SWB Stability?

One word: centripetal force

--
This look left intentionally blank "George MacKenzie" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Tom Sherman <[email hidden]> writes:

< snip - sensible comment from Jeff Wills >

Quoted message said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers RANS "Wavewind" and Rocket,
Earth Cycles Sunset and Dragonflyer

Please explain so that the rest of us may be enlightened.

Doctor: How many fingers am I holding up? Timothy B. Storey: Thursday!

:-)

Dave Larrington - legslarry.beerdrinkers.co.uklegslarry.beerdrinkers.co.ukOpen ↗
===========================================================
Editor - British Human Power Club Newsletter
bhpc.org.ukbhpc.org.ukOpen ↗
===========================================================

Tom Sherman said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a free
body diagram).

They are wrong. Or rather, you are quoting them out of context. If you are working in an inertial
frame there is no such thing as centrifugal force. However, when you get into a rotating reference
frame things behave very differently: things try to fly off from the axis of rotation. This is a
very real effect, and caused by a very real force called centrifugal force. When you write the
equation of motion in a rotating reference frame you definitely need to include centrifugal force.
(And Coriolis force too, for that matter.)

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

Mikael Seierup said:

... Heres my kind of free body diagram: kmmod.comcschifferOpen ↗

Nice ocean and seagulls. I am having trouble seeing the force vectors, however.

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers RANS "Wavewind" and Rocket,
Earth Cycles Sunset and Dragonflyer

Ken Kobayashi must be edykated coz e writed:

Quoted message said:

Tom Sherman said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

They are wrong. Or rather, you are quoting them out of context. If you are working in an inertial
frame there is no such thing as centrifugal force. However, when you get into a rotating reference
frame things behave very differently: things try to fly off from the axis of rotation. This is a
very real effect, and caused by a very real force called centrifugal force. When you write the
equation of motion in a rotating reference frame you definitely need to include centrifugal force.
(And Coriolis force too, for that matter.)

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

Rotational mass is subject to centripedal force, centrifugal force is disproved by the fact that
objects escaping from a spinning mass travel in an arc rather than a straight line, therefore the
escaping object is subject to simple momentum as a result of stored potential energy being allowed
to convert into kinetic energy whilst the objects trajectory is still subject to the action of
centripedal force acting inwards to the rotational mass.

--
Ian

catrike.co.ukcatrike.co.ukOpen ↗

Ken Kobayashi said:

Tom Sherman said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

They are wrong. Or rather, you are quoting them out of context. If you are working in an inertial
frame there is no such thing as centrifugal force. However, when you get into a rotating reference
frame things behave very differently: things try to fly off from the axis of rotation.

The key here is that the object in rotational motion will continue in a straight line [1], i.e.
tangential to the path of circular motion if the force holding it to the center of rotation is
removed. The force that causes the object to travel in a circular path instead of a straight line is
a vector radial from the object to the center of rotation, i.e. centripetal force.

A centrifugal force would be a vector that is radial outwards from the center of rotation to the
object rotating - the free body diagram of the object in motion will clearly show that no such
force exists.

The following is presented for your viewing pleasure, and has nothing to do with the above argument.
< challenge-ligfietsen.nltaifun.jpgOpen ↗

Quoted message said:

[1] If Mr. Newton is correct. 🙂

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers RANS "Wavewind" and Rocket,
Earth Cycles Sunset and Dragonflyer

Ian said:

Rotational mass is subject to centripedal force, centrifugal force is disproved by the fact that
objects escaping from a spinning mass travel in an arc rather than a straight line, therefore the
escaping object is subject to simple momentum as a result of stored potential energy being allowed
to convert into kinetic energy whilst the objects trajectory is still subject to the action of
centripedal force acting inwards to the rotational mass.

That didn't make much sense to me. What exactly do you mean by an "object escaping from a spinning
mass"? Example please.

Centrifugal force is a force felt by all mass when you are working in a rotating reference frame.
Imagine standing in a spinning space station; the force holding down onto the floor is centrifugal
force. When you hold an apple and let go, it's centrifugal force that pulls it towards the floor.

Of course if you step outside the space station and observe things in relation to an inertial frame,
centrifugal force disappears. (Instead you have centripetal force pushing everything towards the
center of the space station, balanced by inertia.) That doesn't mean centrifugal force does not
exist; it means it only exists in certain reference frames.

