shimmy or speed wobble at 35 MPH
Ken Rousslang
So, I've since read many of the posts on this topic, including those
by Jobst Brandt.
My case. I have a 1984 Centurion Comp T/A that has about 25,000 miles
on it, the first 23,000 with the original Shimano 600EX components,
and SR Laprade seatpost, stem, and handlebars. This was a decent
downtube, friction shifter setup. Never experienced speed wobble with
the original bike. I truly love this bike (I also own a 2002 Klein
Quantum Pro Carbon - with Campagnolo Record group).
The handelbars on my old bike were never wide enough, although I made
do. When I had trouble getting replacement parts, I went to eBay, and
found plenty of NOS 600EX. Then I completely rebuilt the bike with
the original frame, new powder coat, and reproduced the original
decals with the aid of a digital camera and graphics design software.
But I got older, and the same hills forced me to upgrade to a triple
crankset, so I went with a Campagnolo triple 10-speed. The next 1000
miles were a step up compared to the old friction set-up, although I
had no complaints with the older generation components. The new,
wider handlebar on my 2002 Klein made me realize the old SR Laprade
gave me a stiff neck and shoulders on 50 mile rides, so I got got a
new Easton carbon bar and stem, and while I was carried away, I also
got a Easton carbon seatpost.
Now that I'm 300 miles into this new arrangement (stem, bar,
seatpost), I'm really liking the changes. Except for tonight. On a
descent I've done many times previously, heading down a long hill
toward Fox Island, in buffeting strong winds, at 38 MPH or so, bumpy
chip-seal road, the bike started gyrating. I applied the brakes
gently, at first thinking it was my imagination. As I slowed, the
oscillation amplitude grew steadily, until as I slowed to a stop, the
bike was nearly uncontrollable. Because the descent is somewhat bumpy,
I assumed I'd taco'ed the rear wheel, 'cuz I could see that the front
wheel was fine. I spun both front and rear wheel, and, of course,
they were perfectly true. So, cautiously I climbed back on, and rode
up Warren Drive, slowly at first. As I rode, I thought, "maybe the
rear dropout is misaligned, or the skewer is loose." So, I stopped
again, and reset the rear wheel, although it was obviously tight and
centered already.
Rather than turn around and ride home, I kept riding. For the rest of
the ride, another 10 miles or so, I experimented. Even though I
reached up to 40 MPH the symptom did not repeat. I thought, "this is
just like Galloping Gertie." I must have gone into resonance with
just the right wind, frequency of bumps, speed, mass, and position (I
live a few miles from the Narrows Bridge).
Anyway, If you've suffered this far through the post, I'm almost done.
Do you think it was the new carbon stuff, or is this one of those
irreproducible results?
Cheers,
Rousslang
Werehatrack
may have said:
>Anyway, If you've suffered this far through the post, I'm almost done.
>Do you think it was the new carbon stuff, or is this one of those
>irreproducible results?
Maybe the carbon stuff might have something to do with it, but I have
my doubts. The only component that I would expect to have any
significant possibility of being a contributor is the seatpost, but
this is hardly supported by any real evidence. If it doesn't happen
often, it's probably just a confluence of conditions in my opinion.
I would point out that the fact that it occurred during a descent may
be significant; your arms were probably in "stiff mode", which has
been posited to be an aggravating condition in regard to this issue.
If it happens again, and if you can bring yourself to do it, loosening
the grip and making your arms as non-resisting as possible may
help...or it may not.
--
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.
Words processed in a facility that contains nuts.
jobst....@stanfordalumni.org
> Anyway, If you've suffered this far through the post, I'm almost
> done. Do you think it was the new carbon stuff, or is this one of
> those irreproducible results?
It is repeatable, and as you say, you have read the FAQ that has been
updated but not yet posted. Hands-on shimmy is caused by the rider,
nothing else.
======================================================================
Subject: 8h.5 Shimmy or Speed Wobble
From: Jobst Brandt <jbr...@hpl.hp.com>
Date: Mon, 25 June 2003 14:13:14 PDT
Shimmy, a spontaneous steering oscillation of the front wheel, usually
occurs at a predictable speed when riding no-hands. The likelihood of
shimmy is greatest when the only rider-to-bicycle contact is at the
saddle and pedals. This position gives the least damping by hands,
arms, and legs. When shimmy occurs on descents, with hands on the
bars, it is highly disconcerting because the most common rider
response, of gripping the bars firmly, only increases it.
Shimmy is not related to frame alignment or loose bearings, as is
often claimed. Shimmy results from dynamics of front wheel rotation,
mass of the handlebars, elasticity of the frame, and where the rider
contacts the bicycle. Both perfectly aligned bicycles and ones with
wheels out of plane to one another shimmy nearly equally well. It is
as likely with properly adjusted bearings as loose ones. The idea
that shimmy is related to bearing adjustment or alignment has been
established by repetition.
Bicycle shimmy is the lateral oscillation of the head tube about the
road contact point of the front wheel and depends largely on frame
geometry and the elasticity of the top and down tubes. It is driven
by gyroscopic forces of the front wheel, making it largely speed
dependent. It cannot be fixed by adjustments because it is inherent
to the geometry and elasticity of the bicycle frame. The longer the
frame and the higher the saddle, the greater the tendency to shimmy,
other things being equal. Weight distribution also has no effect on
shimmy although where that weight contacts the frame does. Bicycle
shimmy is unchanged when riding no-hands, whether leaning forward or
backward.
