Variations on a theme (shimmy)

[Birddog]

Over the weekend I experienced another front end shimmy problem. I was in the front third of a pack of about 40 riders descending at about 40 mph when the front wheel MIGHT have hit a long tar patch (not sure if that's what set it off or not) and the front end started to shimmy. I'm not new to this, and thought I had it figured out (death grip) long ago, but here we go again, except this time surrounded by a pack.

I was pretty calm about it, I just relaxed an already fairly relaxed grip a little more ( I was in the drops) till I was barely holding the bars. The shimmy dissipated a little, but did NOT go away. I next calmly tried the knees pinching the top tube trick with the same result, and then combined the two remedies and still no luck. Apparently others noticed my misfortune, because they were creating some space. I didn't panic, I calmly mentioned that I needed to get over to the right and they gave me some space. The bottom line is that after dropping back and over to the shoulder, everything I tried again didn't work and finally I just pulled over and stopped. The shimmy was never as violent as some I've experienced, but it was definitely uncontrolable. It actually became most severe when I got between approx 10 and 5 mph as I was stopping.

I quickly checked the headset and the front wheel. Nothing out of the ordinary so I took off again. Later I did a straight descent at about 50 mph with no problems (knees were pinched though in anticipation) and still later a couple of hairy twisty descents, again with no problems.

When this started, I might have even been pedaling, I'm not sure about that though (we were descending into a stiff wind). The other thing is, I don't remember if I pushed one foot down hard as an attempt to stop the shimmy.

Anybody have some ideas? This was not a "death grip" phenomena and I haven't had a front end shimmy in a long time. I'm perplexed.

Birddog

[gone]

I had a similar experience, descending a steep, winding road at about 45 mph. Hit a rock that I didn't see just at the entry to a curve which started the shimmy (I may have also tightened my grip on the bars) but in my case the standard set of tricks (relax grip, clamp top tube with knees, etc) worked and the shimmy stopped.

[xlbs]

weird, but I had a similar experience many years ago...I discussed it with an experienced rider, and he suggested that my body had acted like a foil because of the head-wind--lifting the front end of the bicycle ever so slightly--thereby exacerbating the tiny amounts of sympathetic vibration already in the front-end of the frame.

I've never had it happen again (nor do I wish to) and have wondered ever since if the information was correct. Has anyone ever heard of this explanation for shimmy before?

I.

[Serotta PETE]

weird, but I had a similar experience many years ago...I discussed it with an experienced rider, and he suggested that my body had acted like a foil because of the head-wind--lifting the front end of the bicycle ever so slightly--thereby exacerbating the tiny amounts of sympathetic vibration already in the front-end of the frame.

I've never had it happen again (nor do I wish to) and have wondered ever since if the information was correct. Has anyone ever heard of this explanation for shimmy before?

I.

Yes, it causes the front end to get lighter as well as your body(with stiff arms) act as uplift - - - ) if that makes since. Had it happen to me in North East area of Vermont on a l-o-n-g downhill, many years ago. NO FUN AND started my drinking red!!!!

[Ti Designs]

Most bike builders think of shimmy or speed wobble as black magic, some frames have it, others don't - no way of telling. Most have the theory that it's all about frame alignment. I'm dealing with that exact issue right now with a local custom builder. I was told that it's an alignment issue, so they checked the frame, replaced the fork and gave the bike back to the customer. The speed wobble returned...

A number of years ago I had the same problem with a Serotta Legend Ti. Kelly built a replacement frame with a tube taken from the mountain bike, and had the frame back in 6 days. In this time a lot of work at MIT came to a halt as much of the physics department was evaluating speed wobble. Here's the simple version of what I learned.

First, take the case of a pendulum. It has a mass, a radius and gravity. It also has a known period (the time it takes to swing back and forth). The first and second derivatives (velocity and acceleration) are inverses, so when velocity is at zero, acceleration is at it's greates and vise-versa. It's simple harmonic motion. If you change the lenght of the radius you change the period and the frequency.

Now take the case of a spring metal tube. If you were to attach a bar at the end and put a weight on that and start it in motion you would get the same action. The tube twists according to the same rules. It has a set period and frequency based on it's torsional stiffness, it's diameter and it's length.

A bike has two tubes that attach the head tube to the back of the bike, each one has a harmonic frequency. Given the right initial force, a system can be set up so the torsion is additive and causes the head tube to twist side to side, which is what causes speed wobble. The difference between a bike that has speed wobble and one that doesn't is the one that doesn't has a different frequency for it's down tube and top tube. They may start out in the right orientation to cause shimmy, but the tubes quickly get out of phase and the wave is canceled. The kind of speed wobble that requires the bike to come to a full stop is caused by the two tubes having equal resonant frequency or multiples of it.

There are a number of elements that damp the twisting, there are restoring constants from the weight on the saddle, the weight on the bars (the knee on the top tube). There is a restoring constant for the tires on the pavement - which is not a constant as it decreases with speed. Many people have found that making small changes to a bike that has shimmy causes it to go away. What they've done is change the conditions enough to hide the problem - it's still there. If you were to mount two frames to a flat table by the seat tube, extend a bar from the head tube and hang a weight on it, then give the weight a tug to start the twisting motion you would find the bike with the shimmy acts like a pendulum and keeps twisting while the other bike acts quickly loses it's twisting action. This can't be altered by changing out the fork or switching tire brand...

[znfdl]

Yes, it causes the front end to get lighter as well as your body(with stiff arms) act as uplift - - - ) if that makes since. Had it happen to me in North East area of Vermont on a l-o-n-g downhill, many years ago. NO FUN AND started my drinking red!!!!

Pete:

Atleast you had a life altering experience why you started to drinking red. I just like fermented grape juice :D

[Ken Robb]

"started my drinking red": then it was a GOOD thing.

