
the data? nobody seems to know. Has anybody else repeated it? I am not
sure, but others may know.
Sincerely,
Gerard Vroomen, Cervelo Cycles
e-mail: intern...@cervelo.com
website: http://www.cervelo.com
Dave Bailey
On 22 Apr 1998 05:13:57 GMT, Dave Blake <dbl...@phy.ucsf.edu> wrote:
>dbail...@mindspring.com carefully typed:
>>it is a lenticular disk) might be as great as viscous drag.
>>As far as I'm concerned, the jury is out until I solve the
>>equations. Why is it that you're so sure about what you've
>>imagined?
>
>Well, I am basing it on a few things.
>
>The first is empiric. You can verify it yourself if you
>try riding a TT course first with an aero front wheel
>and then a rear. The front makes a much larger difference than
>the rear.
I don't think this matters. All it shows is that the net
drag on any rear wheel is not very big compared to the drag
on other parts of the bike, which is intuitively obvious
anyway. Therefore, eliminating that drag altogether under
very special circumstances (if such is possible) won't make
a huge difference, which we know to be true for exactly the
reason that you state. So I don't think you've ruled out the
possibility.
>Second, I have discussed this issue at length with a friend
>of mine who is a professional aerospace engineer and biker
>racer.
Great, then maybe you can paraphrase what he said to you,
I'd be interested to hear it.
>Third, I have seen wind tunnel testing and know that there
>is nothing resembling laminar flow near the rear wheel.
Which wind tunnel did you visit when you watched the testing?
The statement about laminar flow actually works against you.
The reason is that the turbulent flow generated by the rider
creates a much greater pressure gradient behind the rider
than a laminar flow would. In fact, laminar flow can be
pretty much ruled out due to the fact that the Reynolds
number is on the order of 10^6. Anyway, the large pressure
gradient means that it is possible that the pressure on the
front half of the rear wheel is significantly lower than
that on the rear half of the rear wheel. With a lenticular
disk, this may mean a net force on the wheel from pressure
difference (neglecting viscous drag) which is forward (in
the direction of travel). Also, the viscous drag due to
the presence of the turbulent flow itself is surely almost
completely due to the rider. I doubt removing the rear
wheel has much of an effect - this runs counter to our
intuition that an aero rear wheel doesn't reduce drag all
that much.
>No, there is purportedly a fairly wide range of angles for
>which the net force on a disk wheel is into the wind -
>at least 65-90 degrees. I can easily hold a track stand long
>enough to see if I begin to roll into the wind. I don't have
>the disk wheels though, and even if I did I would not
>waste the time.
You would only roll into the wind if the net drag force on
you plus the bike were negative, which nobody is suggesting.
This is why that experiment is irrelevant. Once again, it
does not allow you to determine the drag force on the rear
wheel.
--
Dave Bailey
dbail...@mindspring.com
Steven Castillo
dbail...@mindspring.com (Dave Bailey) writes:
> On 22 Apr 1998 00:33:20 GMT, Jobst Brandt <jbr...@hpl.hp.com> wrote:
> >Dave Bailey writes:
> >> The bottom line is, somebody is going to have to sit down and
> >> solve the Euler equations for a geometry similar to that of
> >> the bicycle/rider/rear wheel (but simplified for tractability).
> >> Or, better yet, more experimental data should be collected.
> >
> >No one is going to do that and they can't, because the turbulence at
> >the rear wheel defies any systematic analysis within practical
> >computing limits. It also does not warrant such attention because any
> >effect there is not limited to any wheel shape. The airflow has been
> >disturbed by the front wheel, the frame and the rider's legs that are
> >also changing position continuously. You are grasping at straws in a
> >whirlwind.
>
> Could you give me a reference on that? I'm curious because I was
> just in the library and there was a large body of literature dealing
> with computational fluid dynamics. It seems that people have been
> able to compute turbulent flow rather nicely for quite some time now.
> While the microscopic details of the flow will be dependent on machine
> error, the macroscopic properties (its extent, the range of parameters
> for which it exists, the viscous drag generated by it) obtained are
> reliable, especially for relatively simple geometries. Therefore,
> I have to disagree that the turbulence at the rear wheel "defies any
> systematic analysis". Also, what do you mean by your statement that
> the effect "is not limited to any wheel shape"?
>
> Jobst, have you ever solved a differential equation? I suspect not.
> What on earth do you think gives you the authority to make statements
> about the solution of set of nonlinear differential equations when
> you don't even know what they are, much less how to go about solving
> them? All I've said is that the effect is possible, but we can't
> know for sure without more study. Experiments would be the best
> thing, but numerical work is a place to start. We've got to stop
> all this flapping and get some results.