Among parameters that supposedly cause shimmy, spoke pattern and
balance had no effect. Tests with wheels balanced and purposely
unbalanced and ones with paired spokes as well as low spoke count
caused no change in shimmy. Filling the front tire with water,
doubling its mass, had no effect other than to change its frequency of
oscillation slightly.
Shimmy requires a spring and a mass about which to oscillate and these
are furnished by the frame and seated rider. Unloading the saddle
(without standing up) will stop shimmy. Pedaling or rough road will
also reduce the tendency to shimmy. In contrast, coasting no-hands
downhill on a smooth road at more than 20mph with the cranks vertical
seems to be the most shimmy prone condition.
When coasting no-hands, laying one leg against the top tube is the
most common way to inhibit shimmy and also one of the most common ways
to coast no-hands. Compliant tread of knobby tires usually have
sufficient squirming damping to suppress shimmy. Weight of the
handlebar and its extension from of the steering axis also affects
shimmy.
Shimmy is caused by the gyroscopic force of the front wheel whose tilt
is roughly at right angles to the steering axis, making the wheel
steer to the left when it leans to the left. This steering action
twists the toptube and downtube, storing energy that both limits
travel and causes a return swing. Trail (caster) of the fork acts on
the wheel to limit these excursions and return them toward center.
Shimmy that concerns riders occurs with the hands firmly on the bars
is rider generated by muscular effect whose natural response is the
same as the shimmy frequency, about that of Human shivering.
Descending in cold weather can be difficult for this reason. The
rider's "death grip" only enhances the incidence of shimmy. Loosely
holding the bars between thumb and forefinger is a way of avoiding
shimmy when cold.
======================================================================
Jobst Brandt
jobst....@stanfordalumni.org
Hugh Fenton
<jobst....@stanfordalumni.org> wrote in message
news:JrXwb.8037$Wy2.1...@typhoon.sonic.net...
> Ken Rousslang writes:
SNIP
> caused no change in shimmy. Filling the front tire with water,
> doubling its mass, had no effect other than to change its frequency of
> oscillation slightly.
As shimmy is obviously some kind of resonant behaviour, could you enlighten
me why doubling the mass would change the frequency "slightly??? Something
to do with effective pendulum arm springs to mind - but Physics was a LONG
time ago....
>
> Shimmy requires a spring and a mass about which to oscillate and these
> are furnished by the frame and seated rider. Unloading the saddle
> (without standing up) will stop shimmy. Pedaling or rough road will
> also reduce the tendency to shimmy. In contrast, coasting no-hands
> downhill on a smooth road at more than 20mph with the cranks vertical
> seems to be the most shimmy prone condition.
>
> When coasting no-hands, laying one leg against the top tube is the
> most common way to inhibit shimmy and also one of the most common ways
> to coast no-hands. Compliant tread of knobby tires usually have
> sufficient squirming damping to suppress shimmy. Weight of the
> handlebar and its extension from of the steering axis also affects
> shimmy.
>
> Shimmy is caused by the gyroscopic force of the front wheel whose tilt
> is roughly at right angles to the steering axis, making the wheel
> steer to the left when it leans to the left. This steering action
> twists the toptube and downtube, storing energy that both limits
> travel and causes a return swing. Trail (caster) of the fork acts on
> the wheel to limit these excursions and return them toward center.
Can you suggest why my shimmy builds to a fixed level and does not
escalate - I can't imagine that the damping effect from my squirming tyres
or the above mentioned leg would be sufficient.
SNIP
> Jobst Brandt
> jobst....@stanfordalumni.org
At least no-one has suggested fitting a steering damper yet - unlike most
motorcycles suffering from the same problem!
Hugh Fenton
David Mackintosh
> Anyway, If you've suffered this far through the post, I'm almost done.
> Do you think it was the new carbon stuff, or is this one of those
> irreproducible results?
I noticed an increased tendency for the death-wobble after switching
to a carbon bar, which is probably half the weight of my old Cinelli.
Maintaining a loose grip and clamping the top tube with your knees
while descending on bumpy surfaces may help.
-David
jim beam
> irreproducible results?
for my money, it'll be reproducable. i doubt the carbon has any
significant effect at these frequencies.
here are my thoughts on using an older style frame with a 10-speed gruppo:
imo, there are two main factors in shimmy; the respective resonances of
the wheels and the frame. resonance is a function of mass and the
elasticity of the system. [and yes, the driver has a role in this, but,
any bike should be rider-neutral in my opinion.]
in your case, the frame is a constant in your equation, so let's look at
the transition from a 25-year old wheelset, with something like a 6 or 7
speed freewheel/cassette, to a modern highly dished campy 10-speed.
my own experience on "tuning out" resonance on a very shimmy-prone bike
was that if i ran a highly dished campy wheel, butted 2.0/1.8/2.0 spokes
it was close to murder descending that thing. but if i ran it with a
slightly less dished shimano wheel with spokes butted on the lhs,
straight gauge 2.0 on the rhs, it was very stable. rims, tires & tire
pressure were the same. and yes, i was running 9-speed so i had
interoperability on the two cassettes but campy 9 & 10 speed wheels are
dished the same.
i chose this arrangement because, [someone feel free to correct my dodgy
math] the ability of a wheel to support lateral loads is a function of
the elasticity of the spokes - thicker is less elastic - and the angle
the dished spokes make with the hub. i believe it's a function of 1/sin
of the angle the spoke makes with the rim relative to the radial axis.
as this angle gets smaller, lateral loading on the spokes tends radial
support loads to infinity so a truly dishless wheel has zero lateral
stability until there is some distortion.
fwiw, a dura-ace hub is 21.1mm center to rh flange, whereas campy record
is 18.4mm. i figure this makes about 10% difference in lateral loading
characteristics between the two, and spoke thickness makes about 23%.
someone check that.
finally, i think it's also worth mentioning that i now have a different
frame with oversize steel tube, and i can run my old wheelset again
without shimmy, but that is not a variable in your case.
