High-speed shimmy, Speed wobble

Shayne Wissler' may have said:

Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the
violent shimmy?

Any suggestions would be appreciated.

Observing that the problem appeared after the tire change, the connection to the reduced diameter is
possible but not proven.

An observation: Reducing the diameter of the front tire will move the steerer axis more toward the
vertical, which moves the point of intersection of the steerer axis with the ground toward the
tire's contact patch. The distance between the axis intersection and the patch is the "trail" of the
fork. Insufficient trail will cause exactly the problem that you describe...but if the amount of
trail before the tire swap was small enough that the change produced this problem, I would be a bit
surprised. If the bike has a suspension fork, try cranking the preload up to the max, as that will
tend to minimize such a problem.

Somehow, though, I think the lbs is likely to find that there is something else wrong. What I
describe is, in my opinion, not terribly likely to be the problem.

--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.

Quoted message said:

I've read about speed wobbles but have never experienced one. At first I thought my front tire had
gone completely flat and was going out of

control,

Quoted message said:

or as if the front tire had become extremely out of true--I'd guess that

the

Quoted message said:

wobble amplitude was about 2-3", and a frequency of about 5Hz. But the braking was responsive. As
I slowed down it continued to wobble for a

while

Quoted message said:

though lower speeds (I'll guess through 30MPH--but I don't know). I

finally

Quoted message said:

stopped, checked that my front tire was in tact and not out of true. Everything appeared OK, so I
continued down the hill, this time not going over 30 or so.

Once you get a shimmy, my advice is not to slow down, but stop, take a breather, and then get going
again. Lack of confidence doesn't help the situation any.

Quoted message said:

I have made two changes to my bike recently. The most recent was that I lowered my handlebars by
about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is
that I replaced my

front

Quoted message said:

tire with a lighter, skinnier one.

Simplest experiment would be to raise the bars back up again and see if the shimmy goes away. As
you've read in the FAQ, shimmy isn't a function of the bicycle alone, but the entire system, which
includes you on it. The combination of your position being altered and the skinnier tire (which
speeds up response on the front end) sounds like a good place to start, almost a no-brainer.

--Mike-- Chain Reaction Bicycles chainreactionbicycles.comchainreactionbicycles.comOpen ↗

"Shayne Wissler" <[email hidden]> wrote in message
news:70Zdb.634208$o%2.291570@sccrnsc02...

Quoted message said:

This weekend I was descending a canyon, and at around 42MPH my LeMond

Buenos

Quoted message said:

Aires started to get a bad speed wobble. This is the first time it's ever happened (and I've been
at that speed many times over the summer).

I've read about speed wobbles but have never experienced one. At first I thought my front tire had
gone completely flat and was going out of

control,

Quoted message said:

or as if the front tire had become extremely out of true--I'd guess that

the

Quoted message said:

wobble amplitude was about 2-3", and a frequency of about 5Hz. But the braking was responsive. As
I slowed down it continued to wobble for a

while

Quoted message said:

though lower speeds (I'll guess through 30MPH--but I don't know). I

finally

Quoted message said:

stopped, checked that my front tire was in tact and not out of true. Everything appeared OK, so I
continued down the hill, this time not going over 30 or so.

My owners manual says that if I experience this, to take it into the shop.

I

Quoted message said:

called the shop and they want to see the bike. But based on my reading of the shimmy FAQ, I'm half
expecting that they won't find anything.

I have made two changes to my bike recently. The most recent was that I lowered my handlebars by
about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is
that I replaced my

front

Quoted message said:

tire with a lighter, skinnier one.

I have descended with the tire before, but I can't say whether I reached 42MPH or not. I can say
that ever since I put that tire on, I have noticed there being less stability while riding
no-hands--it does start to shimmy

a

Quoted message said:

little, but at lower speeds and not so violently. Is it probable that this low-speed (~20MPH?)
no-hands shimmy is from the same source as the violent shimmy?

Any suggestions would be appreciated.

