Another shimmy question

I've read quite a few articles on shimmy, here and other places and
none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter bend.
The changes made for a more comfortable position, but I've had a
couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to shimmy?
I'd like to keep the current set-up, but I'll change it back if I have
to.

The bike can be seen as it was built at:
http://members.cox.net/orngcat/Bike1.htm

Thanks very much for any help.

Dave who? writes:

I've read quite a few articles on shimmy, here and other places and
none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to shimmy?
I'd like to keep the current set-up, but I'll change it back if I
have to.

I think you'll find that adding a mass to the handlebar/stem junction
will change the onset of shimmy if not get rid of it entirely. The
important point is when the shimmy occurs. Is while riding no-hands
or with hands on the bars? The real test is what is toes when riding
no-hands with the pedals vertical and horizontal.

If this is occurring with hands on the bars, then it is being caused
by muscular feedback and is enhanced by gripping the bars more firmly,
often called the death grip. 40MPH is a fairly high speed for onset
of natural shimmy. Find a 30MPH smooth continuous grade and test it
riding no-hands. I am interested in what you find.

The bike can be seen as it was built at:
http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in motion.
As I have reported, my bicycle shimmies any time I let it above 22 or
so MPH and it doesn't if I don't want it to. It coasts nicely at all
speeds up to and including 60MPH. Surprisingly, filling the front
tire with water instead of air does not change its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt

On 26 Nov 2005 03:34:58 GMT, wrote:

Dave who? writes:

I've read quite a few articles on shimmy, here and other places and
none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to shimmy?
I'd like to keep the current set-up, but I'll change it back if I
have to.

I think you'll find that adding a mass to the handlebar/stem junction
will change the onset of shimmy if not get rid of it entirely. The
important point is when the shimmy occurs. Is while riding no-hands
or with hands on the bars? The real test is what is toes when riding
no-hands with the pedals vertical and horizontal.

If this is occurring with hands on the bars, then it is being caused
by muscular feedback and is enhanced by gripping the bars more firmly,
often called the death grip. 40MPH is a fairly high speed for onset
of natural shimmy. Find a 30MPH smooth continuous grade and test it
riding no-hands. I am interested in what you find.

The bike can be seen as it was built at:
http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in motion.
As I have reported, my bicycle shimmies any time I let it above 22 or
so MPH and it doesn't if I don't want it to. It coasts nicely at all
speeds up to and including 60MPH. Surprisingly, filling the front
tire with water instead of air does not change its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt

Thanks very much Jobst.

The shimmy episodes have always occurred while decending with my hands
in the drops. The bike will first feel a bit "loose," as if the front
tire were underinflated. Then the shimmy starts and yes, at that
point "death grip" properly describes my reaction. Hmmm. Maybe I
"death grip" the bars when I feel the instability and *cause* the
progression to full shimmy?

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I can
induce the same feeling of pre-shimmy instability by riding with my
hands at the ends of the (drop) handlebars, back toward my body.

Thanks again.

Dave who? writes:

The shimmy episodes have always occurred while descending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and
*cause* the progression to full shimmy?

Yes, but what does the bicycle do when you do the no-hands test?
That's the point where you can't manually generate shimmy and the
bicycle speaks for itself.

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.

As I said, changing the inertia of the stem and bars, by adding mass
(the same as lengthening the stem) would show an effect but only with
your hands not on the bars. All this is dependent on whether the
bicycle shimmies by itself, so the no-hands test is essential to the
analysis. Hands on the bars shimmy is rider induced.

Jobst Brandt

Dave wrote:
On 26 Nov 2005 03:34:58 GMT, wrote:


Dave who? writes:


I've read quite a few articles on shimmy, here and other places
and none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to
shimmy? I'd like to keep the current set-up, but I'll change it
back if I have to.

I think you'll find that adding a mass to the handlebar/stem
junction will change the onset of shimmy if not get rid of it
entirely. The important point is when the shimmy occurs. Is while
riding no-hands or with hands on the bars? The real test is what
is toes when riding no-hands with the pedals vertical and
horizontal.

If this is occurring with hands on the bars, then it is being
caused by muscular feedback and is enhanced by gripping the bars
more firmly, often called the death grip. 40MPH is a fairly high
speed for onset of natural shimmy. Find a 30MPH smooth continuous
grade and test it riding no-hands. I am interested in what you
find.