Matt O'Toole
If a bike manufacturer employed you to eliminate shimmy from their line of
bikes, how would you go about it? (Beyond telling them to stiffen their
frames.)
Matt O.
<jobst....@stanfordalumni.org> wrote in message
news:JrXwb.8037$Wy2.1...@typhoon.sonic.net...
John Everett
wrote:
>
>Shimmy that concerns riders occurs with the hands firmly on the bars
>is rider generated by muscular effect whose natural response is the
>same as the shimmy frequency, about that of Human shivering.
>Descending in cold weather can be difficult for this reason. The
>rider's "death grip" only enhances the incidence of shimmy. Loosely
>holding the bars between thumb and forefinger is a way of avoiding
>shimmy when cold.
Funny thing, in years of riding I've never experienced shimmy and have
only seen it once. We were descending on Pomorado Road heading north
into Poway early one morning when the bike next to me started
shimmying violently. That was at least ten years ago. I remember my
feet were so cold at the bottom of the descent we stopped so I could
put toe heaters in my shoes. It wasn't until this morning reading
Jobst's post that I made the connection to temperature.
jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3
Mike Jacoubowsky
>
> If a bike manufacturer employed you to eliminate shimmy from their line of
> bikes, how would you go about it? (Beyond telling them to stiffen their
> frames.)
A: What's wrong with stiffening a frame?
B: Would you want to eliminate the possibility of high-speed shimmy at the
expense of handling at normal speeds?
--Mike-- Chain Reaction Bicycles
http://www.ChainReactionBicycles.com
Matt O'Toole
"Mike Jacoubowsky" <mik...@ix.netcom.com> wrote in message
news:4z5xb.466$yx5...@newssvr27.news.prodigy.com...
> > If a bike manufacturer employed you to eliminate shimmy from their line of
> > bikes, how would you go about it? (Beyond telling them to stiffen their
> > frames.)
> A: What's wrong with stiffening a frame?
Nothing, it's just so obvious, and there are other factors too.
> B: Would you want to eliminate the possibility of high-speed shimmy at the
> expense of handling at normal speeds?
I don't believe that's a necessary tradeoff.
Matt O.
Richard Chan
> ...
> my own experience on "tuning out" resonance on a very shimmy-prone bike
> was that if i ran a highly dished campy wheel, butted 2.0/1.8/2.0 spokes
> it was close to murder descending that thing. but if i ran it with a
> slightly less dished shimano wheel with spokes butted on the lhs,
Is this another saying of Shimano as ShimmyNO? Holiday Cheers!
jobst....@stanfordalumni.org
>> caused no change in shimmy. Filling the front tire with water,
>> doubling its mass, had no effect other than to change its frequency
>> of oscillation slightly.
> As shimmy is obviously some kind of resonant behaviour, could you
> enlighten me why doubling the mass would change the frequency
> "slightly??? Something to do with effective pendulum arm springs to
> mind - but Physics was a LONG time ago....
When the oscillating mass increases the gyroscopic forces increase and
the amount of stored energy per half-cycle increases using the same
spring (the same frame) causing a lower frequency. I expected that
but I wasn't sure until I tried it. I have no mathematical model on
which to try these things. Maybe Damon Rinard will put something
together in that direction, it being his job, in a way, to do so. If
I knew more about it, I wouldn't hesitate to write about it.
>> Shimmy requires a spring and a mass about which to oscillate and
>> these are furnished by the frame and seated rider. Unloading the
>> saddle (without standing up) will stop shimmy. Pedaling or rough
>> road will also reduce the tendency to shimmy. In contrast,
>> coasting no-hands downhill on a smooth road at more than 20mph with
>> the cranks vertical seems to be the most shimmy prone condition.
>> When coasting no-hands, laying one leg against the top tube is the
>> most common way to inhibit shimmy and also one of the most common
>> ways to coast no-hands. Compliant tread of knobby tires usually
>> have sufficient squirming damping to suppress shimmy. Weight of
>> the handlebar and its extension from of the steering axis also
>> affects shimmy.
>> Shimmy is caused by the gyroscopic force of the front wheel whose
>> tilt is roughly at right angles to the steering axis, making the
>> wheel steer to the left when it leans to the left. This steering
>> action twists the toptube and downtube, storing energy that both
>> limits travel and causes a return swing. Trail (caster) of the
>> fork acts on the wheel to limit these excursions and return them
>> toward center.
> Can you suggest why my shimmy builds to a fixed level and does not
> escalate - I can't imagine that the damping effect from my squirming
> tyres or the above mentioned leg would be sufficient.
This is not my experience nor that of Damon Rinard, who instrumented a
bicycle with an accelerometer mounted on the side of the head tube.