Shayne Wissler

shayne

i used to have similar experiences on an older road frame and i managed to "tune" it out.

as you read in the faq, there are many reasons for the shimmy, but basically, you're looking at a
dynamic system which has a resonance, just like that famous footage of the tacoma narrows bridge.

the initiation of the resonance aside, and tires are one, the frame and the wheels facilitate the
resonance. just like the bridge, the wind initiated the oscillation, but the resonant frequency of
the bridge was disasterously "in tune".

so, if you are to address this problem with your bike, you have to address these two main areas -
frame & wheels. i was lucky enough to have several rear wheels and noticed that different wheels had
different effects on the resonance. in my case, the lighter spoked wheels were much more prone then
the heavier spoked ones. following this pattern, i ended up building a "stiff" rear wheel with
straight gauge 2.0mm spokes on the drive side and butted 2.0/1.8/2.0mm on the non-drive. it's very
rigid and quite a harsh ride, but it killed the shimmy completely.

i have since replaced that frame and can report that while i can induce some degree of shimmy in any
of the three others i now have if i really try, none of them are susceptible in the same way.

ymmv, but in my opinion, start with wheel variants and if that doesn't work, move on to a different
frame. my aluminum frame is the most stable.

jb

Shayne Wissler said:

My owners manual says that if I experience this, to take it into the shop. I called the shop and
they want to see the bike. But based on my reading of the shimmy FAQ, I'm half expecting that they
won't find anything.

Quite lkely.

Quoted message said:

I have made two changes to my bike recently. The most recent was that I lowered my handlebars by
about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is
that I replaced my front tire with a lighter, skinnier one.

The maddening thing about shimmy is that it can be just about anything that will cause, or cure, it.
It's as likely the bar height (and the change in your position that results) as the tire.

Quoted message said:

I have descended with the tire before, but I can't say whether I reached 42MPH or not. I can
say that ever since I put that tire on, I have noticed there being less stability while
riding no-hands--it does start to shimmy a little, but at lower speeds and not so violently.
Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the
violent shimmy?

I'd say it's possible.

Quoted message said:

Any suggestions would be appreciated.

Try it with the bars a bit higher, or with another front tire. If it _does_ shimmy, press one knee
against the top tube. That should stop it. Also, unloading your weight from the saddle will stop
the shimmy.

--

David L. Johnson

__o | "Business!" cried the Ghost. "Mankind was my business. The _`\(,_ | common welfare was my
business; charity, mercy, forbearance, (_)/ (_) | and benevolence, were, all, my business. The
dealings of my trade were but a drop of water in the comprehensive ocean of my business!"
--Dickens, "A Christmas Carol"

"Shayne Wissler" <[email hidden]> wrote in message
news:<70Zdb.634208$o%2.291570@sccrnsc02>...

Quoted message said:

The other is that I replaced my front tire with a lighter, skinnier one.

I bet this is it. When I developed a "high-speed" no-hands shimmy earlier this year, I read
everything on rb* on shimmies. It seemed like the fact I had replaced the rear tire with a slightly
fatter one was the likely culprit. I returned to a skinnier rear tire, and the high speed shimmy
disappeared.

Warm Regards,

Claire Petersky ([email hidden]) Home of the meditative cyclist:
home.earthlink.netWelcome.htmOpen ↗

Books just wanna be FREE! See what I mean at: bookcrossing.comCpeterskyOpen ↗

Quoted message said:

Quoted message said:

Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the

violent

Quoted message said:

Quoted message said:

shimmy?

Any suggestions would be appreciated.

. I'll tell you this just once, and I assume you don't want the [censored] scared out of you again. The
skinny tire did it. Put a 25 width tire on the front and enjoy life.

It seemed

Quoted message said:

like the fact I had replaced the rear tire with a slightly fatter one was the likely culprit. I
returned to a skinnier rear tire, and the high speed shimmy disappeared.

It doesn't matter what you have on the back wheel, it's the front wheel that counts.

I suppose we need another FAQ item on this. What is missing in most of these testimonials is how and
at what speed these shimmy episodes occurred. In prior airing of this sort of thing, it was claimed
that balancing the front wheel would prevent shimmy or that a different weight wheel would do it.

I put a large imbalance on a front wheel and made test runs with and without imbalance (I balanced
the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade, smooth half mile section of road,
coasting at between 25 and 30 mph no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This changed the shimmy frequency
slightly but did not stop it from occurring.

That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my
hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and
should not be allowed to occur by the rider. SInce most people cannot ride no-hands, they are
not aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster
about to happen.

draco.acs.uci.edu8h.5.htmlOpen ↗

Jobst Brandt [email hidden]

