The problem of shimmy explained

jur wrote:
'here' (http://en.wikipedia.org/wiki/Speed_wobble) is my explanation
in words.

I think you're going down the wrong path. I haven't read it all, but
here are a few paragraphs from that link that need some attention, IMO.

| It doesn't happen on xyz bike: Not all bikes have enough springyness
| to provide the necessary feedback for nutation to be amplified, which
| explains why many riders have never experienced it. Or they have not
| reached that critical speed where the nutation Q factor is high
| enough, or where the nutation frequency matches the natural frequency
| of any springy mass.

How does the nutation get "amplified?" What role does the "springyness"
of a bike play in that amplification?

| The back wheel will usually be flexing the most since 1) it is not as
| stiff as the other components; 2) it is under rider load, so the lower
| vertical spokes' tension is reduced, and with a dished wheel the
| non-drive side spokes are under even less tension; and 3) it is subject
| to a lever action. It requires only a small amount of sideways flexing
| to account for the head tube movement.

You're saying that the rear wheel stiffness depends on local spoke
tension? I don't agree.

| All these predict that a stiffer wheel, and an equally dished wheel
| will be less prone to shimmy. Double-butted spokes should be more prone
| to shimmy, and likewise heavier riders will reduce bottom spoke tension,
| increasing shimmy.

Similar comments to above. In addition, how would an equally dished
wheel be less prone to shimmy? I believe it has been discussed on this
newsgroup that dishing *increases* the lateral stiffness of a wheel,
which would push the resonant frequency of the system upwards.

Have you done the math for the caster effect?

--
Dave
dvt at psu dot edu

Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)

In article .com,
wrote:

jur wrote:
RonSonic Wrote:
I don't see anything new in the wikipedia entry, but you might
have pointed to it earlier, before becoming so committed to you
little game of "I've got a secret."

You don't see anything new? How about the key word NUTATION????

Nope, sorry. That's an old word. ;-)

Honestly, from here, the nutation idea sounds like another flower on
a well-decorated cake. AFAICT, you haven't really proven it's a
major cause, and you haven't shown how knowing about it makes a
critical difference.

Well, he's added a term swiped from its usual use in astronomy (along
with gimbal moment). But the concept as described in the Wikipedia
entry is nothing new, we've been talking about it for years on
rec.bicycles.tech. He's (assuming jur is the author or latest editor of
the entry) tried to bring in a lot of factors and to be comprehensive,
so kudos for that. All that said, though, there seems to be nothing new
in concept only the addition of a new term or two.

Put your knee against the top tube. It's that simple. Almost forgot
this weekend, and almost went into the trees. Then I remembered, and
the shimmy stopped instantly (I was surprised how fast).

Yes, this was mentioned in your reluctantly-given wiki citation. Should
be mentioned more often, as a kindness to new cyclists.

[BTW, it was on a fast and *very* bumpy descent; I was a bit cold and
possibly shivering - I think Jobst said in the faq that human shivers
are unfortunately at a shimmy's driving frequency.]

Mark

Tim McNamara Wrote:

Well, he's added a term swiped from its usual use in astronomy (along
with gimbal moment). But the concept as described in the Wikipedia
entry is nothing new, we've been talking about it for years on
rec.bicycles.tech. He's (assuming jur is the author or latest editor
of
the entry) tried to bring in a lot of factors and to be comprehensive,
so kudos for that. All that said, though, there seems to be nothing
new
in concept only the addition of a new term or two.
Nothing doing. The term Nutation has its definition in the theory of
rigid spinning bodies. It has been applied in astronomy, not the other
way around. A bit of Google research will show it. As for gimbal, look
up its meaning (link given in article). I am in a position of having
designed feedback loops for dynamically tuned gyros so I know what I'm
talking about.

As for having talked about it on r.b.t before, show me. I searched and
the only other reference to it in this context, was also by myself. It
has been mentioned in connection with unscrewing of screw threads,
erroneously at that.

Nothing new? The big point I'm making is that the original FAQ cannot
explain sustained oscillation. Besides that, it leaves out completely
the effect of nutation which is dominant, and the influence of various
factors in a bicycle is now described in terms of their influence on
gimbal moments and feedback; this approach not only completely explains
_sustained_ instability but also every other effect that has been
mentioned on these forums, including 2 separate modes of instability
which has been consistently denied by Brand despite having been
reported by many here.