His graphs grow from a straight line in an expanding envelope of
lateral excursions that get increasingly greater while maintaining
the same frequency. I stop the (no-hands) test when the shimmy
becomes so large that it seems unsafe for the wheel. Bicycle wheels
are notoriously weak laterally, their usual mode of failure.
> At least no-one has suggested fitting a steering damper yet - unlike
> most motorcycles suffering from the same problem!
Of course not. It doesn't occur with hands on the bars unless the
rider induces it and riding no-hands it is easily damped by placing a
leg against the top tube. So who wants to add weighty mechanism to a
bicycle gratuitously.
Jobst Brandt
jobst....@stanfordalumni.org
jobst....@stanfordalumni.org
> If a bike manufacturer employed you to eliminate shimmy from their
> line of bikes, how would you go about it? (Beyond telling them to
> stiffen their frames.)
I don't know. All my bicycles have shimmied on demand and it has
never bothered me because I don't need to let it shimmy. It seems
that in the days of yore, when only young can-do racers rode racing
(light weight) Italian and French frames, it was their business to
take care of such anomalies. We never talked about it.
I had a chance to ride with some of those guys not long ago on a
recreation of a Sunday "Jobst ride" to celebrate Dave Prion's wedding.
We rode up Alpine Rd (Portola Valley), an unpaved road and partially
steep rutted trail to Skyline Blvd. No one complained and most rode
without dismounting up the 35% grade dirt bumps. We crossed skyline
and descended paved but bumpy Alpine Road (west) at speeds that
reassured me I haven't been exaggerating when describing the good old
racing days. We had about 15 or more riders who descended like
motorcycles. We turned off onto Tarwater Trail, an old logging road
with rolling drains (sharp whoop-de-doos), and forest debris. Here
the speed was more like dirt motos than bicycles. Some of these guys
were riding steel frames from the past, others carbon but all on road
bicycles. None had seen this trail before except maybe Tom Ritchey
and none of these riders have ever mentioned shimmy, it being
something you just take care of.
Jobst Brandt
jobst....@stanfordalumni.org
Ken Rousslang
wind, no wobble. No eyeglasses or water-bottle either ;-) too much
of a hurry........ had my LBS check out the bike beforehand anyway.
My favorite mechanic said it was a "straight" bike with no obvious
problems. The owner of the LBS said he sold a nice bike he owned when
it shimmied. Didn't trust it anymore.
This time, at least, I was prepared to hug the top tube with my knee,
or come off the seat slightly, or any of the other suggestions for
nipping wobble in the bud. But no need..........we'll see
Cheers,
Rousslang
Carl Fogel
> Matt O'Toole writes:
>
> > If a bike manufacturer employed you to eliminate shimmy from their
> > line of bikes, how would you go about it? (Beyond telling them to
> > stiffen their frames.)
>
> I don't know. All my bicycles have shimmied on demand and it has
> never bothered me because I don't need to let it shimmy. It seems
> that in the days of yore, when only young can-do racers rode racing
> (light weight) Italian and French frames, it was their business to
> take care of such anomalies. We never talked about it.
[snip]
Dear Jobst,
Here's a really naive question from someone
to whom shimmy on a bicycle is theoretical.
Does frame height have anything to do with it?
I just looked through your Tour of the Alps
photos for something else and kept noticing
that your bike seems to be much taller than
everyone else's bike.
Now I see you saying that all your bikes have
shimmied on demand, that you don't know how
you'd go about eliminating it, and that "we
never talked about it."
Come to think of it, do tandems shimmy?
Carl Fogel
Sergio SERVADIO
> Matt O'Toole writes:
> > If a bike manufacturer employed you to eliminate shimmy from their
> > line of bikes, how would you go about it? (Beyond telling them to
> > stiffen their frames.)
> I don't know. All my bicycles have shimmied on demand and it has
> never bothered me because I don't need to let it shimmy. It seems
> that in the days of yore, when only young can-do racers rode racing
> (light weight) Italian and French frames, it was their business to
> take care of such anomalies. We never talked about it.
Since I am almost Jobst's age, having been riding almost for as long as
he, though surely not nearly the same 'kilometrage', I feel urged to add
my two Euro cents.
I have never experienced shimmy.
Why?
Perhaps, I never dared to go fast enough with city bikes, but not so with
my racing bikes. I am quite used to descend steep roads.
About posture: I never lean my knee on the top tube, I don't think I ever
shiver in cold weather (I have done substantial descents at -8C), I never
cling strongly to the handlebar and I stand on the pedals, alleviating
the weight from the seat, when coasting down.
So, where is the trick?
Sergio
Pisa
jobst....@stanfordalumni.org
> Does frame height have anything to do with it?
Yes. Taller frames have longer top and down tubes and have a longer
lever from road to top tube on which to twist that tube as a spring.
Beyond that, the higher seat makes a more stable anchor for the
action, it also having a larger lever with which to counter the
twisting moment from the front of the bicycle.
> I just looked through your Tour of the Alps photos for something
> else and kept noticing that your bike seems to be much taller than
> everyone else's bike.
> Now I see you saying that all your bikes have shimmied on demand,
> that you don't know how you'd go about eliminating it, and that "we
> never talked about it."
This distracts from the issue and that is that riders who suffer from
this characteristic of the bicycle are not riding tall frames and are
shimmying with hands on the bars. I only take my bicycle as an
example because it falls into the most likely to shimmy
category... and it doesn't.