>
> --
> Dave Bailey
> dbail...@mindspring.com
Dave:
I assure you that attempting to solve the Navier Stokes equations for
the complex 3-d problem of flow over the wheel in the presence of the
frame and the rider would tax the largest computational resources at
any of the DOE facilities in the country. Generation of the geometry
and the finite element mesh before any simulation would also be
extremely time consuming. Of course, if there is grant money
available somewhere, perhaps a proposal could be written to address
the problem?:)
In this case, experiments would be more appropriate. I believe that
velonews did some very systematic studies in a wind tunnel a few years
ago with several different types of wheels using the wind direction
(attack angle) as one of the variables. The article was well done.
Unfortunately, I don't save my old velo news anymore.
Steve C.
Jobst Brandt
Dave Bailey lathers us with the following jargon:
> Let's try and solve the incompressible Navier-Stokes equations in 2D
> for the following geometry:
> A solid cylinder of radius a located at the origin (the seat tube).
> Two solid cylinders, each of radius b, located at x = +/- 3b/2,
> y = b (the legs).
> An ellipsoid with major axis c, minor axis d, centered at x = 0,
> y = -c/2 - a (the rear wheel - a lenticular disk). The major axis
> is perpendicular to the line connecting the two solid cylinders...
I think I've had enough. Whom are you trying to impress? You propose
a disk bicycle wheel with improbable aerodynamic abilities that you
cannot substantiate, so you respond with unrelated techno-jargon that
might be an entertaining caricature were it in a different context.
> I'm actually serious, by the way. I think it would be fun for
> once to see us stop bitching at each other and try and work together
> to solve a problem which is somewhat relevant. If the results aren't
> completely ridiculous, maybe we can take a shot at 3D.
You shouldn't have added this comment because it leaves you no way out
of your buffoonery. You may master the jargon but you don't understand
where and how to use it.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
Dave Bailey writes:
>>> The bottom line is, somebody is going to have to sit down and
>>> solve the Euler equations for a geometry similar to that of
>>> the bicycle/rider/rear wheel (but simplified for tractability).
>>> Or, better yet, more experimental data should be collected.
>> No one is going to do that and they can't, because the turbulence
>> at the rear wheel defies any systematic analysis within practical
>> computing limits. It also does not warrant such attention because
>> any effect there is not limited to any wheel shape. The airflow
>> has been disturbed by the front wheel, the frame and the rider's
>> legs that are also changing position continuously. You are
>> grasping at straws in a whirlwind.
> Could you give me a reference on that? I'm curious because I was
> just in the library and there was a large body of literature dealing
> with computational fluid dynamics.
How about YOU giving a reference on that. You may believe that
someone would spend the time to design an aerodynamic computer model
of a bicycle to include the front wheel, forks, frame tubes, cranks
and pedals in every position, the rider's legs, also in every position
and finally the rear wheel to determine whether or not a disk wheel
produces forward thrust in certain angles of winds. No one has done
this and I doubt that anyone would attempt it, considering the utility
of the exercise.
> It seems that people have been able to compute turbulent flow rather
> nicely for quite some time now. While the microscopic details of
> the flow will be dependent on machine error, the macroscopic
> properties (its extent, the range of parameters for which it exists,
> the viscous drag generated by it) obtained are reliable, especially
> for relatively simple geometries. Therefore, I have to disagree
> that the turbulence at the rear wheel "defies any systematic
> analysis". Also, what do you mean by your statement that the effect
> "is not limited to any wheel shape"?
If airflow around a rear wheel pushes it forward then it pushes it
forward whether it is a disk or conventional wheel.
> Jobst, have you ever solved a differential equation? I suspect not.
On what basis do you come to this conclusion. Would you feel better
if I used more jargon. Maybe you you have not heard of Richard
Feynman who in his Physics lectures followed the principle of "if you
can't explain it in plain language, then you probably don't understand
it yourself." This isn't a matter of differential or even partial
differential equations. All your proposed computation has no value if
the model is incorrect. Garbage in = garbage out, should be familiar
to you.
> What on earth do you think gives you the authority to make statements
> about the solution of set of nonlinear differential equations when
> you don't even know what they are, much less how to go about solving
> them? All I've said is that the effect is possible, but we can't
> know for sure without more study. Experiments would be the best
> thing, but numerical work is a place to start. We've got to stop
> all this flapping and get some results.
I don't see that I made any statements about nonlinear things on earth
or otherwise. You have concocted these hypothetical problems, not I.
Jobst Brandt <jbr...@hpl.hp.com>
David Casseres
In article <slrn6jpv2h.5...@cycleops.localdomain>,
dbail...@mindspring.com wrote:
>...the rear wheel
>sits in the wake generated by everything in front of it (the
>rider's legs, seat tube, etc.). This is why I think all the
>comparisons with sails and wings are a little off.