I'm sorry to nitpick like this but I'm so sick of people saying "there's no such thing as
centrifugal force" without sufficient explanation and when there's no real need to make a
distinction.

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

I wonder how much of the stability difference between the streetmachine and vision r40 is the rider position. The SMGT has the rider reclined at about 35 degrees. The vision I test rode, and the linear I currently ride (which is very twitchy), both have the rider in a fairly upright position. This would raise the center of gravity (no, I can't enter the Sherman/Kobayashi physics debate), and make them more unstable.

Also, from my test ride on an SMGT, I found the shocks added a lot of stability because they reduced the correction the rider needed to make.

Quoted post said:

Originally posted by Mads Hilberg
> > I can't rule out that some SWBs are poorly designed stabilitywise but if

Quoted message said:

so

Quoted message said:

it certainly hasn't been any of those I have ridden. Old wives tale if

you

Quoted message said:

ask me.

Just pulling data from my own experiences.

I own a Bjaellby and a Streetmachine (both SWB). The Bjaellby is far less stable than the SM, which
handles really well even at very low speeds. It's all down to geometry and weight distribution I
guess. The Bjaellby is still a good bike as it's lighter and faster, but the Streetmachine is just
so sure-footed (sure-wheeled??) it's a dream.

Quoted message said:

Quoted message said:

Maybe it was the USS? I hear USS is horribly unstable. *cough cough* ;o)

:-) USS is very stable, IMO. I loved my Ryan Vanguard. Sometimes I wish

I

Quoted message said:

hadn't sold it. <sniff>

I agree - USS is more stable than OSS. I find that many OSS bikes steering twitches from side to
side as one rides along on them.

Quoted message said:

I was curious about SWB's in particular. The short wheelbase does create some extremes of steering
sensitivity. I've wondered if maybe they should have steering dampers or something. Might make me
feel a little less worried on fast descents/sprints (not that there are too many of those

here

Quoted message said:

in flat Illinois...)

Having gone down a few mountains in Nova Scotia recently on my Streetmachine (loaded with 4
panniers), I can only say that it handled very well. At 75km/h I felt no instability whatsoever and
going round the switch-backs at ridiculous speeds didn't worry me at all. You suggest steering
dampening, but perhaps just having fairly wide handlebars like the SM is all that is needed. The
only time I have felt a little instability was when I was pulling a BOB YAK which was ridiculously
heavily loaded, but even then I found I could control it - I just had to be a little more awake.

Mads

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

Quoted message said:

Jeff Wills said:

...

<snippedy-doo-dah>

Quoted message said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

Tom

*I* know that... but "centrifugal force" is shorthand for "inertia will tend throw the object in a
direction opposite of the restraining,
i.e. centripetal, force."

Golly-gee-whillickers...

Jeff

"Ken Kobayashi" skrev...

Quoted message said:

Centrifugal force is a force felt by all mass when you are working in a rotating reference frame.
Imagine standing in a spinning space station; the force holding down onto the floor is centrifugal
force. When you hold an apple and let go, it's centrifugal force that pulls it towards the floor.

phors.locost7.infophors04.htmOpen ↗ My freshman physics book supports this btw. but I'm too lazy
to type it. (Mechanics; Berkely Physics Course; Volume 1 Second Edition pp. 111-112 Paragraph is
called "Fictitious Forces." or pseudoforces)

Quoted message said:

I'm sorry to nitpick like this but I'm so sick of people saying "there's no such thing as
centrifugal force" without sufficient explanation and when there's no real need to make a
distinction.

They are saying it because there really is no such thing as centrifugal force. Its a convenient way
of looking at things to make Newtons equations work in a noninertial frame. Hence pseudo- or
fictitious force.

M.

Mikael Seierup said:

phors.locost7.infophors04.htmOpen ↗ My freshman physics book supports this btw. but I'm too lazy
to type it. (Mechanics; Berkely Physics Course; Volume 1 Second Edition pp. 111-112 Paragraph is
called "Fictitious Forces." or pseudoforces)

They are saying it because there really is no such thing as centrifugal force. Its a convenient way
of looking at things to make Newtons equations work in a noninertial frame. Hence pseudo- or
fictitious force.