I've had several bikes that wobbled beneath me and on one of them the only way I could stop it was to take a death grip on the bars and lock my elbows against my sides. Not the usually suggested technique but it was all that worked on that bike.

[atmo]

Most bike builders think of shimmy or speed wobble as black magic, some frames have it, others don't - no way of telling. Most have the theory that it's all about frame alignment.

what builders think of it as being related to frame alignment?
i know of none that do atmo. fwiw, we had a shimmy thread
a year (?) ago and i pasted this link (http://search.bikelist.org/getmsg.asp?Filename=classicrendezvous.10207.0504.e ml) to a 2002 thread regarding
same that appeared on another listserve atmo.

[Ken Robb]

still mostly on-topic: old motorcycles often had friction steering dampers on the head tubes to cure wobbles and headshake. If they were over-tightened it sometimes produced a slow weave but never, in my experience, wobbles. I mention this because someone here said he had reduced wobbles on his bike by having increased friction in his headset. Might work sometimes?

[catulle]

what builders think of it as being related to frame alignment?
i know of none that do atmo. fwiw, we had a shimmy thread
a year (?) ago and i pasted this link (http://search.bikelist.org/getmsg.asp?Filename=classicrendezvous.10207.0504.e ml) to a 2002 thread regarding
same that appeared on another listserve atmo.

So, exactly where is the problem located?

[Serotta PETE]

still mostly on-topic: old motorcycles often had friction steering dampers on the head tubes to cure wobbles and headshake. If they were over-tightened it sometimes produced a slow weave but never, in my experience, wobbles. I mention this because someone here said he had reduced wobbles on his bike by having increased friction in his headset. Might work sometimes?

Drink red and enjoy the "wobbles".

In all seriousness, it can be very scary when it happens.

PETE

[trophyoftexas]

Hate it that everyone seems to have experienced this at some time in their cycling career's but glad to know that I'm not nuts or doing something wrong! My Specialized Roubaix Pro has done this to me a couple of times and I was concerned that perhaps this was a "carbon" thing, now, after reading all of the other posts here, seems not. I do know this, my Roubaix approaches a nice downhill and seems to whisper to me "easy now, eeeeaaasssyyyy!" On the other hand by Ottrott ST seems to say "what's the matter with you, NO GUTS??.....faster....I SAID FASTER!" MY question here is what will I hear the bike say if I get the Legend Ti that I'm now thinking about?

[Needs Help]

ya' know what i think the real issue is? some people don't
have balls. they don't know how to go down a hill without freezing
up. period.
In my opinion, anyone that believes that is ignorant.

[atmo]

So, exactly where is the problem located?
my feeling is based on seeing all sides of
this issue (to date) as well as experiencing
the phenonenom itself at jiminy peak (ed
will know that descent). it's all in the link i
pasted, yet will reiterate.

shimmy never happens the same way twice.
it is hardly duplicate-able on the same hill by
the same person. give said bicycle to another
and the resultant achy-shaky will likely not be
present. if a person encounters a wind shift, a
crown in the road, a momentary lapse of attention,
ice, whatever, and cannot react calmly and
accordingly, fear sets in. after that, at ??? mph,
that trepidation leads to an inbalance above the
bicycle that causes the speeding wheels to begin
oscilating (sp?). once that happens, it gets worse
before it gets better atmo. iow, pilot error.

read that link i pasted. odd that with all the descents
that pros do over a calendar year, a story about shimmy
never surfaces. these guys are comfortable at speed.
it's also telling that little kids can do all sorts of gyrations
on a 50 dollar sears bike whilst delivering papers, etc.

last time this subject surfaced in a thread, i took heat
for my opinions on this. i already have this time offlist.
i'll eagerly await other explanations about the cause of
shimmy atmo.
In my opinion, anyone that believes that is ignorant.
ya' see. it's starting again.

[Serotta PETE]

Hate it that everyone seems to have experienced this at some time in their cycling career's but glad to know that I'm not nuts or doing something wrong! My Specialized Roubaix Pro has done this to me a couple of times and I was concerned that perhaps this was a "carbon" thing, now, after reading all of the other posts here, seems not. I do know this, my Roubaix approaches a nice downhill and seems to whisper to me "easy now, eeeeaaasssyyyy!" On the other hand by Ottrott ST seems to say "what's the matter with you, NO GUTS??.....faster....I SAID FASTER!" MY question here is what will I hear the bike say if I get the Legend Ti that I'm now thinking about?

Legend ti especially with F2 or F3 fork will say - - smile and enjoy :banana:

[Allez!]

A bike has two tubes that attach the head tube to the back of the bike, each one has a harmonic frequency.

Interesting.... I have a hammer with a tuning fork built into the handle to dampen vibration. Wonder if two differently tuned forks in the frame tubes could control shimmy?

[zap]

snipped

shimmy never happens the same way twice.
it is hardly duplicate-able on the same hill by
the same person.


Wanna bet.

Ooooops. Can't. Frame died.

But when this particular frame was rideable I could duplicate shimmy on the same hill. I could make it go away too.

About Pro's, you need to look into the early days of carbon composite bikes.

[atmo]

But when this particular frame was rideable I could duplicate shimmy on the same hill. I could make it go away too.
are we talking about shimmy that you would actually
induce into your ride? what would be the point of
causing your own speed wobble? my opinions are
with respect to what others would consider naturally
occuring speed wobbles atmo.




About Pro's, you need to look into the early days of carbon composite bikes.
can you narrow that down to a year, or a team?
or a brand?

[Needs Help]

if a person encounters a wind shift, a
crown in the road, a momentary lapse of attention,
ice, whatever, and cannot react calmly and
accordingly, fear sets in. after that, at ??? mph,
that trepidation leads to an inbalance above the
bicycle that causes the speeding wheels to begin
oscilating (sp?). once that happens, it gets worse
before it gets better atmo. iow, pilot error.