>
>Because the rear wheel is always in the wake, it may be possible
>for the pressure to be greater on the rear half of the rear wheel
>than on the front half (I'm imagining streamlines coming together
>from being spread apart around the rider's legs). If the rear
>wheel is a lenticular disk, this pressure differential may
>actually be great enough to give a net force of zero on the
>rear wheel. Note that by this argument, a flat disk or spoked
>wheel will not exhibit that effect.
I think this is just another way of saying a lenticular disk wheel results
in less drag on the whole configuration. That's different from the claim
that it generates forward aerodynamic thrust.
Tim McNamara
In article <6hjcnj$15...@itssrv1.ucsf.edu>, Dave Blake
<dbl...@phy.ucsf.edu> wrote:
>Does anyone really believe that disc wheels will provide
>thrust into the wind ?
Obviously yes, since they continue to argue that this is the case. Some
folks clearly want this to be true (either that or they want Jobst, et al,
to be wrong).
I still cannot bring myself to believe that a bicycle wheel rotating
forward into a direct or side wind (whether a dics wheel, trispoke type
wheel or conventional wheel) can generate a *forward* force from
aerodynamic effects. It violates common sense, for one thing, and the
comparisons with a sailboat, land yacht or full-fairing HPV are obviously
invalid. Any sail-driven vehicle is of is drastically different
geometrical configuration: the sail's angle to the direction of travel
*and* the direction of the wind is constantly variable, a sail is not
symmetrical, and it it located on a large lever arm above the main body of
the vehicle (which may or may not have anything to do with anything). The
surface area of a sail is *huge* compared to the rest of the vehicle. But
if the proposition is true that a net thrust in provided in a headwind,
and if these wheels are symmetrical, wouldn't it result in an equal net
*drag* in a tailwind?
On the bicycle, the surface area of a trispoke type wheel is small (that
of a disc wheel is much larger). It rotates relative to the bicycle, and
only one wheel is really "in the wind" as the rear wheel is largely in a
turbulent slipstream caused by the parts of the bike that precede it.
Even if there was some net forward force gained, which seems impossible to
me, it would only be gained through a very small part of the rotation of
the wheel (in the case of a trispoke type wheel, anyways, since the
airfoil would only be properly oriented to the wind at one point in its
rotation and would probably be cancelled out 180 degrees later).
Basically it seems to me that this idea is somehow a contravention of a
few laws of physics.... Now, OTOH, I would be quite willing to believe
that disc wheels and trispoke type wheels provide greater benefit in a
full or partial *tailwind* owing to their greater surface area.
--
One pane of glass in the window. No one is complaining, though,
come in and shut the door. Faded is the crimson of the ribbons
that she wore, and it's strange how no one comes round any more.
-Robert Hunter
Enduro Sport Dan
> how it got here this time I don't know, but a previous reason (in 1995)
> for a thread on this was the posting of (British??) windtunnel results
> that showed a very small thrust at a very specific angle. The conclusion
> definitely wasn't that as long as you have disk wheels, you might as
> well take off your pedals, but it was a small thrust. How accurate was
> the data? nobody seems to know. Has anybody else repeated it? I am not
> sure, but others may know.
I have been following this thread and while I have no scientific
background to contribute to the flying formulae I have also seen a wind
tunnel test summary showing lift from a HED disc (but not from the Zipp
disc or Specialized Trispoke in the same test).
The test was conducted at the Bath University Dept. of Mechanical Eng. and
summarized in Cycling Plus (UK) in July 1993. The summary of the wind
tunnel test on the HED disc reads as follows:
"Far and away the best performer in the wind tunnel. Though not quite as
fast as the Zipp with a headwind, as soon as the wind was angled at more
than 15 degrees it outperformed the other wheels. Under certain conditions
- from 30 degrees onwards - it gave a slight lift, acting like a sail and
exerting a positive force forwards. This force was really quite small but
it's clearly there on the graph"
It was implied that in the Bath U tests that the wheels we tested alone,
not in the context of a complete bicycle or bike and rider.
Listening to the arguments I wouldn't discount the posibility of
generating lift (and forwards thrust)from a lenticular disc wheel. The
amount would likely be small and would not be enough to make a bike/rider
system move forwards into the wind, however, that doesn't take away from
the fact that there can be a force working in your favor due to using a
lenticular disc wheel. The issue of which way is forwards in also
misleading. We use words like forwards and backwards that "the wind"
doesn't relate to but the poster's intent of forwards is "into the wind"
and it would be reasoned that in a net tailwind that a disc (or anything
else that makes a bike more aero from the back) wouldn't help you. But
when do we get a _net_ tailwind? In those wind conditions surely the rider
would be traveling fast enough to generate an aparent headwind (forward
quarter).