> Come to think of it, do tandems shimmy?
I suppose not because the front rider is about half way between the
front and rear wheel and therefore acts as a damper.
Jobst Brandt
jobst....@stanfordalumni.org
Sergio SERVADIO
> > Does frame height have anything to do with it?
> Yes. Taller frames have longer top and down tubes and have a longer
> lever from road to top tube on which to twist that tube as a spring.
Admittedly, noone has ever 'computed' shimmy; neither to provoke nor,
rather, to prevent it for sure by judicious design.
Here is a basic question, in fact, that springs to my mind.
Why should speed come into play at all?
Is there a characteristic length built in the road, any road?
Surely not.
'Speed relative to what?, would Galileo ask.
Sergio
Pisa
David Kerber
@servadio.df.unipi.it>, serv...@mail.df.unipi.it says...
> On Thu, 27 Nov 2003 jobst....@stanfordalumni.org wrote:
> > Carl Fogel writes:
> > > Does frame height have anything to do with it?
> > Yes. Taller frames have longer top and down tubes and have a longer
> > lever from road to top tube on which to twist that tube as a spring.
> ........
>
> Admittedly, noone has ever 'computed' shimmy; neither to provoke nor,
> rather, to prevent it for sure by judicious design.
>
> Here is a basic question, in fact, that springs to my mind.
> Why should speed come into play at all?
If any wheel imbalance contributed to it, it would be speed-dependent.
--
Dave Kerber
Fight spam: remove the ns_ from the return address before replying!
REAL programmers write self-modifying code.
Carl Fogel
Dear Sergio,
Eppur si muove.
(I wouldn't dare hazard the Italian for "shimmies"
and "moves" means the same thing.)
Carl Fogel
Sergio SERVADIO
> If any wheel imbalance contributed to it, it would be speed-dependent.
Very good point, indeed.
So, perfectly balanced wheels might be a cure?
Sergio
Pisa
Sergio SERVADIO
Taking up David's quite appropriate hint I sort of computed the
wheel revolution frequency at 60 kms/h.
It is something like 9 wheel-revs/sec.
This is way higher than any resonance frequency I would expect the bike,
or the bike+rider coupled system, to have.
What is missing, then?
Are we at total loss?
Sergio
Pisa
Tim McNamara
Hmm. Now, not being an engineer I am speculating out in left field.
It seems to me that out of balanced wheels, typically being out by
only a few grams, would not offer significant vibrational forces. It
also seems to me that any vibration from out of balance wheels would
be in the plane of the wheels, that is to say, always vertical with
respect to the bike.
Since shimmy is a *lateral* vibration, not a vertical one, I don't see
how an out of balance wheel could contribute. In shimmy, the main
triangle is whipping back and forth- the front wheel is tracking
essentially a straight line on the ground, while above it the steerer
is turning slightly back and forth relative to the frame. It seems
like the natural self-centering tendency of the steering geometry
would contribute: the steering assembly overshoots, recenters and
overshoots, recenters and overshoots, etc. thanks to the spring action
of the frame, anchored by the rider's weight on the saddle.
Would an "indexed" headset contribute to shimmy as the bearing climbs
in and out of the detent? One can make a severely indexed headset
sort-of shimmy by holding the front end of the bike off the ground and
gently nudging the bars- the steering assembly will wobble back and
forth, eventually settling into the "detented" position.
dianne_1234
> Taking up David's quite appropriate hint I sort of computed the
> wheel revolution frequency at 60 kms/h.
> It is something like 9 wheel-revs/sec.
> This is way higher than any resonance frequency I would expect the bike,
> or the bike+rider coupled system, to have.
>
> What is missing, then?
> Are we at total loss?
>
> Sergio
> Pisa
Thanks for doing that math! In my experience, 9 Hz seems to be about
what my bike shimmies at. Is there some way we could measure this
frequency?
jobst....@stanfordalumni.org
>>>> Does frame height have anything to do with it?
>>> Yes. Taller frames have longer top and down tubes and have a
>>> longer lever from road to top tube on which to twist that tube as
>>> a spring.
>> Admittedly, noone has ever 'computed' shimmy; neither to provoke
>> nor, rather, to prevent it for sure by judicious design.
>> Here is a basic question, in fact, that springs to my mind. Why
>> should speed come into play at all?
> If any wheel imbalance contributed to it, it would be
> speed-dependent.
It doesn't. We've been through that at length with purposely
imbalanced and balanced wheels.
By the way, noon is when we eat lunch but tell no one about it.
Jobst Brandt
jobst....@stanfordalumni.org
Rick Onanian
<serv...@mail.df.unipi.it> wrote:
>he, though surely not nearly the same 'kilometrage', I feel urged to add
That abomination is enough reason to convince me to abandon any
attempt to adopt the metric system. Long live mileage!
>Sergio
>Pisa
--
Rick Onanian
Carl Fogel
Dear Jobst,
I can't seem to help myself. What kind of
shimmy (if any) would you expect from an
old-fashioned lady's frame, the kind with
two down-tubes and no top-tube?
That is, would it shimmy without the normal
top-tube? And if so, would pressing a leg
against either of the two ascending tubes
help?
seat handlebars
___ __
| / |
/ \ / / \
/ \ / / \
/ \/ / \_front
/_______\/ wheel
rear pedal
wheel
It's getting close to 6pm, so time for my
breakfast (really).