That seems to imply that if I gave you a certain frame, you could ride downhill in the wind, over crowns in the road, and by not paying attention you could induce shimmy. Do you really believe that? Maybe the next time the Serotta test fleet is in your area, you could give it a try, and then post the results.

[atmo]

That seems to imply that if I gave you a certain frame, you could ride downhill in the wind, over crowns in the road, and by not paying attention you could induce shimmy. Do you really believe that?
you got that from the text of mine that you pasted?!
i implied no such thing at all.

Maybe the next time the Serotta test fleet is in your area, you could give it a try, and then post the results.
huh?

[zap]

are we talking about shimmy that you would actually
induce into your ride? what would be the point of
causing your own speed wobble? my opinions are
with respect to what others would consider naturally
occuring speed wobbles atmo.

I don't know about actually inducing shimmy. You sort of ride along and things would wobble a wee bit. But I could make it worse. Just take the hands of the bars on a 40+mph decent. I could make it go away by putting the hands back in the drops and lean further forward.

I just put this down as a noodly frame.

Re Pros. I think it was the '86 TdF. Look or TVT frame.

[stevep]

the only time ive ever experienced shimmy on a bike was cold induced.
descending in the high mts on windy or damp and cold days if i failed to bring enopugh warm clothing ...a little shiver really turns the bike into a noodle for me.
the rest of the time i descend pretty well at speed and comfortable even on big fast switchback descents... no trouble letting the same bike go when warm... a cold shiver makes the thing unrideable at any speed for me
has only happened twice in 35 years.

[atmo]

aw heck why not redux this (http://forums.thepaceline.net/showthread.php?t=17052&page=1) encore again atmo.

[Louis]

A bike has two tubes that attach the head tube to the back of the bike, each one has a harmonic frequency. Given the right initial force, a system can be set up so the torsion is additive and causes the head tube to twist side to side, which is what causes speed wobble. The difference between a bike that has speed wobble and one that doesn't is the one that doesn't has a different frequency for it's down tube and top tube. They may start out in the right orientation to cause shimmy, but the tubes quickly get out of phase and the wave is canceled. The kind of speed wobble that requires the bike to come to a full stop is caused by the two tubes having equal resonant frequency or multiples of it.

Ti D,

This is the first time I have heard bike-related shimmy described in this manner. Do you have any references for this?

As best I can tell, frequencies of the DT and TT and their values relative to each other are not a direct driver of shimmy. For example, I could design a frame with a massive DT or a massive TT and eliminate the other tube completely and not get shimmy. (This would not be efficient weight wise, but is doable.) I could then take that frame and gradually reduce the stiffness of the DT or TT until I got the frame to shimmy. The key is the system torsional stiffness, not whether or not the DT and TT frequencies (for whichever mode you happen to be referring to, since each has an infinite number of modes) happen to tune.

Louis

[J.Greene]

In my opinion, anyone that believes that is ignorant.

I believe atmo and I'm not ignorant. Why would you say that his opinion is ignorant, his frames have an impressive palmares?

JG

[nm87710]

Birddog, Sorry to read of your shimmy and shakin pains. I assume it was up at the EC ride this past weekend? Which downhill? FWIW and as one who has experienced the shimmies I agree w/atmo as to the root causes. Don't overthink it. Just keep practicing w/good downhill techniques.

Cheers and wishing I was in AF instead of this heat!

[catulle]

Sometimes, when sprinting really hard on the 53X11 and holding the drops too hard for leverage, I can make the front end shimmy. But that most definitely is my fault, I think. :eek:

[Ti Designs]

As best I can tell, frequencies of the DT and TT and their values relative to each other are not a direct driver of shimmy. For example, I could design a frame with a massive DT or a massive TT and eliminate the other tube completely and not get shimmy. (This would not be efficient weight wise, but is doable.) I could then take that frame and gradually reduce the stiffness of the DT or TT until I got the frame to shimmy. The key is the system torsional stiffness, not whether or not the DT and TT frequencies (for whichever mode you happen to be referring to, since each has an infinite number of modes) happen to tune.


Let's oversimplify it for a second. Say you have one tube. Yes, you could make it massive enough to eliminate shimmy, but what you're really doing is making the frequency outside of the sustainable range, so even if you could induce it the restoring constants would damp it out. Bringing down the diameter would cause shimmy as the frequency gets into a sustainable range, it's a simple Sin() function. With two tubes there are two torsional frequencies to look at, they are multipied by the Sin(angle they intersect the head tube), and added together. If the frequency is the same or close, the additive values (it's really subtractive values as the top and bottom of the head tube go in different directions during shimmy) create a larger wave. If they are different, the come out of phase and as soon as the wave dies down to a threshold it's damped by the rider and wheels on the ground.

Graph Sin(x), that's a single tube. Graph Sin(x)+Sin(x), that's two tubes that share a frequency. Graph Sin(3x)+Sin(7x), that's two tubes with different frequencies - not the sections where there's phase cancelation...

[chrisroph]

There seem to be lots of opinions on this issue. FWIW, the only shimmies I've experienced were definitely rider induced. I once made my guerciotti track bike shimmy when I was riding a miss'n out and pulled a bonehead by accelerating and trying to squeeze to the inside. Of course, as the lead riders approached the line, they slowed. Since I was accelerating and on the inside, I rapidly overtook riders and had nowhere to go but the infield. Since it was alpenrose, which has quite steep banks, and since I was on the infield heading for the bank under a whole group of riders, my body tensed causing a really nice shimmy. It could just have easily been a soiled chamois but instead it was a stiff arm shimmy. To finish the story, I flashed back to my motocross days and hit the bank like a berm effectively turning the bike and missing everybody. I did that one once.