Nothing new? I see you have not thoroughly understood the entire
concept or the dominant effect of nutation.


--
jur

dvt Wrote:
jur wrote:
'here' (http://en.wikipedia.org/wiki/Speed_wobble) is my explanation
in words.

I think you're going down the wrong path. I haven't read it all, but
here are a few paragraphs from that link that need some attention,
IMO.

I'm not sure what you mean by wrong path. I already have a complete
explanation in mathematical terms which explains everything and takes
properly into account nutation which is dominant. Surely you can't mean
my explanation is wrong after all that?

Anyway, criticism of the entry is of course welcomed since I want it as
clear as it can be.
dvt Wrote:

| It doesn't happen on xyz bike: Not all bikes have enough springyness
| to provide the necessary feedback for nutation to be amplified,
which
| explains why many riders have never experienced it. Or they have not
| reached that critical speed where the nutation Q factor is high
| enough, or where the nutation frequency matches the natural
frequency
| of any springy mass.

How does the nutation get "amplified?" What role does the
"springyness"
of a bike play in that amplification?
This will clearly be shown once I post the model on the wiki entry.
Basically a disturbance of the effective gimbals have the effect of
feeding back to the front wheel the required impetus of amplifying
nutation which acts very much like a underdamped spring to external
inputs.
dvt Wrote:

| The back wheel will usually be flexing the most since 1) it is not
as
| stiff as the other components; 2) it is under rider load, so the
lower
| vertical spokes' tension is reduced, and with a dished wheel the
| non-drive side spokes are under even less tension; and 3) it is
subject
| to a lever action. It requires only a small amount of sideways
flexing
| to account for the head tube movement.

You're saying that the rear wheel stiffness depends on local spoke
tension? I don't agree.

| All these predict that a stiffer wheel, and an equally dished wheel
| will be less prone to shimmy. Double-butted spokes should be more
prone
| to shimmy, and likewise heavier riders will reduce bottom spoke
tension,
| increasing shimmy.

Similar comments to above. In addition, how would an equally dished
wheel be less prone to shimmy? I believe it has been discussed on this
newsgroup that dishing *increases* the lateral stiffness of a wheel,
which would push the resonant frequency of the system upwards.
I will admit to being on unfamiliar territory here, and if that idea is
incorrect, then I will of course reconsider it. With stiffness of the
wheel I mean the ability of the axle to _twist_ away from the null, not
to be displaced sideways _along_ the axis. This mode of stiffness is
perhaps different to the lateral one you mean?
dvt Wrote:

Have you done the math for the caster effect?I have only shown that it is not required for instabilty; additionally
the castor effect as found on trolley wheels is a different phenomenon,
well understood, which does not have application in a bicycle wheel
because the dynamics are different. I am entertaining the possibility
that castor/trail may have an effect on the amplitude or damping factor
of nutation, but not as a mechanism for origin.


--
jur

jur wrote:

dvt Wrote:

jur wrote:

'here' (http://en.wikipedia.org/wiki/Speed_wobble) is my explanation
in words.

I think you're going down the wrong path. I haven't read it all, but
here are a few paragraphs from that link that need some attention,
IMO.

I'm not sure what you mean by wrong path. I already have a complete
explanation in mathematical terms which explains everything and takes
properly into account nutation which is dominant. Surely you can't mean
my explanation is wrong after all that?

So show us the money... um ... math! Your Bike Forums post asks to
borrow equipment to test hypotheses - if you *have* the math, run a
simulation, no equipment needed.

Or do you mean that you "have an explanation in mathematical terms" that
*might* "explain[s] everything", and you need to test your explanation?

Mark

In article ,
jur wrote:

Tim McNamara Wrote:

Well, he's added a term swiped from its usual use in astronomy
(along with gimbal moment). But the concept as described in the
Wikipedia entry is nothing new, we've been talking about it for
years on rec.bicycles.tech. He's (assuming jur is the author or
latest editor of the entry) tried to bring in a lot of factors and
to be comprehensive, so kudos for that. All that said, though,
there seems to be nothing new in concept only the addition of a new
term or two.

Nothing doing. The term Nutation has its definition in the theory of
rigid spinning bodies. It has been applied in astronomy, not the
other way around. A bit of Google research will show it. As for
gimbal, look up its meaning (link given in article). I am in a
position of having designed feedback loops for dynamically tuned
gyros so I know what I'm talking about.