Carl Fogel
David Kerber
tim...@bitstream.net says...
...
> It seems to me that out of balanced wheels, typically being out by
> only a few grams, would not offer significant vibrational forces. It
I would say it's more than a few grams. On my wheels, it's enough to
consistently turn the wheel so the reflector is down, even against the
drivetrain friction if it's in a high gear when I lift it up.
> also seems to me that any vibration from out of balance wheels would
> be in the plane of the wheels, that is to say, always vertical with
> respect to the bike.
I didn't really think it would contribute significantly to the
shimmying, but mentioned it just in case.
> Since shimmy is a *lateral* vibration, not a vertical one, I don't see
> how an out of balance wheel could contribute. In shimmy, the main
> triangle is whipping back and forth- the front wheel is tracking
> essentially a straight line on the ground, while above it the steerer
> is turning slightly back and forth relative to the frame. It seems
> like the natural self-centering tendency of the steering geometry
> would contribute: the steering assembly overshoots, recenters and
> overshoots, recenters and overshoots, etc. thanks to the spring action
> of the frame, anchored by the rider's weight on the saddle.
That definitely describes the shimmy I had on my old bike when I would
ride fast without hands. At lower speeds or with hands on the bars, it
wouldn't show up. On my new bike, I have never experienced a shimmy,
but the new one is a smaller size, and has larger diameter tubing, which
might make it stiff enough laterally not to shimmy.
....
jim beam
> Why?
it might be a size thing - how big are your frames? my experience of
larger frames is that some of them can be chronic, smaller less so.
> About posture: I never lean my knee on the top tube.
in my experience, this is one of the most sure-fire ways of stopping it,
once it's started.
jb
Tim McNamara
> In article <m21xrs3...@localhost.bitstream.net>,
> tim...@bitstream.net says...
>
> ...
>
>> It seems to me that out of balanced wheels, typically being out by
>> only a few grams, would not offer significant vibrational forces.
>> It
>
> I would say it's more than a few grams. On my wheels, it's enough
> to consistently turn the wheel so the reflector is down, even
> against the drivetrain friction if it's in a high gear when I lift
> it up.
A properly adjusted wheel will center the heavy side down if the
difference is as little as 5 grams. It will find a balance bewteen
two or more heavy points. We're talking, even in the case of your
reflectors, a few grams. Not enough to create a vibrational problem-
the wheel just isn't rotating fast enough for a few grams to make a
difference. There have been many threads on the uselessness of wheel
balancing with bicycles, please Google them and read for yourself.
Balancing is an issue in car wheels, because a car wheel spins much
faster. At 35 mph, a 700C wheel is rotating at 424 rpm; a car wheel
at the same speed is rotating faster as it's smaller in diameter.
Most cars don't experience significant shimmy from an unbalanced wheel
until 60 to 70 mph- few bicycles ever get anywhere near that speed,
the hysterics of Phil Liggett notwithstanding.
>> Since shimmy is a *lateral* vibration, not a vertical one, I don't
>> see how an out of balance wheel could contribute. In shimmy, the
>> main triangle is whipping back and forth- the front wheel is
>> tracking essentially a straight line on the ground, while above it
>> the steerer is turning slightly back and forth relative to the
>> frame. It seems like the natural self-centering tendency of the
>> steering geometry would contribute: the steering assembly
>> overshoots, recenters and overshoots, recenters and overshoots,
>> etc. thanks to the spring action of the frame, anchored by the
>> rider's weight on the saddle.
>
> That definitely describes the shimmy I had on my old bike when I
> would ride fast without hands. At lower speeds or with hands on the
> bars, it wouldn't show up. On my new bike, I have never experienced
> a shimmy, but the new one is a smaller size, and has larger diameter
> tubing, which might make it stiff enough laterally not to shimmy.
I've only once experienced hands-on shimmy, which was descending in a
cross-wind at about 35 mph; I was playing with the Marco Pantani
butt-off-the-back-of-the-saddle style at the time. I've had hands-off
shimmy on many bikes, lighter and "better" quality bikes being
paradoxically more prone to this. The exception is my Rivendell
All-Rounder, which has yet to shimmy on me; my Ritchey road bike,
OTOH, which is several pounds lighter, does shimmy when being ridden
hands off over about 22 or 23 mph.
I still wonder about the effect of mass on the bars- with longer
stems and brifters being the norm, are bikes more or less prone to
shimmy? Or does this make no difference?
David L. Johnson
>> Admittedly, noone has ever 'computed' shimmy; neither to provoke nor,
>> rather, to prevent it for sure by judicious design.
The only way to do that would be to design the rider as well. Don't
forget that much of the "system" that contributes to shimmy is the rider
-- position, grip on the bars, etc.
>>
>> Here is a basic question, in fact, that springs to my mind. Why should
>> speed come into play at all?
The impetus to shimmy -- the input force -- is that of the small
balancing/counterbalancing motions we make in order to stay upright. That
is speed dependent. It is also reinforced by things like the wind, which
is also speed dependent. Whether or not a shimmy occurs depends on many
factors in the "system" -- too many to control, and the response is too
sensitive to small changes of these parameters to reliably control it even
if you were to know and analyze all inputs. That is why almost anything
someone does to the bike will "fix" a shimmy. People say that replacing a
seatpost has cured a shimmy, but that is evidently only a minor player in
any model.