The second time was approaching the finish at the end of a championship road race. About 500 meters from the line, one of my "buddies" decided he wanted to be a 1/2 bike length in front of where I was so he came down on me hard. I had about 1mm of clearance on each side of my front wheel and I tried to squeeze the bars a little closer together so I would fit and it caused a real strong shimmy. Fortunately, the gap opened as quickly as it closed and the shimmy disappeared as I relaxed my arms, sprinted to the line and won the filed sprint for the silver.

My serotta ti was a warranty return because it shimmied for its original owner. This was fully disclosed by "dude" when he sold it to me with a money back guarantee. The bike is rock solid under me.

So, my only shimmy experiences were that I caused it and the bike that shimmied under somebody else is fine for me. Thus, ATMO, I tend to agree with ATMO but I also don't disbelieve anybody who has had the dreaded shimmy under certain conditions as YMMV.

[Louis]

TiD,

Like I said, do you have any references that discuss your approach ???

Everything I can find out there handles it in the "conventional" manner. Two examples: Brandt (http://www.sheldonbrown.com/brandt/shimmy.html) and Wikipedia (http://en.wikipedia.org/wiki/Speed_wobble)

Louis

[Ti Designs]

Like I said, do you have any references that discuss your approach ???

Just myself (degrees in physics and computer science), a lot of computer model work, a few working examples in controlled testing and a deck of tarot cards...

This gets back to the discussion about being certified as a coach, most certs (or in this case references) are worth far less than what they back up. If my findings mean nothing 'cause Sheldon Brown doesn't write about them (his wife was my computer science advisor), it's time for me to get the hell off the Serotta board.

I did some testing and asked some friends to model the system because I wanted to know if it was the frame or the rider. My conclusions, based on testing, were that there are some frames that do hold a wave and are prone to picking up a wobble that doesn't stop until the bike does. I got my answer - no references needed.

[atmo]

TiD,
Two examples: Brandt (http://www.sheldonbrown.com/brandt/shimmy.html)<cut>

Louis
text lifted, boldface added:
Shimmy that concerns riders the most occurs with hands firmly on the bars and it is rider generated by muscular effect whose natural response is the same as the shimmy frequency, about that of Human shivering.

wow.
pilot error?
mon dieu.

[Ginger]

Bikes that are too long in the top tube and don't fit me shimmy on descents over 30/35mph. Depending on how much they don't fit me, sort of equates to the amount of shimmy. I suspect it has to do with muscles twitching starting it off. Don't know...it can be spooky but entertaining in a sick sort of way.

Bikes that fit me and have proper weight distribution don't shimmy. Except for once. And that was my own fault (listening to nervous nellies who told me a hill was really difficult...once I relaxed, everything was wonderful...).

Now...perhaps because I'm so stretched out on a bike that doesn't fit that silly things that I don't notice are amplified and the bike shimmies.

Can't be freezing up...I love to descend...heck...I'll try to climb ANY hill if there's a good descent on the other side. :) Faster FASTER she said! (Of course, I'm older now and I take a staid approach to descending...very sad...)

There you go. Ginger's non-scientific, ancedotal, yet somewhat non-technical view of shimmy... I even stayed away from the wikipedia definition of Shimmy...

[Birddog]

Well as I suspected, lots of reasons on the cause of "shimmy", but no real answers to my question "why didn't the old remedies work"?

I've had several bikes shimmy over the years, but not in the recent past since I was enlightened to the fact that it was likely caused by me and my "death grip". Based on my own experience, I've been quick to tell others that shimmy was likely caused by rider input and "death grip". As stated previously, I absolutely did NOT have a "death grip" in this case, but I can't rule out "pilot error" either, in fact, it probably was.
This was not what I would call a sketchy pack that I was in, everybody was actually riding pretty sensibly. We had just gotten into a section of pavement that had several long tar patches running in the direction of travel, you knowm those 1" wide jobs, that if they aren't filled will swallow a tire. I was in the process of moving a little to the right to avoid same when the shimmy started. (Yes NM87710, it was at the EC on the fastest section a little past the mine on the way to Questa). Although disconcerting, I wasn't scared, didn't panic, and didn't do anything rash. I calmly loosened my already fairly loose grip, then I tried the knee on top tube, then both knees pinching, then all three remedies. The shimmy diminished some, but did not go away. I feel like I was pretty calm during all this. My reaction to the shimmy in the first place was more "aw crap" than "Holy $#!+". I felt like my best option was to get out of the pack and come to a stop, so that's what I did. No sense having some other rider panic when they see what's happening to my bike and my situation and do something stupid.

About a half hour later, on a very long straight descent (Garrapata Ridge), I let it hang out, and nothing happened at about 50 mph (this sorta proves ATMO's pt ). Still later on the twisty descent down Palo Flechedo, I thought the bike handled as well as it could and I maybe went as fast as I ever have down that section with no problems and again down Bobcat pass into Red River, although much of that was into the wind so not quite as fast. The question was, why didn't the regular corrections work?

Maybe it was because I run Shimano on that bike and not Campy!
That ought to be worth a few more pages heh, heh, heh.

Birddog

[Needs Help]

wow.
pilot error?
Is it pilot error to shiver in the cold? Is it pilot error to contract a specific set of muscles when riding? Does shimmy occur every time a rider shivers in the cold or contracts a given set of muscles?

I think the rider input is complete happenstance and converges with other factors to induce shimmy. In my opinion, shimmy is caused by a combination of the rider's weight and what they happen to be doing the moment before the shimmy starts, the frame, wind, uneven pavement, and speed. If they all combine in the right way, then the frame will begin oscillating. Whether you can successfully dampen the oscillations by clamping your knee against the top tube is anyone's guess. It seems plausible that the oscillations could be severe enough that the damping provided by a knee would not stop the shimmy.