I was also experimenting with descending shimmy this past weekend, not by choice at first. I just
built a bicycle with upright steel handlebars (older schwinn type) using an older Specialized
Expedition touring bike (long wheelbase). The wheels are Mavic MA-2 with Campagnolo 36h hubs, 126mm
spacing in the rear 13-24 6sp. The tire are the older Avocet Duro Plus 32's, tire pressure 85
psi. On a descent grade of 3-6 % for about 1/3 mile I was able to induce shimmy with my hands on the
handlebars. My speed was 26-32mph. I've also noticed that with upright bars the steering is very
responsive from what I was used to, riding drop bars. I'm not sure if this is because of the
handlebars being further behind the stem. In any-case it was easy to induce shimmy. I thought this
was interesting, so I rode back up the hill and repeated it again. As I got the bike to shimmy I
wanted to see if I could correct this. I took weight off the handlebars by every so slightly
removing both hands about an inch away from the bar. My speed was about 27 mph, and before I knew
it, the shimmy corrected itself and was completely eliminated. I repeated this a 3rd time, to make
sure it wasn't a fluke and actually timed how long it took for the shimmy to correct itself. The
time was about 5 to 7 seconds for the shimmy to completely vanish. I'm going to try this again
since it's one of my training routes. This time I changed tires, with Avocet Carbon 12 Road 700 x
25c.100psi.tire pressure. These are the tires I normally ride with my roadbike. Strange, but my
roadbike doesn't shimmy at all on this stretch of my training ride. -tom

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

Quoted message said:

I suppose we need another FAQ item on this. What is missing in most of these testimonials is how
and at what speed these shimmy episodes occurred. In prior airing of this sort of thing, it was
claimed that balancing the front wheel would prevent shimmy or that a different weight wheel
would do it.

I put a large imbalance on a front wheel and made test runs with and without imbalance (I balanced
the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade, smooth half mile section of road,
coasting at between 25 and 30 mph no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This changed the shimmy frequency
slightly but did not stop it from occurring.

That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my
hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and
should not be allowed to occur by the rider. SInce most people cannot ride no-hands, they are
not aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster
about to happen.

draco.acs.uci.edu8h.5.htmlOpen ↗

Jobst Brandt [email hidden]

Tom Nakashima said:

On a descent grade of 3-6 % for about 1/3 mile I was able to induce shimmy with my hands on the
handlebars.

I have had a similar experience (to this, and to the OP) recently. I replaced my Avocet Fasgrip
700x28 tires (to tie into yet another thread, the rear was completely worn through after 1500 miles,
120000 feet of climbing) with an Avocet Criterium 700x23 (the "real" 28 doesn't fit my frame, and I
couldn't find a 25).

The second day out, riding no-hands at 35-40 mph I had an extremely violent wobble, far worse than
the shimmy I've had before at around 18mph. I've since noticed that I feel this wobble even with my
hands on the bars, though it's easily controlled in this situation.

This was contrary to what I've read here and in the FAQ, which seems to imply that shimmy doesn't
happen with one's hands on the bars. Changing my position on the bike, including using the aero
bars, makes little difference. Unlike yours, my shimmy seems not to self-correct, though I have only
let it go for 1-2 seconds because it feels like it's about to throw me off the bike at 40mph, and
that's bad for morale.

Sam

"David L. Johnson" <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

Shayne Wissler said:

I have made two changes to my bike recently. The most recent was that I lowered my handlebars by
about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other
is that I replaced my front tire with a lighter, skinnier one.

The maddening thing about shimmy is that it can be just about anything that will cause, or cure,
it. It's as likely the bar height (and the change in your position that results) as the tire.

I've encountered shimmy exactly once, after I assembled a used eBay bike and took a first ride. I
had not tightened the handlebars enough. I found this when I went quickly over a rise in the road on
a downhill. I didn't get airborne, but I did get very light, and then came down upon the bars with
some force, which caused them to rotate down and lower my hand position by several inches. For a few
seconds I could not reach the brake levers. I beleive that my speed was basically unaltered - the
only real change in the dynamics of the bike was that my mass on the front of the bike was noticably
lower. The resulting shimmy was immediate and violent, but stopped quickly with the knees on the top
tube method.

From this I think it is possible that your problem may be caused by lowering your bars alone, or
from the combo of the two. I'll second the recommendation that you set the bars back up and try the
same hill with the new tire.

Good luck. Erik Brooks Seattle

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

Quoted message said:

I suppose we need another FAQ item on this. What is missing in most of these testimonials is how
and at what speed these shimmy episodes occurred. In prior airing of this sort of thing, it was
claimed that balancing the front wheel would prevent shimmy or that a different weight wheel
would do it.

I put a large imbalance on a front wheel and made test runs with and without imbalance (I balanced
the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade, smooth half mile section of road,
coasting at between 25 and 30 mph no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This changed the shimmy frequency
slightly but did not stop it from occurring.

That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my
hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and
should not be allowed to occur by the rider. SInce most people cannot ride no-hands, they are
not aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster
about to happen.

I've read the FAQ and everything else I could find on this subject, and what seems apparant is that
nobody has a really good handle on the causes of the usually mild no-hands shimmy that many
bike/rider systems exhibit, or the much more violent "deathwobble" hands-on headshake that,
fortunately, only a few of us seem to experience. Weight distribution, steering geometry, and
structural variable may all contribute to varying degrees.