The easiest, surest, simplest way to cure a shimmy is to unload the saddle
if the bike starts to shimmy. It will _always_ stop the shimmy, since the
butt-saddle joint is a major pivot for the side-to-side motions of the
bike. This should be taught in any cycling training class, but seems to
never happen, because folklore like balancing the wheels or replacing the
frame persists. It persists because it "works", but for me, a free,
guaranteed solution beats replacing a perfectly good frame.
To put it bluntly, shimmy is a result of riding technique, not bike
design. Any bike can shimmy, under certain conditions. Any rider
experiencing shimmy can quickly and safely stop it if he/she knows what to
do.
> If any wheel imbalance contributed to it, it would be speed-dependent.
But wheel balance does not produce shimmy, and balancing a wheel will not
stop it.
--
David L. Johnson
__o | As far as the laws of mathematics refer to reality, they are not
_`\(,_ | certain, and as far as they are certain, they do not refer to
(_)/ (_) | reality. -- Albert Einstein
Ken Rousslang
> Ken Rousslang writes:
>
> > Anyway, If you've suffered this far through the post, I'm almost
> > done. Do you think it was the new carbon stuff, or is this one of
> > those irreproducible results?
>
> It is repeatable, and as you say, you have read the FAQ that has been
> nothing else.
The only problem I have with this is: If this hands-on phenomenon is
PURELY RIDER INDUCED, why didn't this show up in the first 23,000
miles with the bike? Seems the probability is quite low for a new
event occurring after such a long time. However, if you mean that
with the slightly changed components the bike is now more susceptible
to shimmy, then justification of your hypothesis would require future
episodes of shimmy. Unless of course shimmy is occasionally a once in
a lifetime phenomenon. And, why is it impossible for the confluence
of wind, speed, and road surface to HELP induce shimmy? I did say the
winds were buffeting, in fact, with gusts nearly blowing me off the
road. Being a physicist (physical chemist) I'm well aware of force
vectors culminating in a resultant force. Angular momentum can be
coupled, wind can produce rim forces, torque, etc., can have
contributing components. Oh, and I was not shivering...........nor
gyrating unusually.
Cheers,
Rousslang
Sergio SERVADIO
> To put it bluntly, shimmy is a result of riding technique, not bike
> design. Any bike can shimmy, under certain conditions. Any rider
> experiencing shimmy can quickly and safely stop it if he/she knows what to
> do.
So, please, forgive the ignorant and enlighten me.
What are the typical characteristic frequencies of the shimmying
normal modes?
Sergio
Pisa
Sergio SERVADIO
> > I have never experienced shimmy.
> > Why?
> it might be a size thing - how big are your frames? my experience of
> larger frames is that some of them can be chronic, smaller less so.
My frames vary from 49x51 to 51x54, and they have never betrayed me.
This morning at 11.45, Colosseo time, on my way from Lucca to Pisa,
I drove by Mario Cipollini descending the gentle slope from Madonna
dell'Acqua.
There he was, as high as the World knows he can raise, riding no-hands,
talking on his cellular phone, in full glory and ... relaxed.
No shimmy.
Perhaps, asking his wife to get the spaghetti ready?
Sergio
Pisa
dianne_1234
David is partly right: I have never found a bike that won't shimmy if
I find for the right (wrong?) conditions.
But my experience is that bike design has a lot to do with it, too. My
more flexible bikes have always been more likely to shimmy than my
stiffer ones.
To answer Sergio: my most recent observation has been that 9 Hz is
about the shimmy frequency. How can we measure this frequency?
jim beam
that's my point. all the people i know who ride smaller frames say it
never happens, and those of us that do [mine's 62x58] have had bikes
where it can be brought on "on demand".
noone would doubt the validity of your experience - they merely seek the
recognition for their own.
jb
Sergio SERVADIO
> To answer Sergio: my most recent observation has been that 9 Hz is
> about the shimmy frequency. How can we measure this frequency?
I would do this.
I would memorize the rhythm and try to sing it, in my mind or aloud
like a percussionist would.
That rhythm, then, should be easy to reproduce.
Then, anyone with some musical education would come up
with the answer.
Sergio
Pisa
Sergio SERVADIO
Recognition granted, from my part.
Then, I would blame the occurrence of shimmy on inadequate scaling up
of the design (tubing) for the larger frames.
As simple as that.
Sergio
Pisa
Tim McNamara
Yes, well, given the contraints resulting from lugged construction on
tubing diameter, the industry never got around to scaling up the
tubes. If one scaled them up, steel frames would have been using
Cannondale-sized tubes for bikes 60 cm and up 50 years ago. The
trend towards "oversize" tubes has reduced shimmy somewhat, IME (I
ride roughly a 62 x 59).
There were some photos of Jobst's current bike on a Web site a yearor
two ago, taken at his "40 Years in the Alps" presentation at Palo Alto
Cycles. It looked like he had down tubes for both the down tube and
top tube- looked like 1 3/8" tubes, maybe even 1 1/4".
dianne_1234
Thank you. That seems like an excellent suggestion.
David L. Johnson
> The only problem I have with this is: If this hands-on phenomenon is
> PURELY RIDER INDUCED, why didn't this show up in the first 23,000 miles
> with the bike? Seems the probability is quite low for a new event
> occurring after such a long time. However, if you mean that with the
> slightly changed components the bike is now more susceptible to shimmy,
> then justification of your hypothesis would require future episodes of
> shimmy.