If some riders don't get shimmy because they are very skilled riders, then it seems to me that an expert rider could ride less skillfully and purposefully induce shimmy in a frame. To test that hypothesis, it would be interesting to see a group of skilled riders take the Serotta test fleet down a bunch of descents and lock their arms, swing their hips from side to side, or jump up and down on the pedals. Whatever they want. Then report back on how many frames they could get to shimmy. If they can't get a single frame to shimmy, then to me that would be good evidence that rider skill has nothing to do with shimmy. If they can get every frame to shimmy, then that would be good evidence that lack of riding skill is what causes shimmy. On the other hand, if they can get certain frames to shimmy and not others, then that would seem like good evidence that rider skill is but one factor in causing shimmy.

Another enlightening test would to take a 54cm frame that is made of super stiff tubing and a 60cm frame that is made of very compliant tubing and see which frame expert riders can cause to shimmy easier.

but no real answers to my question "why didn't the old remedies work"?Because the damping you provided was not enough to overcome the oscillations of the shimmy.

[RPS]

Why not concentrate on the root cause of a wobble? Without knowing the dynamics of what causes it, how can it be prevented from ever occurring in the first place? Clamping the top tube with knees or holding the bars in a certain way are remedies and not solutions for a problem that shouldn’t exist.

If a high-speed wobble is an oscillation out of control, what feeds it, what supplies the energy to keep it going? Without a source of energy, would it not damp out quickly?

Once initiated, a wobble must be sustained by either the rider or outside forces. IMO at low speeds of 5 to 10 MPH rider input is the likely fuel (probably from trembling associated with an adrenaline rush). The original high-speed (30-50 MPH) wobble that caused the rush is a different matter.

[Needs Help]

If a high-speed wobble is an oscillation out of control, what feeds it, what supplies the energy to keep it going?
The rider is losing potential energy as they descend, right?

[NateM]

Birddog,
did you get off the saddle? I have found that getting off the saddle will calm the dreaded shimmy once it gets going.One knee against the top tube, relax the grip on bars, lift up the butt. Takes a little practice and not too much fun when speeds are over 45mph,but it has saved me more than once.

[Ti Designs]

Is it pilot error to shiver in the cold?


Speed wobble is the second most overdiagnosed problem after ADD. The bike reacting to the rider shivering isn't speed wobble, it's just the bike reacting. By speed wobble I'm talking about and oscilation set up acorss the plane of the bike. My guess is that far less than 1% of the frames out there can sustain such oscilations. (then again, I would have guessed that very few riders need 180mm cranks, but it seems half the forum has 'em). The system is powered by forward momentum, the head tube twists, the front wheel is displaced off centerline and the trail causes it to cut back acorss the centerline. Just like a pendulum it doens't just stop at the center, it has mass, it continues to twist. The result is it scrubs speed but increases the amplitude of the wobble. Let's say the amplitude of the wobble was 4" at the tire/road contact point, and the frequency was 8 cycles per second. Knowing the riders mass and verlocity we could calculate the energy in the system, and it's gonna be a hell of a lot more than any human can damp by holding onto the bars.

[atmo]

atmo high speed wobble is initiated during a momentary
lapse of attention or being in control when a rider hits
an obstacle, wind shift, crown in the road, or a certain
speed, and subsequently loses it. it can happen to anyone,
anywhere, and it usually does. another way to explain it
atmo is that you're skiing down an incline and the tactile
sense of rubber to pavement is analagous to board on ice.
some folk have experienced enough to know when they're
in deep should that danger zone reer (sp?) its head, and
these cats are comfortable there. others, usually on a hill
or in an area with which they're unfamiliar, freeze up once
the peril sets in. that's not to suggest this is brought on by
being cold, though that certainly would not help. it's more
about that comfort zone we all live in, and what happens
when we leave the margins atmo.

[Birddog]

did you get off the saddle
Nah, I didn't think of that one. Once I had slipped out of the pack that would have been a good thing to try, unfortunately, my bag of tricks was limited to the two mentioned, and they have always worked before.
Since I was on a downhill, I had to use the brakes to slow down, so in the process, I had somewhat of a grip on the bars while braking, not real tight though. As I was slowing, it was sort of interesting to note how the occillations changed with the change in forward speed. The highest or strongest shimmy occurred at something like under 10 and above 5 mph when the side to side movement was most pronounced. I actually let it pick up speed again and did it twice to check it out. Very weird.

Birddog

[Ti Designs]

atmo high speed wobble is initiated during a momentary
lapse of attention or being in control when a rider hits
an obstacle, wind shift, crown in the road, or a certain
speed, and subsequently loses it.

That's what I would call rider error. By that definition anything and everything will have the potential for speed wobble. I've seen cases of speed wobble where the bike had to be brought down to almost zero MPH before it went away - that's speed wobble.

[obtuse]

i have never experienced shimmy in my life. ever notice how it always seems to happen to people on bicycles that look like they belong in the circus?

not enough weight on the front wheel generally, inexperienced rider, poor bike design....i have no idea. i'm still agnostic about the whole concept and i'm not even sure "shimmy" isn't some made up thing to excuse poor descending skills.

obtuse

ps. and fstrthnu can vouch for this, remember the lotto boys showing the rest of the peloton how shi'ite thier litespeeds were by slapping the top tube and making the whole bike shake out of control?

[RPS]

Ti Design,

Assuming your theory is correct (and I’m not disagreeing), what is your recommended solution? Would you simply make the frame stiffer in torsion to keep the head tube from rotating relative to the rider?

[zeroking17]

If no obvious mechanical cause for the bike’s behavior is found, then maybe we can explain it by means of chaos theory, which investigates why apparently stable and predictable dynamic systems suddenly become unstable and unpredictable under certain conditions.