This site disagrees (and gives a good explanation):
observe.arc.nasa.govcentrifugal index.htmlOpen ↗

To quote:

"Because the centrifugal force exists only in rotating reference frames, but not in inertial
reference frames, it's sometimes called a 'fictitious' or 'pseudo' force.

"We don't like this characterization because there is nothing fictitious or pseudo about it when
your car goes off the road and crashes, or when your bicycle skids out from under you when cornering
a slippery curve. The Earth's equatorial bulge is not a fiction, nor is the problem an engineer
confronts when designing turbine blades of jet engines that have to stay together at rotation rates
of up to 100,000 revolutions per minute."

The "we," presumably, being NASA scientists working on public outreach programs. (Yes they employ
"real" scientists to do public outreach - I know a few of them.)

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

"Ken Kobayashi" skrev

Quoted message said:

"We don't like this characterization because there is nothing fictitious or pseudo about it when
your car goes off the road and crashes, or when your bicycle skids out from under you when
cornering a slippery curve. The Earth's equatorial bulge is not a fiction, nor is the problem an
engineer confronts when designing turbine blades of jet engines that have to stay together at
rotation rates of up to 100,000 revolutions per minute."

Sure if you're just dumbing it down for the average Joe on the streets there no reason to muddle the
waters. Its not physics though.

Regards Mikael

Your physics instructor probably still thinks that the wheel gyro effects stability.
--
Bill "Pop Pop" Patterson Retired and riding my front drive low racer and our M5 tandem.

See some Bikes At:

home.earthlink.netindex.htmlOpen ↗

PC

roadkillbill.comr135.htmlOpen ↗

Class and Helicopter

calpoly.edu~wpattersOpen ↗

Reply to [email hidden]

Jeff Wills said:

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

Quoted message said:

Jeff Wills said:

...

<snippedy-doo-dah>

Quoted message said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

Tom

*I* know that... but "centrifugal force" is shorthand for "inertia will tend throw the object in a
direction opposite of the restraining,
i.e. centripetal, force."

Golly-gee-whillickers...

No it will not. The object will move in its plane of rotation PERPENDICULAR to the direction of the
centripetal force, if the centripetal force is removed. Or has Newton's First Law become invalid?

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers RANS "Wavewind" and Rocket,
Earth Cycles Sunset and Dragonflyer

Ken Kobayashi must be edykated coz e writed:

Quoted message said:

Ian said:

Rotational mass is subject to centripedal force, centrifugal force is disproved by the fact that
objects escaping from a spinning mass travel in an arc rather than a straight line, therefore the
escaping object is subject to simple momentum as a result of stored potential energy being
allowed to convert into kinetic energy whilst the objects trajectory is still subject to the
action of centripedal force acting inwards to the rotational mass.

That didn't make much sense to me. What exactly do you mean by an "object escaping from a spinning
mass"? Example please.

Centrifugal force is a force felt by all mass when you are working in a rotating reference frame.
Imagine standing in a spinning space station; the force holding down onto the floor is centrifugal
force. When you hold an apple and let go, it's centrifugal force that pulls it towards the floor.

Of course if you step outside the space station and observe things in relation to an inertial
frame, centrifugal force disappears. (Instead you have centripetal force pushing everything
towards the center of the space station, balanced by inertia.) That doesn't mean centrifugal force
does not exist; it means it only exists in certain reference frames.

I'm sorry to nitpick like this but I'm so sick of people saying "there's no such thing as
centrifugal force" without sufficient explanation and when there's no real need to make a
distinction.

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

The example would be a bicycle wheel braking apart during use, the basic premise of centrifugal
force states that objects will move in a straight line away from a median point of rotation, however
this was disproved before I was born, so centrifugal force is a myth. What you describe is inertia,
if centrifugal force existed then why am I walking in the surface of a spinning mass?

--
Ian

catrike.co.ukcatrike.co.ukOpen ↗

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

Quoted message said:

Mikael Seierup said:

... Stick a kid on a bike for the first time without any trainingwheels and ask what he feels
about its stability after a few tries and we might start calling DFs unstable too.

All single-track vehicles are unstable, since they all require rider input to create reactions
perpendicular to the vehicle's track to keep the vehicle upright.

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers

I dunno, Tom... didn't you ever "ghost-ride" your bike when you were a kid? A *well-designed*
single-track machine can coast upright *on its own* given enough initial speed. The front-end
geometry will correct any tendency for the bike to fall over.