My daily rides involve descents that allow me to easily reach 50mph, and I've been up to about
58mph. I can ride comfortably no-hands and, on many bikes, easily find the no-hands shimmy speed,
and then stop it with a knee, or by putting one hand on the bar. This shimmy is mostly just annoying
when one is trying to open a snack or take off a jacket while riding near the shimmy speed. Ideally,
a bike would not do it at all.

I don't know if the deathwobble is related or not. This most often occurs at higher speeds (35mph+),
rough pavement, and while braking hard in a straight line, or lightly through the first half of a
corner. It seems to happen less frequently if I concentrate on maintaining a very light grip on the
bars, but on bumpy pavement this is not always possible. Also, the grip on the bars and forward
weight transfer while braking may set it off. It is not easily stopped by the methods mentioned in
the FAQ, and makes steering control very difficult. Usually, I just have to brake hard enough to
slow the bike out of the deathwobble zone and hope that I don't run out of road. This seems to be
more common on larger, more flexible frames, but I've also experienced it of very rigid frames,
which leads me to suspect it's also related to steering geometry or weight distribution. Suggestions
I've heard include very stiff top tubes, and more relaxed head angles (almost all large frames have
74 degree head angles) with longer rake forks.

-David

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

Quoted message said:

The second day out, riding no-hands at 35-40 mph I had an extremely

violent

Quoted message said:

wobble, far worse than the shimmy I've had before at around 18mph. I've since noticed that I feel
this wobble even with my hands on the bars,

though

Quoted message said:

it's easily controlled in this situation.

This was contrary to what I've read here and in the FAQ, which seems to

imply

Quoted message said:

that shimmy doesn't happen with one's hands on the bars. Changing my

position

Quoted message said:

on the bike, including using the aero bars, makes little difference.

Unlike

Quoted message said:

yours, my shimmy seems not to self-correct, though I have only let it go

for

Quoted message said:

1-2 seconds because it feels like it's about to throw me off the bike at 40mph, and that's bad
for morale.

Sounds like my experience: 42MPH and a very violent wobble. And my hands were definitely on the
handlebars (I was descending a canyon and going fast).

I wonder if it has anything to do with reduced spoke count wheels. My front wheel has 20 spokes.

Shayne Wissler

"Shayne Wissler" <[email hidden]> wrote in message news:pireb.2020$%h1.2293@sccrnsc02...

Quoted message said:

I wonder if it has anything to do with reduced spoke count wheels. My

front

Quoted message said:

wheel has 20 spokes.

Here is a thought: At 42MPH, the wheel rotates at about 9 times per second. This is in the frequency
range of the wobble I experienced (I guessed 5, but it could have been 9). A reduced spoke count
wheel is going to have less lateral stability than a wheel with more spokes, and so will have more
tendency to vibrate laterally at lower frequencies. Perhaps this wobble is a lateral vibration of
the rim itself, and it happens at the particular speed where the wheel rotation rate corresponds to
the frequency of vibration.

If this were happening it would explain the violence of the wobble. It is hard for me to imagine how
it could have been so radical if the wheel were not bending, storing the energy of the oscillation.
The question is: how would one test this hypothesis?

Someone brought up the Tacoma Narrows bridge. Perhaps this example is even more apt: maybe this
death-wobble is caused from designing rims that aren't stiff enough.

Shayne Wissler

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

All of this seems consistent with my hypothesis:

Quoted message said:

I don't know if the deathwobble is related or not. This most often occurs at higher speeds
(35mph+), rough pavement, and while braking hard in a straight line, or lightly through the first
half of a corner.

Weight shifting onto the front wheel would cause a greater lateral deflection, storing more energy
in the front wheel. If the braking was absolutely straight and the wheel were absolutely true, this
wouldn't happen, but neither are ever the case. Rough pavement would be more prone to setting up the
lateral vibrations in the rim.

Quoted message said:

It seems to happen less frequently if I concentrate on maintaining a very light grip on the bars,
but on bumpy pavement this is not always possible.

A light grip could easily correspond to less weight on the front wheel.

Quoted message said:

Also, the grip on the bars and forward weight transfer while braking may set it off.

Yes...

Quoted message said:

It is not easily stopped by
the methods mentioned in the FAQ,

If my hypothesis is right, it is not at all the same shimmy, so the same methods would not work.
Moving your knee against the top tube would not dampen much if the vibration is mostly going on in
the rim itself. In fact, if my hypothesis is right, the correct action is to shift some weight off
of the front tire, not off of the seat (actually the correct action is to get a stiffer rim).