In this case, why does the bike’s behavior change radically at a particular speed? It’s probably some combination of just that size frame and just those frame angles, just that rider’s position and weight distribution, just that particular speed, just that road surface, just that tire pressure, and a butterfly flapping its wings just that way in Provo. Change any of these variables, and the wobble goes away (or re-emerges at some other combination of these factors). What could be more obvious?

[atmo]

Originally Posted by atmo
atmo high speed wobble is initiated during a momentary
lapse of attention or being in control when a rider hits
an obstacle, wind shift, crown in the road, or a certain
speed, and subsequently loses it.That's what I would call rider error. By that definition anything and everything will have the potential for speed wobble. I've seen cases of speed wobble where the bike had to be brought down to almost zero MPH before it went away - that's speed wobble.


alas - you're feelin' me atmo.

[atmo]

If no obvious mechanical cause for the bike’s behavior is found, then maybe we can explain it by means of chaos theory, which investigates why apparently stable and predictable dynamic systems suddenly become unstable and unpredictable under certain conditions.
guilty.
my wife, aka the lovely deb, oftens explains
me away as being walking entropy atmo.

[bironi]

the only time ive ever experienced shimmy on a bike was cold induced.
descending in the high mts on windy or damp and cold days if i failed to bring enopugh warm clothing ...a little shiver really turns the bike into a noodle for me.
the rest of the time i descend pretty well at speed and comfortable even on big fast switchback descents... no trouble letting the same bike go when warm... a cold shiver makes the thing unrideable at any speed for me
has only happened twice in 35 years.

I agree with Steve. Anyone can test the effect of shivering on a descent, just experiment by setting up a fake shiver that vibrates down your lower arms, you can really get your stable road bike to wobble nearly uncontrollably. This test can be done a low speed for safety. There may be other causes, but all I have experienced are cold shivers, and loose headsets. :beer:
Byron

[nm87710]

atmo high speed wobble is initiated during a momentary
lapse of attention or being in control when a rider hits
an obstacle, wind shift, crown in the road, or a certain
speed, and subsequently loses it. it can happen to anyone,
anywhere, and it usually does. another way to explain it
atmo is that you're skiing down an incline and the tactile
sense of rubber to pavement is analagous to board on ice.
some folk have experienced enough to know when they're
in deep should that danger zone reer (sp?) its head, and
these cats are comfortable there. others, usually on a hill
or in an area with which they're unfamiliar, freeze up once
the peril sets in. that's not to suggest this is brought on by
being cold, though that certainly would not help. it's more
about that comfort zone we all live in, and what happens
when we leave the margins atmo.

true dat

let's pluto this thread

[zeroking17]

guilty.
my wife, aka the lovely deb, oftens explains
me away as being walking entropy atmo.

You're a true entropyneurial spirit!

[zeroking17]

true dat

let's pluto this thread

It's a cold universe.


The widow of the astronomer who discovered Pluto 76 years ago said Thursday she was frustrated by the decision to strip it of its planetary status, but she added that Clyde Tombaugh would have understood.

"I'm not heartbroken. I'm just shook up," Patricia Tombaugh, 93, said in a telephone interview from her home in Las Cruces.

Clyde Tombaugh was 24 when he discovered Pluto while working at Lowell Observatory in Flagstaff, Ariz., in 1930. He spent months meticulously examining images of the sky, looking for a planet observatory founder Percival Lowell theorized was affecting the orbit of Uranus. Lowell was wrong _ Pluto is too small to affect giant Neptune's orbit _ but Tombaugh found it anyway.

[davids]

guilty.
my wife, aka the lovely deb, oftens explains
me away as being walking entropy atmo.
Join the club.

http://hew.ca/yapc/exceptions/slides/images/things2.jpg

[nm87710]

It's a cold universe.

The widow of the astronomer who discovered Pluto 76 years ago said Thursday she was frustrated by the decision to strip it of its planetary status, but she added that Clyde Tombaugh would have understood.

"I'm not heartbroken. I'm just shook up," Patricia Tombaugh, 93, said in a telephone interview from her home in Las Cruces.

Clyde Tombaugh was 24 when he discovered Pluto while working at Lowell Observatory in Flagstaff, Ariz., in 1930. He spent months meticulously examining images of the sky, looking for a planet observatory founder Percival Lowell theorized was affecting the orbit of Uranus. Lowell was wrong _ Pluto is too small to affect giant Neptune's orbit _ but Tombaugh found it anyway.

#134340 :crap: :crap: :crap: A galactic travesty!!!!

[Kahuna]

I've been able to initiate speed wobble while descending in a reproduceable fashion on certain frames. Doing so involves getting into a position that no one typically descends in. It's dangerous and I don't recommend it, but it works if you want to see if your bike is prone to shimmy. It's simple. Here's how:

Get yourself going on a long steep downhill section. The smoother and straighter the road, the better. When you reach 30 or 35mph while still accellerating, put all your weight on the saddle, take your hands off the bars and sit upright. Don't pinch the top tube with your knees. Hold it like that and continue to accellerate. If your bike is prone to wobble it will begin here and get progressively worse. Then play a game of chicken with yourself and see how long it takes before fear takes over and you snatch the bars to stop the shakin'.

I'm of the opinion that certain bikes do this more easily than others and bikes that are stiffer torsionally are the least likely to wobble.

I don't think I've ever had a bike oscillate like that when my hands were on the bars.

-K

[David Kirk]

I was the lucky bastard that for many years got to take out Serottas that were thought to have wobble issues.

I'd get a ride to the top of some nasty hill (Spier Falls Road most of the time) and take the bike down to see how fast one had to go to get it to wobble. This was fun in that crap your pants kind of way.