I'd do this with a recumbent, but I've gotten too old and crotchety to subject fine machinery to
crashing into signposts.

Jeff

Jeff Wills said:

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

Quoted message said:

Mikael Seierup said:

... Stick a kid on a bike for the first time without any trainingwheels and ask what he feels
about its stability after a few tries and we might start calling DFs unstable too.

All single-track vehicles are unstable, since they all require rider input to create reactions
perpendicular to the vehicle's track to keep the vehicle upright.

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers

I dunno, Tom... didn't you ever "ghost-ride" your bike when you were a kid? A *well-designed*
single-track machine can coast upright *on its own* given enough initial speed. The front-end
geometry will correct any tendency for the bike to fall over.

However, once the bike comes to rest it will fall over. A single-track vehicle is certainly
statically unstable.

Tom Sherman - Near the confluence of the Mississippi and Rock Rivers RANS "Wavewind" and Rocket,
Earth Cycles Sunset and Dragonflyer

Tom Sherman said:

Jeff Wills said:

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

Quoted message said:

Jeff Wills wrote:
> ...

<snippedy-doo-dah>

Quoted message said:

According to my physics texts, centrifugal force does not exist (this can also be shown with a
free body diagram).

Tom

*I* know that... but "centrifugal force" is shorthand for "inertia will tend throw the object in
a direction opposite of the restraining,
i.e. centripetal, force."

Golly-gee-whillickers...

No it will not. The object will move in its plane of rotation PERPENDICULAR to the direction of the
centripetal force, if the centripetal force is removed. Or has Newton's First Law become invalid?

It depends on which reference frame you are looking at. If you are experiencing centrifugal force,
that means you are in a rotating reference frame. If an object is realeased, it will indeed travel
away from the center of rotation. (Though it will soon be deflected by Coriolis force.) See the
third and forth diagrams here:

hyperphysics.phy-astr.gsu.educorf.htmlOpen ↗

If you are looking at the system from an inertial frame, then centripetal force is originally
keeping the object in a circular motion. When the force is removed, the object travels perpendicular
to the original centripetal force.

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

Ian said:

The example would be a bicycle wheel braking apart during use, the basic premise of centrifugal
force states that objects will move in a straight line away from a median point of rotation,
however this was disproved before I was born, so centrifugal force is a myth. What you describe is
inertia, if centrifugal force existed then why am I walking in the surface of a spinning mass?

I still have no idea what you're talking about. If you are working in an inertial frame, pieces of
an exploding bicycle wheel would travel in a straight line and that is caused by inertia. If you are
working on the rotating reference frame (i.e. the frame fixed to the wheel), then it's centrifugal
force pulling the pieces away from the axis. Plus Coriolis force which deflects the trajectory from
a straight line.

Yes, I fully realize that centrifugal force is not a fundamental force. It's an inertial force, a
"pseudo-force" if you insist, an extra term in the equation of motion that appears when working in a
rotating reference frame but not in an inertial frame. That doesn't mean it's "not real." When you
are sitting on a bicycle and turning, YOU ARE IN A ROTATING REFERENCE FRAME! The force you feel is
very real.

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗

Mikael Seierup said:

"Ken Kobayashi" skrev

Quoted message said:

observe.arc.nasa.govcentrifugal index.htmlOpen ↗

"We don't like this characterization because there is nothing fictitious or pseudo about it when
your car goes off the road and crashes, or when your bicycle skids out from under you when
cornering a slippery curve. The Earth's equatorial bulge is not a fiction, nor is the problem an
engineer confronts when designing turbine blades of jet engines that have to stay together at
rotation rates of up to 100,000 revolutions per minute."

Sure if you're just dumbing it down for the average Joe on the streets there no reason to muddle
the waters. Its not physics though.

Did you read the link at all? It makes a very clear distinction between fundamental vs. inertial
forces. Of course it's sometimes vital to make that distinction. But it's stupid to say that
"centrifugal force does not exist" - that ain't physics either.

Do you also insist that magnetism does not exist? It's not a fundamental force either - it's just a
manifestation of relativity and electrostatic force.

Ken Kobayashi [email hidden] solarwww.mtk.nao.ac.jppersonalOpen ↗