The bike can be seen as it was built at:

http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in
motion. As I have reported, my bicycle shimmies any time I let it
above 22 or so MPH and it doesn't if I don't want it to. It coasts
nicely at all speeds up to and including 60MPH. Surprisingly,
filling the front tire with water instead of air does not change
its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt


Thanks very much Jobst.

The shimmy episodes have always occurred while decending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and *cause*
the progression to full shimmy?

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.

Thanks again.

i think the stem length affects your bike the same way a mass on a pole
affects the resonant frequency with how far it is from the end. as
jobst says, if you put a weight on the bar, it would probably tune it
out, but i think it's better to address the root causes and tune out the
resonances of the whole system of frame torsion combining with the
lateral springiness of the [primarily] rear wheel. i think there are
two major factors to address shimmy:

1. big tube frame. yes, steel is real, and your serotta is a great
bike, etc., but trust me on this, big tube aluminum frames /don't/
shimmy because of much higher torsional stiffness. i have three big
tube al frames, and they're all stable as rocks, even with flexy wheels
[which we'll come to in a second]. my steel bikes, even my gorgeous
della santa, can all be made to shimmy. the d*s is the best of the
steels as it uses an oversize tube set, but even bigger is even better.
frame replacement may be a drastic step, but it works.

2. stiffer rear wheel. dished rears, as a function of their geometry,
tend to have a lot of lateral flexibility. after some experimentation,
i resorted to a dura-ace hub [the best of the spoke bracing angles
available] and straight gauge 2.0mm spokes on the drive side. thicker
is stiffer, and it makes quite a good difference. with a wheel like
this, even my most shimmy-prone steel frame would be much better
behaved, although the problem could not be completely eliminated.
conversely, the standard "shimmy" wheels that were so bad on my steel
frames work great on my al frames.

so, contrary to the usual admonishments to stop your "biomechanical
feedback" and live with the problem, i think this can be addressed in
the two ways above. messing about with weights and seat positions is
just rearranging the deck chairs on a sinking yacht. fix the leak and
you won't be worrying about the life boats.

wrote:
Dave who? writes:


The shimmy episodes have always occurred while descending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and
*cause* the progression to full shimmy?


Yes, but what does the bicycle do when you do the no-hands test?
That's the point where you can't manually generate shimmy and the
bicycle speaks for itself.


You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.


As I said, changing the inertia of the stem and bars, by adding mass
(the same as lengthening the stem) would show an effect but only with
your hands not on the bars. All this is dependent on whether the
bicycle shimmies by itself, so the no-hands test is essential to the
analysis. Hands on the bars shimmy is rider induced.

Jobst Brandt

"rider induced". sure, blame the rider. examine the facts, then figure
out the cure won't you?

On Sat, 26 Nov 2005 16:34:30 -0800, jim beam
wrote:

Dave wrote:
On 26 Nov 2005 03:34:58 GMT, wrote:


Dave who? writes:


I've read quite a few articles on shimmy, here and other places
and none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to
shimmy? I'd like to keep the current set-up, but I'll change it
back if I have to.

I think you'll find that adding a mass to the handlebar/stem
junction will change the onset of shimmy if not get rid of it
entirely. The important point is when the shimmy occurs. Is while
riding no-hands or with hands on the bars? The real test is what
is toes when riding no-hands with the pedals vertical and
horizontal.

If this is occurring with hands on the bars, then it is being
caused by muscular feedback and is enhanced by gripping the bars
more firmly, often called the death grip. 40MPH is a fairly high
speed for onset of natural shimmy. Find a 30MPH smooth continuous
grade and test it riding no-hands. I am interested in what you
find.


The bike can be seen as it was built at:

http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in
motion. As I have reported, my bicycle shimmies any time I let it
above 22 or so MPH and it doesn't if I don't want it to. It coasts
nicely at all speeds up to and including 60MPH. Surprisingly,
filling the front tire with water instead of air does not change
its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt


Thanks very much Jobst.

The shimmy episodes have always occurred while decending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and *cause*
the progression to full shimmy?

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.