I've found that most any bike will wobble if you get the bike going about 30mph, sit bolt upright, and then (this is the fun part...try this at your own risk) take your clenched fist and give the stem right above the head tube a good whack to the side. Hit it hard and watch it go.

Fun with science....I feel like Bill Nye.

Dave

[Needs Help]

I was the lucky bastard that for many years got to take out Serottas that were thought to have wobble issues.
Were there any common themes that you were able to discern?

[Ginger]

I suppose that whack to the stem might be one...

[David Kirk]

Were there any common themes that you were able to discern?

I can tell you that the problem became much more common with the advent of STI/Ergo stuff (extra weight on the bars) and more upright rider positions.

Very heavy early clip on aero bars were the worst. All that weight swinging around out there was not good.

I can also say that I've never found a bike that wobbled due to poor alignment or improperly adjusted headsets. One can actually get a wobbler to stop wobbling by screwing up the alignment. Cock the front wheel in the fork or the rear in it's dropouts and it will almost always stop. Once you get the bike to pull to one side or the other it won't want to go back and forth and set up a wobble.

Dave

[vaxn8r]

Having never experienced SW in 25 years and having owned and or at least ridden dozens of race bikes (with personal best top speed of 67mph), I would be curious about bike set up of those who have experienced it. For example: stem height, stem length, length of saddle set back, as well as rider technique, descending on tops, hoods or drops etc.

ATMO, at speed, bikes descend best (most stability) when seated low and in the drops. I don't descend like that anymore, 50+/- is my about my limit anymore. Got family.

[RPS]

I can tell you that the problem became much more common with the advent of STI/Ergo stuff (extra weight on the bars) and more upright rider positions.

Very heavy early clip on aero bars were the worst. All that weight swinging around out there was not good.

Dave

This pattern of extra mass at the front end makes sense to me. I had a similar experience after attaching a small pack to my bars for a long unsupported ride. That bike had never wobbled before and hasn’t since.

Perhaps the greater inertia has the effect of lowering the wobble frequency and/or increasing its amplitude; which could also be expected of a more flexible frame.

[Orin]

I was the lucky bastard that for many years got to take out Serottas that were thought to have wobble issues.

I'd get a ride to the top of some nasty hill (Spier Falls Road most of the time) and take the bike down to see how fast one had to go to get it to wobble. This was fun in that crap your pants kind of way.

I've found that most any bike will wobble if you get the bike going about 30mph, sit bolt upright, and then (this is the fun part...try this at your own risk) take your clenched fist and give the stem right above the head tube a good whack to the side. Hit it hard and watch it go.

Fun with science....I feel like Bill Nye.

Dave

My Rapid Tour took less than 22 mph, didn't need a hill and didn't need the stem whacking... I didn't have to be bolt upright either.

Hands on the bars, knee against the top tube stopped it. But I always felt that it wasn't completely stopped, merely damped. Not a problem going straight, but in a corner...

Yes, I switched wheels back and forth with my early 80s Koga-Miyata Full Pro. The K-M rode straight with no shimmy hands off at the same speed.

There are some bikes that ride 'straight' and some that seem to be doing gentle S curves down the road. Yes, even the 'straight' riding bikes are deviating from a straight line, but not in a regular manner. My feeling is that the bike that is doing the S curves is trying to shimmy, the bars are oscillating slightly, but the oscillation is being damped by hands on the bars. Take your hands off the bars and the amplitude of the oscillation rapidly gets larger and you have shimmy.

Orin.

[atmo]

there's a difference between a bicycle that steers a certain
way at not-so-fast speeds and it's attributable to a low
trail measurement; these can often seem hard to steer
(no hands, or when the front is unweighted) at slow speeds.

i think of speed wobble as that personal jesus moment that
occurs when the bike tells you what it wants to do, and this
would be the case on a downhill more often than not atmo.

[rwkahn3]

I bought my roubaix base model in 04, my first road bike. I have loved it.
Several weeks ago, went on my longest ride, 70 miles. I went down one hill early on in the ride, 38 mph. No problems. Towards the end at around 65 miles in, I was going downhill and BAM! All of sudden front end shimmy. Not knowing anything about this, I grapped as tight as I could. It wouldn't go away. I slowly braked and realized I didn't need to stay clipped in so I unclipped my right foot.
It didn't stop until I stopped the bike. My computer said 41.1 mph! The fastest I have ever gone. I found out later that I had a loose front spoke. No wobble in the wheel though.

My buddies that I ride with suggest I upgrade my bike. And I am now looking at an 06 roubaix pro. Some of your posts are making me nervous about this decision. I like comfort and I am not a racer.

Anyones thoughts would be much appreciated.
Please feel free to email me.
Thanks

[Peter P.]

Ti Design's explanation of shimmy using the MIT group's work was the best explanation I've seen for what I've suspected all along-the frame has a resonant frequency that, if it's associated with a rideable speed, and is excited, will cause shimmy. The idea of the frame design is to put that resonant frequency into a speed range that the rider will not encounter. Of course, component and rider interaction must be taken into account.

And to refute atmo's belief that shimmy can't be induced twice under the same conditions, I tell ya'll this:

I have a commuter bike with a rear rack. I've ridden that bike at all speeds under all conditions and have never experienced shimmy on it.

However, if I put a load on top of the rack of about 5lbs., the bike is GUARANTEED to shimmy at all speeds over about 10mph, and the higher the speed the worse it gets. But I'll only notice it IF I take at least one hand off the bars-I don't dare take boths hands off. This tells me that center of gravity and/or weight distribution is also a factor in shimmy, and by changing some bike dimensions, perhaps shimmy can be eliminated without affecting rider position.