Quoted message said:

and makes steering control very difficult. Usually, I just have to brake hard enough to slow the
bike out of the deathwobble zone and hope that I don't run out of road.

This is what happened to me. The front of the bike was violently vibrating from side to side. It
seemed to be the rim (I suspected a flat and/or a severely bent rim).

Quoted message said:

This seems to be more common on larger, more flexible frames, but I've also experienced it of very
rigid frames, which leads me to suspect it's also related to steering geometry or weight
distribution.

If it's the front rim, the frame would make little difference.

Quoted message said:

Suggestions I've heard include very stiff top tubes, and more relaxed head angles (almost all
large frames have 74 degree head angles) with longer rake forks.

A longer fork might help, if it absorbed some of the energy of the lateral vibration.

Shayne Wissler

Shayne Wissler said:

"Shayne Wissler" <[email hidden]> wrote in message
news:pireb.2020$%h1.2293@sccrnsc02...

Quoted message said:

I wonder if it has anything to do with reduced spoke count wheels. My

front

Quoted message said:

wheel has 20 spokes.

Here is a thought: At 42MPH, the wheel rotates at about 9 times per second. This is in the
frequency range of the wobble I experienced (I guessed 5, but it could have been 9). A reduced
spoke count wheel is going to have less lateral stability than a wheel with more spokes, and so
will have more tendency to vibrate laterally at lower frequencies. Perhaps this wobble is a
lateral vibration of the rim itself, and it happens at the particular speed where the wheel
rotation rate corresponds to the frequency of vibration.

I've got 36 spokes. Do I need to add 4 more?

Sam

David Mackintosh said:

Quoted message said:

That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my
hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and
should not be allowed to occur by the rider. Since most people cannot ride no-hands, they are not
aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster about to
happen.

Quoted message said:

I've read the FAQ and everything else I could find on this subject, and what seems apparent is
that nobody has a really good handle on the causes of the usually mild no-hands shimmy that many
bike/rider systems exhibit, or the much more violent "deathwobble" hands-on headshake that,
fortunately, only a few of us seem to experience. Weight distribution, steering geometry, and
structural variable may all contribute to varying degrees.

I think you missed the point that bicycle shimmy is in the frequency of normal human shivering,
something that is related to the natural response of the human muscles. Therefore, trying to resist
shimmy by grabbing the bars firmly and attempting to counter the oscillation can easily lead to
amplifying it. My early experience in this matter was that a friend could not get started on a cold
morning before breakfast because he shimmied before the bicycle got moving and it only got worse
from there.

I believe that the terrified "death grip" is a major contributor to shimmy because the tense muscles
naturally respond in the shimmy frequency. When I have ridden too far without food and a warm up in
cold weather, I can detect the tendency to shimmy on descents and I take every precaution to not get
caught in the feedback loop. This includes a loose grip on the bars.

Jobst Brandt [email hidden]

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

Quoted message said:

I think you missed the point that bicycle shimmy is in the frequency of normal human shivering,
something that is related to the natural response of the human muscles.

Just because the shimmy frequency is close to the shiver frequency does not imply that
shivering or other muscle instability causes a shimmy. I have descended this particular canyon
many times, and this time was not particularly cold, nor was I nervous or particularly tired.
The bike was definitely doing this vibrating on its own, though certainly my weight
distribution at that particular speed could have been a factor. I was part of the system, but
the instability was in the bike.

Shayne Wissler

may have said:

...bicycle shimmy is in the frequency of normal human shivering, something that is related to the
natural response of the human muscles. Therefore, trying to resist shimmy by grabbing the bars
firmly and attempting to counter the oscillation can easily lead to amplifying it. My early
experience in this matter was that a friend could not get started on a cold morning before
breakfast because he shimmied before the bicycle got moving and it only got worse from there.

I believe that the terrified "death grip" is a major contributor to shimmy because the tense
muscles naturally respond in the shimmy frequency. When I have ridden too far without food and a
warm up in cold weather, I can detect the tendency to shimmy on descents and I take every
precaution to not get caught in the feedback loop. This includes a loose grip on the bars.

One rider's perfect bike can be another's nightmare, as well; I've seen shimmy only rarely (and
haven't experienced it in a while), but one memorable instance involved a road bike that shimmied
horribly under one particular rider, but not at all under me or the other three who were present
at the time.

The rider is part of the system; this is why from an engineering standpoint, some of the possible
problems may not be predictable.

Has anyone tried using a steering damper on a bicycle to combat this problem? It's a ubiquitous
solution on motorcycles.

--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.