Thanks again.

i think the stem length affects your bike the same way a mass on a pole
affects the resonant frequency with how far it is from the end. as
jobst says, if you put a weight on the bar, it would probably tune it
out, but i think it's better to address the root causes and tune out the
resonances of the whole system of frame torsion combining with the
lateral springiness of the [primarily] rear wheel. i think there are
two major factors to address shimmy:

1. big tube frame. yes, steel is real, and your serotta is a great
bike, etc., but trust me on this, big tube aluminum frames /don't/
shimmy because of much higher torsional stiffness. i have three big
tube al frames, and they're all stable as rocks, even with flexy wheels
[which we'll come to in a second]. my steel bikes, even my gorgeous
della santa, can all be made to shimmy. the d*s is the best of the
steels as it uses an oversize tube set, but even bigger is even better.
frame replacement may be a drastic step, but it works.

2. stiffer rear wheel. dished rears, as a function of their geometry,
tend to have a lot of lateral flexibility. after some experimentation,
i resorted to a dura-ace hub [the best of the spoke bracing angles
available] and straight gauge 2.0mm spokes on the drive side. thicker
is stiffer, and it makes quite a good difference. with a wheel like
this, even my most shimmy-prone steel frame would be much better
behaved, although the problem could not be completely eliminated.
conversely, the standard "shimmy" wheels that were so bad on my steel
frames work great on my al frames.

so, contrary to the usual admonishments to stop your "biomechanical
feedback" and live with the problem, i think this can be addressed in
the two ways above. messing about with weights and seat positions is
just rearranging the deck chairs on a sinking yacht. fix the leak and
you won't be worrying about the life boats.

Well, I think there's something to what you're saying. I think this
bike probably is more prone to shimmy because of its thin tubes and
long wheelbase. Thing is that I've ridden it many thousands of miles
and never had the problem 'till I changed the stem and bars. I have
to think that that has something to do with it.

I DID have my wheels rebuilt a while ago. They have Dura Ace hubs but
butted spokes. They seem true and well tensioned but maybe I should
get them checked.

Thanks very much for your input.

Dave wrote:
On Sat, 26 Nov 2005 16:34:30 -0800, jim beam
wrote:


Dave wrote:

On 26 Nov 2005 03:34:58 GMT, wrote:



Dave who? writes:



I've read quite a few articles on shimmy, here and other places
and none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to
shimmy? I'd like to keep the current set-up, but I'll change it
back if I have to.

I think you'll find that adding a mass to the handlebar/stem
junction will change the onset of shimmy if not get rid of it
entirely. The important point is when the shimmy occurs. Is while
riding no-hands or with hands on the bars? The real test is what
is toes when riding no-hands with the pedals vertical and
horizontal.

If this is occurring with hands on the bars, then it is being
caused by muscular feedback and is enhanced by gripping the bars
more firmly, often called the death grip. 40MPH is a fairly high
speed for onset of natural shimmy. Find a 30MPH smooth continuous
grade and test it riding no-hands. I am interested in what you
find.



The bike can be seen as it was built at:

http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in
motion. As I have reported, my bicycle shimmies any time I let it
above 22 or so MPH and it doesn't if I don't want it to. It coasts
nicely at all speeds up to and including 60MPH. Surprisingly,
filling the front tire with water instead of air does not change
its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt


Thanks very much Jobst.

The shimmy episodes have always occurred while decending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and *cause*
the progression to full shimmy?

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.

Thanks again.

i think the stem length affects your bike the same way a mass on a pole
affects the resonant frequency with how far it is from the end. as
jobst says, if you put a weight on the bar, it would probably tune it
out, but i think it's better to address the root causes and tune out the
resonances of the whole system of frame torsion combining with the
lateral springiness of the [primarily] rear wheel. i think there are
two major factors to address shimmy:

1. big tube frame. yes, steel is real, and your serotta is a great
bike, etc., but trust me on this, big tube aluminum frames /don't/
shimmy because of much higher torsional stiffness. i have three big
tube al frames, and they're all stable as rocks, even with flexy wheels
[which we'll come to in a second]. my steel bikes, even my gorgeous
della santa, can all be made to shimmy. the d*s is the best of the
steels as it uses an oversize tube set, but even bigger is even better.
frame replacement may be a drastic step, but it works.