[OldDog]

I see no mention to the effects of an unbalanced wheel, how much does a tube stem (and loss of material in the rim for the stem hole) contribute to wobble? Would not a few grams cause a wheel to begin oscalation at X speed? Could the effect of imbalanced f/r wheels, rotating out of sync, set up the harmonic effects in the frame?

[RPS]

I see no mention to the effects of an unbalanced wheel, how much does a tube stem (and loss of material in the rim for the stem hole) contribute to wobble? Would not a few grams cause a wheel to begin oscalation at X speed? Could the effect of imbalanced f/r wheels, rotating out of sync, set up the harmonic effects in the frame?Not likely IMO because imbalance of this type acts vertically for the most part, which should not affect steering directly. Forces that help twist the frame and other components are more important.

[barry1021]

I was the lucky bastard that for many years got to take out Serottas that were thought to have wobble issues.

I'd get a ride to the top of some nasty hill (Spier Falls Road most of the time) and take the bike down to see how fast one had to go to get it to wobble. This was fun in that crap your pants kind of way.

I've found that most any bike will wobble if you get the bike going about 30mph, sit bolt upright, and then (this is the fun part...try this at your own risk) take your clenched fist and give the stem right above the head tube a good whack to the side. Hit it hard and watch it go.

Fun with science....I feel like Bill Nye.

Dave

To kiss my butt goodbye??

[David Kirk]

To kiss my butt goodbye??

Depends on how quickly you can kiss your butt. Practice makes perfect.

In all seriousness I bet you'd be fine. I DO NOT endorse this course of action but....that said....it's fun to play with.

You need to be a good lose bike handler and you absolutely can not miss hitting the stem. If you swing and miss it has it's own issues.

Try it at low speeds . . ....20mph-ish and go up from there.

Dave

[Erik.Lazdins]

Depends on how quickly you can kiss your butt. Practice makes perfect.

In all seriousness I bet you'd be fine. I DO NOT endorse this course of action but....that said....it's fun to play with.

You need to be a good lose bike handler and you absolutely can not miss hitting the stem. If you swing and miss it has it's own issues.

Try it at low speeds . . ....20mph-ish and go up from there.

Dave

I tried this the day after I read the original post and it worked - thanks! I did not feel that I was about to crash and learned something cool in the process.

[ss-jimbo]

Here's a question. Does anyone know someone who's crashed because of SW? I was sure I was going to crash last year at 6 Gap in GA. I was looking for a reasonable place on the side of the road to ride off into the bushes (rather than a guard rail), but I managed to get it back under control.

This year, btw, on a different bike, I rode down the same hill, much colder and wetter, with no SW.

jimbo

[atmo]

alas -
it comes full circle atmo.

sw is practically a message from above, rather than a poorly
executed detail that somehow crept into an otherwise fine bicycle
design. pilot error? maybe. maybe not. but it's hardly due to a
design element that someone has actually pinpointed atmo.

there is a god.
proof: sinatra and dylan.

[nm87710]

Bobby Z's thoughts on the shimmies...

Mama, put my bikes in the ground
I can't ride them anymore.
That long black cloud is comin' down
I feel like I'm knockin' on heaven's door.

Knock, knock, knockin' on heaven's door
Knock, knock, knockin' on heaven's door
Knock, knock, knockin' on heaven's door
Knock, knock, knockin' on heaven's door

[atmo]

Bobby Z's thoughts on the shimmies...

and the only sound that's left
after the ambulances go
is cinderella sweeping up
on desolation row atmo

[Fivethumbs]

Around 1989 or 1990 I owned a Centurion Dave Scott Iron Man road bike outfitted with Shimano 600 and Araya rims. My friend, who I always road with, had a 1989 Bottecchia Team ADR (which I later took off his hands) made with Columbus SPX tubing, outfitted with full Campy C-Record and Campy wheels.

We used to ride a lot of hills. On the decents my bike would develop speed wobble as soon as I hit 40 mph. The first time it happened my friend was riding next to me and I asked him if his bike was wobbling and it wasn't - his bike was solid as a rock. I slowed down to about 35 mph and he kept accelerating until we met up at the bottom of the hill. This happened to me everytime my bike got up to 40 mph.

I finally asked my friend to let me ride his bike the next time we rode over the hill. On the decent with me on his bike there was not one hint of wobble and I went up to about 48 mph. With him on my bike, he did not experience wobble either. Over the years we switched bikes a few times always with the same result. He told me he read that to eliminate speed wobble the rider should squeeze the top tuble between the legs. I started doing that on my Centurion on the decents and I could feel the bike trying to wobble, but I was able to keep it under control and go as fast as I wanted, though it was more unnerving than riding my friend's bike with no wobble. I don't think I became a better rider the times I rode his bike. Thats my experience with speed wobble.

[soulspinner]

I've been able to initiate speed wobble while descending in a reproduceable fashion on certain frames. Doing so involves getting into a position that no one typically descends in. It's dangerous and I don't recommend it, but it works if you want to see if your bike is prone to shimmy. It's simple. Here's how:

Get yourself going on a long steep downhill section. The smoother and straighter the road, the better. When you reach 30 or 35mph while still accellerating, put all your weight on the saddle, take your hands off the bars and sit upright. Don't pinch the top tube with your knees. Hold it like that and continue to accellerate. If your bike is prone to wobble it will begin here and get progressively worse. Then play a game of chicken with yourself and see how long it takes before fear takes over and you snatch the bars to stop the shakin'.

I'm of the opinion that certain bikes do this more easily than others and bikes that are stiffer torsionally are the least likely to wobble.

I don't think I've ever had a bike oscillate like that when my hands were on the bars.

-K
Had an old Cannondale that I could make shimmy all the time. Subsequent bikes I cant get to do it ever....my Strong with lotsa bottom bracket drop is very stable no hands downhill-dont try this at home...