No. A particular set of circumstances is unique. You can never step in
the same river twice, grasshopper.
> Unless of course shimmy is occasionally a once in a lifetime
> phenomenon. And, why is it impossible for the confluence of wind, speed,
> and road surface to HELP induce shimmy?
It's not impossible. It is a complex system; many factors contribute.
--
David L. Johnson
__o | Enron's slogan: Respect, Communication, Integrity, and
_`\(,_ | Excellence.
(_)/ (_) |
David L. Johnson
> On Sat, 29 Nov 2003 18:58:14 +0100, Sergio SERVADIO
> <serv...@mail.df.unipi.it> wrote:
>
>>On Sat, 29 Nov 2003, David L. Johnson wrote:
>>> To put it bluntly, shimmy is a result of riding technique, not bike
>>> design. Any bike can shimmy, under certain conditions. Any rider
>>> experiencing shimmy can quickly and safely stop it if he/she knows what
>>> to do.
>>
>>So, please, forgive the ignorant and enlighten me. What are the typical
>>characteristic frequencies of the shimmying normal modes?
Who cares? What would that answer? You don't address my point, which is
that, rather than being a damnation against a particular bike, a shimmy is
simply a situtation any rider can encounter. It's how the rider deals
with it that matters.
> But my experience is that bike design has a lot to do with it, too. My
> more flexible bikes have always been more likely to shimmy than my stiffer
> ones.
And some bikes are perhaps more prone to endos than others as well, but if
you see some guy flying over the handlebars, you don't blame the
equipment.
Mike Jacoubowsky
> a lifetime phenomenon.
Count me in that club. One time, back in 1992 on the Steinbeck
Century,descending Los Laureles Grade towards Highway 89 (could be wrong
about the number, but it's the one that goes past Laguna Seca). I'd ridden
countless zillions of miles prior to that, and countless zillions since, but
that was the only time I've experienced something like that. Something so
unusual and scary that I didn't just slow down, but stopped... to check out
my bike and try to get a handle on what had gone one. I went over it
carefully, found nothing, and figured (apparently correctly) that it was
just something about the way I'd configured myself on the descent and
adjusted accordingly. Started back up and had no further problems.
I should add that that was my very first ride (yeah, on a century, no less)
on a brand new bike. Could be that there's a normal, sub-conscious process
of constantly adapting to how a bike handles under various circumstances
that ensures stability... but too many variables at once might temporarily
overwhelm one's ability to deal with it.
--Mike-- Chain Reaction Bicycles
http://www.ChainReactionBicycles.com
"Ken Rousslang" <rous...@harbornet.com> wrote in message
news:35b8e50a.0311...@posting.google.com...
jobst....@stanfordalumni.org
>>>>>> Does frame height have anything to do with it?
>>>>> Yes. Taller frames have longer top and down tubes and have a
>>>>> longer lever from road to top tube on which to twist that tube
>>>>> as a spring.
>>>> Admittedly, noone has ever 'computed' shimmy; neither to provoke
>>>> nor, rather, to prevent it for sure by judicious design.
>>>> Here is a basic question, in fact, that springs to my mind. Why
>>>> should speed come into play at all?
>>> If any wheel imbalance contributed to it, it would be
>>> speed-dependent.
>> It doesn't. We've been through that at length with purposely
>> imbalanced and balanced wheels.
>> By the way, noon is when we eat lunch but tell no one about it.
> I can't seem to help myself. What kind of shimmy (if any) would you
> expect from an old-fashioned lady's frame, the kind with two
> down-tubes and no top-tube?
These tubes can be as rigid as horizontal ones but I suppose they
would shimjy fine in test. As I said, I once rode a "Holland" bicycle
, the long frame, heavy, all black shopping bicycle of northern Europe
and could not pedal it no-hands even at 15-20km/h it shimmied so
badly. You don't hear folks in the flat northland complaining.
> That is, would it shimmy without the normal top-tube? And if so,
> would pressing a leg against either of the two ascending tubes help?
How can you do that. You need to get soft tissue as a damper in there.
My lower legs are shin bones. Hands on the bars is plenty of damping.
> It's getting close to 6pm, so time for my breakfast (really).
After breakfast comes noone and lunch for others!
Jobst Brandt
jobst....@stanfordalumni.org
Hugh Fenton
"Mike Jacoubowsky" <mik...@ix.netcom.com> wrote in message
news:1fgyb.28031$%64.2...@newssvr29.news.prodigy.com...
> > Unless of course shimmy is occasionally a once in
> > a lifetime phenomenon.
>
(yeah, on a century, no less)
> on a brand new bike. Could be that there's a normal, sub-conscious
process
> of constantly adapting to how a bike handles under various circumstances
> that ensures stability... but too many variables at once might temporarily
> overwhelm one's ability to deal with it.
>
> --Mike-- Chain Reaction Bicycles
> http://www.ChainReactionBicycles.com
>
I find the one in a lifetime thing very strange. Shimmy in (my old and
poorly designed - hmmm my bikes Italian - oops) motorcycles normally occurs
as roughly the same speed, can be replicated at any time by starting a
resonance by quickly pushing on the handlebars. Interestingly it never
occurs when accelerating or hard braking - but that might either be because
of the increased loading on the handlebars (damping) or the change in
steering geometry as the suspension works.
For the motorcycle (Moto Morini if you must know) you just crank up the
steering damper another notch
Hugh Fenton