Not saying you don't know what you're talking about. But I am pointing
out that as of yet you have added nothing new to the discussion other
than a couple of previously unused terms. Sorry if that offends your
ego, as it seems to have done.

As for having talked about it on r.b.t before, show me. I searched
and the only other reference to it in this context, was also by
myself. It has been mentioned in connection with unscrewing of screw
threads, erroneously at that.

Do your own homework. There have been dozens if not hundreds of threads
on shimmy, speed wobble, death wobble, etc. Should be thousands of
posts for you to look at.

Nothing new? The big point I'm making is that the original FAQ cannot
explain sustained oscillation.

The discussions on the newsgroup have gone far beyond the FAQ, then,
since sustained oscillation and its mechanism(s) has (have) been
discussed.

Besides that, it leaves out completely the effect of nutation which
is dominant, and the influence of various factors in a bicycle is now
described in terms of their influence on gimbal moments and feedback;
this approach not only completely explains _sustained_ instability
but also every other effect that has been mentioned on these forums,
including 2 separate modes of instability which has been consistently
denied by Brand despite having been reported by many here.

Brandt (not Brand) has mentioned both of the modes of shimmy that are
described in the Wikipedia article. Indeed, he has taken a lot of flack
from people over the second mode. Normal muscular tremor,
overcorrection and shivering have all been identified as impetus for
shimmy by Jobst, so he has already recognized your second mode of
shimmy. Furthermore he has also identified hands-off shimmy, your first
mode. Indeed, your article pretty much tracks along with what Jobst has
been talking about for years in connection to this issue. But since you
haven't yet done your research on the prior art, you apparently don't
know that.

Nothing new? I see you have not thoroughly understood the entire
concept or the dominant effect of nutation.

No, you've just failed to demonstrate your theory as of yet. I will
enjoy reading your proofs in hopes of learning something new. I'm
looking forward to your discussion with Jobst on this topic, since both
of you like to adopt the authoritative tone to quell any disagreement.

You are unfortunately making the common error of believing and defending
your idea before it's been proven. As others have pointed out, we have
seen dozens of guys turn up in the newsgroup shouting "eureka!" only to
have reinvented the wheel or to have gotten the wrong end of the stick
entirely. So you will have to forgive us if we adopt a wait-and-see
attitude towards your breakthrough theory. Saviors come and go all the
time.

I want to test issues like wheel and frame flex; frame flex in
particular is not needed in my analysis but I'm not willing to dismiss
it without some testing. I want to find out if the effect is important
for some frames or not.

Another big thing I want to test is death wobble and film it with a
high speed camera to see if my explanation for it matches reality. I
guess I'm too chicken to deliberately try and induce it while riding at
speed on the road.


Oh yes, and the math I am busy writing it down in publishable format,
busy learning how to use the wiki formatting, and refining and
extending the model. Plagued by too little time. Once TdF is over, I'll
have more time for this.


--
jur

In article ,
jur wrote:

dvt Wrote:
Have you done the math for the caster effect?

I have only shown that it is not required for instabilty;
additionally the castor effect as found on trolley wheels is a
different phenomenon, well understood, which does not have
application in a bicycle wheel because the dynamics are different. I
am entertaining the possibility that castor/trail may have an effect
on the amplitude or damping factor of nutation, but not as a
mechanism for origin.

You do have some homework to do which might help.

http://www.amazon.com/gp/product/026...9902?v=glance&
n=283155

On Tue, 18 Jul 2006 10:29:04 +1000, jur
wrote:


I want to test issues like wheel and frame flex; frame flex in
particular is not needed in my analysis but I'm not willing to dismiss
it without some testing. I want to find out if the effect is important
for some frames or not.

Another big thing I want to test is death wobble and film it with a
high speed camera to see if my explanation for it matches reality. I
guess I'm too chicken to deliberately try and induce it while riding at
speed on the road.


Oh yes, and the math I am busy writing it down in publishable format,
busy learning how to use the wiki formatting, and refining and
extending the model. Plagued by too little time. Once TdF is over, I'll
have more time for this.

Dear Trevor,

Be sure to let us know when you pull something together.

Cheers,

Carl Fogel