2. stiffer rear wheel. dished rears, as a function of their geometry,
tend to have a lot of lateral flexibility. after some experimentation,
i resorted to a dura-ace hub [the best of the spoke bracing angles
available] and straight gauge 2.0mm spokes on the drive side. thicker
is stiffer, and it makes quite a good difference. with a wheel like
this, even my most shimmy-prone steel frame would be much better
behaved, although the problem could not be completely eliminated.
conversely, the standard "shimmy" wheels that were so bad on my steel
frames work great on my al frames.

so, contrary to the usual admonishments to stop your "biomechanical
feedback" and live with the problem, i think this can be addressed in
the two ways above. messing about with weights and seat positions is
just rearranging the deck chairs on a sinking yacht. fix the leak and
you won't be worrying about the life boats.


Well, I think there's something to what you're saying. I think this
bike probably is more prone to shimmy because of its thin tubes and
long wheelbase. Thing is that I've ridden it many thousands of miles
and never had the problem 'till I changed the stem and bars. I have
to think that that has something to do with it.

of course. it's the icing on the cake of what was probably a metastable
system anyway. but to be sure, have you done anything else like add a
rack? those can add to the shimmy equation considerably.


I DID have my wheels rebuilt a while ago. They have Dura Ace hubs but
butted spokes. They seem true and well tensioned but maybe I should
get them checked.

ok, but don't get stuck on the tension concept - it won't fix anything!
contrary to popular belief, once the wheel structure is stable,
increasing spoke tension does not increase lateral wheel stiffness - the
3 stiffness factors are spoke bracing angle, spoke gauge and rim
stiffness. a spoke is just as *elastic* under #10 load as it is under
#100 load.


Thanks very much for your input.

On Sat, 26 Nov 2005 19:36:02 -0800, jim beam
wrote:

Dave wrote:
On Sat, 26 Nov 2005 16:34:30 -0800, jim beam
wrote:


Dave wrote:

On 26 Nov 2005 03:34:58 GMT, wrote:



Dave who? writes:



I've read quite a few articles on shimmy, here and other places
and none have addressed my question.

I have a custom Serotta touring bike. It's a Colorado CRL frame
stretched and lowered a bit to touring geometry (WB=104cm, BB
ht=26.5cm). I've ridden it for many years without any problems.
Since I'm riding less now and *sigh* getting a little older, I
shortened the stem by 1cm and put on a handlebar with a shorter
bend. The changes made for a more comfortable position, but I've
had a couple episodes of 40mph shimmy since the change.

Is shortening the stem/handlebar a recognized contributor to
shimmy? I'd like to keep the current set-up, but I'll change it
back if I have to.

I think you'll find that adding a mass to the handlebar/stem
junction will change the onset of shimmy if not get rid of it
entirely. The important point is when the shimmy occurs. Is while
riding no-hands or with hands on the bars? The real test is what
is toes when riding no-hands with the pedals vertical and
horizontal.

If this is occurring with hands on the bars, then it is being
caused by muscular feedback and is enhanced by gripping the bars
more firmly, often called the death grip. 40MPH is a fairly high
speed for onset of natural shimmy. Find a 30MPH smooth continuous
grade and test it riding no-hands. I am interested in what you
find.



The bike can be seen as it was built at:

http://members.cox.net/orngcat/Bike1.htm

Shimmy tendency is not readily visible, it must be tested in
motion. As I have reported, my bicycle shimmies any time I let it
above 22 or so MPH and it doesn't if I don't want it to. It coasts
nicely at all speeds up to and including 60MPH. Surprisingly,
filling the front tire with water instead of air does not change
its shimmy tendencies.

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt


Thanks very much Jobst.

The shimmy episodes have always occurred while decending with my
hands in the drops. The bike will first feel a bit "loose," as if
the front tire were underinflated. Then the shimmy starts and yes,
at that point "death grip" properly describes my reaction. Hmmm.
Maybe I "death grip" the bars when I feel the instability and *cause*
the progression to full shimmy?

You don't mention in either your reply or the article that you
referred me to, to handlebar reach as a contributing factor. Can I
discount it? One reason that I suspected reach (other than that
shimmy had never occurred before I changed the position) is that I
can induce the same feeling of pre-shimmy instability by riding with
my hands at the ends of the (drop) handlebars, back toward my body.