I have followed this thread with interest but the level of antagonism in
this "discussion" makes it difficult to follow at times.
Dan Rishworth
Enduro Sport - Canada's Multi-Sport Pro Shop
1 800 448 4678
http://www.endurosport.com
mwe...@mother.com
Holy cow! what a dick! Why don't you just say
"I know everything and you suck. Kneel and whimper, simple cyclists. Bow to
the annointed fucking fountain of all knowledge."
Wait, I have a better idea. Why don't you just shut the fuck up, Jobst!
In article <6hl1jf$b...@hplntx.hpl.hp.com>#1/1,
jbr...@hpl.hp.com (Jobst Brandt) wrote:
>
> I think I've had enough. Whom are you trying to impress? You propose
> a disk bicycle wheel with improbable aerodynamic abilities that you
> cannot substantiate, so you respond with unrelated techno-jargon that
> might be an entertaining caricature were it in a different context.
>
> You shouldn't have added this comment because it leaves you no way out
> of your buffoonery. You may master the jargon but you don't understand
> where and how to use it.
>
> Jobst Brandt <jbr...@hpl.hp.com>
>
mwe...@mother.com
Holy cow! what a dick! Why don't you just say
"I know everything and you suck. Kneel and whimper, simple cyclists. Bow to
Jay Wenner
Tim McNamara (tim...@minn.net) wrote:
: invalid. Any sail-driven vehicle is of is drastically different
: geometrical configuration: the sail's angle to the direction of travel
: *and* the direction of the wind is constantly variable, a sail is not
: symmetrical, and it it located on a large lever arm above the main body of
: the vehicle (which may or may not have anything to do with anything). The
: surface area of a sail is *huge* compared to the rest of the vehicle. But
: if the proposition is true that a net thrust in provided in a headwind,
: and if these wheels are symmetrical, wouldn't it result in an equal net
: *drag* in a tailwind?
Has anyone said their wouldn't be a drag is the wind switched directions?
I have no knowledge in this area (but doesn't seem to stop others...)
Jobst's arguement leads me to believe that a sailboat could not
sail with a rigid circular sail (shape of a large disk wheel).
True?
Finally, if you're going to model this question, why would you
make it complicated and model a rider on a bike? The difference between
spoked and disk wheels is due to the wheel not the rider.
I say Euler the chain not the wheel.
Couldn't resist.
Jay Wenner
Jobst Brandt
Dan (who?) writes:
> Listening to the arguments I wouldn't discount the possibility of
> generating lift (and forwards thrust)from a lenticular disc
> wheel. The amount would likely be small and would not be enough to
> make a bike/rider system move forwards into the wind, however, that
> doesn't take away from the fact that there can be a force working in
> your favor due to using a lenticular disc wheel. The issue of which
> way is forwards in also misleading. We use words like forwards and
> backwards that "the wind" doesn't relate to but the poster's intent
> of forwards is "into the wind" and it would be reasoned that in a
> net tailwind that a disc (or anything else that makes a bike more
> aero from the back) wouldn't help you. But when do we get a _net_
> tailwind? In those wind conditions surely the rider would be
> traveling fast enough to generate an apparent headwind (forward
> quarter).
That's all interesting but the term forward and backward are with
respect to the bicycle, not the wind or the wheels. Therefore, the
disk wheel being radially (and therefore fore and aft) symmetrical,
tailwinds would have to cause retarding forces if headwinds cause
forward forces.
Jobst Brandt <jbr...@hpl.hp.com>
Jeffrey J. Potoff
Dave Bailey wrote:
>
> Anyway, the large pressure
> gradient means that it is possible that the pressure on the
> front half of the rear wheel is significantly lower than
> that on the rear half of the rear wheel. With a lenticular
> disk, this may mean a net force on the wheel from pressure
> difference (neglecting viscous drag) which is forward (in
> the direction of travel). Also, the viscous drag due to
> the presence of the turbulent flow itself is surely almost
> completely due to the rider. I doubt removing the rear
> wheel has much of an effect - this runs counter to our
> intuition that an aero rear wheel doesn't reduce drag all
> that much.
>
I don't get it. You keep digging your hole deeper and
deeper. The more you post, the more incorrect you become.
The pressure on the front half of the rear wheel is not
significantly lower than on the rear half.
Since you seem to be convinced that disk wheels
generate lift or some net forward motion, I suggest you
perform an experiment and prove it. Put the wheel in
question in a fork mounted on something that floats. Face
the 'boat' into the wind and see if it travels into the
wind. Vary the orientation of the wheel and see if the
boat moves in any direction other that the direction of
the wind.
Jeff