Thanks again.

i think the stem length affects your bike the same way a mass on a pole
affects the resonant frequency with how far it is from the end. as
jobst says, if you put a weight on the bar, it would probably tune it
out, but i think it's better to address the root causes and tune out the
resonances of the whole system of frame torsion combining with the
lateral springiness of the [primarily] rear wheel. i think there are
two major factors to address shimmy:

1. big tube frame. yes, steel is real, and your serotta is a great
bike, etc., but trust me on this, big tube aluminum frames /don't/
shimmy because of much higher torsional stiffness. i have three big
tube al frames, and they're all stable as rocks, even with flexy wheels
[which we'll come to in a second]. my steel bikes, even my gorgeous
della santa, can all be made to shimmy. the d*s is the best of the
steels as it uses an oversize tube set, but even bigger is even better.
frame replacement may be a drastic step, but it works.

2. stiffer rear wheel. dished rears, as a function of their geometry,
tend to have a lot of lateral flexibility. after some experimentation,
i resorted to a dura-ace hub [the best of the spoke bracing angles
available] and straight gauge 2.0mm spokes on the drive side. thicker
is stiffer, and it makes quite a good difference. with a wheel like
this, even my most shimmy-prone steel frame would be much better
behaved, although the problem could not be completely eliminated.
conversely, the standard "shimmy" wheels that were so bad on my steel
frames work great on my al frames.

so, contrary to the usual admonishments to stop your "biomechanical
feedback" and live with the problem, i think this can be addressed in
the two ways above. messing about with weights and seat positions is
just rearranging the deck chairs on a sinking yacht. fix the leak and
you won't be worrying about the life boats.


Well, I think there's something to what you're saying. I think this
bike probably is more prone to shimmy because of its thin tubes and
long wheelbase. Thing is that I've ridden it many thousands of miles
and never had the problem 'till I changed the stem and bars. I have
to think that that has something to do with it.

of course. it's the icing on the cake of what was probably a metastable
system anyway. but to be sure, have you done anything else like add a
rack? those can add to the shimmy equation considerably.

Nope. No other changes. I have ridden with a rear rack with as much
as 25 pounds of stuff in it in the past with no problems.

I DID have my wheels rebuilt a while ago. They have Dura Ace hubs but
butted spokes. They seem true and well tensioned but maybe I should
get them checked.

ok, but don't get stuck on the tension concept - it won't fix anything!
contrary to popular belief, once the wheel structure is stable,
increasing spoke tension does not increase lateral wheel stiffness - the
3 stiffness factors are spoke bracing angle, spoke gauge and rim
stiffness. a spoke is just as *elastic* under #10 load as it is under
#100 load.


Thanks very much for your input.

"Dave" (clip) Thing is that I've ridden it many thousands of miles and
never had the problem 'till I changed the stem and bars. I have to think
that that has something to do with it. (clip)
^^^^^^^^^^^^^^
To me it is obvious that it does. "Shimmy" is the cycling word for a more
general phenomenon: oscillation, which can occur in mechanical and
electrical systems. In electrical systems it is often a design feature. In
mechanical systems, including the bicycle/rider combination, it is generally
needs to be avoided. It occurs when a small disturbance produces more
corrective force than necessary. This causes an overcorrection which is
larger than the original disturbance, and results is a series of deviations
which increase in amplitude. THAT's your shimmy.

In your case, it appears that the change in your stem and bars changed the
dynamics of your bike/body system in such a way that they can become
unstable at some speeds and riding conditions. I am virtually certain that
you could get rid of the problem by changing things back. You may also be
able to reduce or eliminate the problem by fooling with other things. You
could wear lead gloves, or hold the handlebars in a different place, or
relax your arms instead of stiffening up. Some changes may help, and some
may make things worse. The fact that the minor change you made, for the
sake of comfort, set this problem off means that you have been of the edge
of this unstable condition all along, and did not know it. You may be able
to get rid of the shimmy by making other adjustments, or you may have to put
things back the way they were. This is not like finding the leak in a tube
and patching it. You are dealing with a dynamic system, and every part
affects all the other parts.