From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <n3Ua8.20238$TI3.189780@typhoon.sonic.net>
Date: Thu, 14 Feb 2002 19:17:39 GMT

Thomas Harrigan writes:

> Are you saying that you had one of those violent speed-wobbles where
> you can barely hang onto your handlebars and simply stood up off your
> saddle and the shimmy stopped?

Don't make it sound so dramatic.  All you need to do is unload the
saddle.  That's far from standing up.  Without the rider mass on the
bicycle saddle, shimmy cannot resonate.

> I've had speed wobbles so violent that I wasn't sure whether I was
> standing or not.  I've also had speed wobbles where I've stood up
> only to cause my rear wheel to skip from side to side.  I've never
> had a wobble that simply disappeared the instant I stood up.  The
> only thing that really works is to slow down.

You'll have to try it.  Your tales of wheels skipping from side to
side make your accounts of shimmy to sound like imagined events.
There are many riders who have experienced shimmy and it isn't the way
you tell it.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <xHcc8.1411$hb6.9439@typhoon.sonic.net>
Date: Mon, 18 Feb 2002 19:34:53 GMT

Thomas Harrigan writes:

>> I have not found a bicycle that doesn't shimmy if I coast no-hands
>> down a hill with increasing speed.  At some point, less than 40mph,
>> I have experienced shimmy.  I have not tried this on frames smaller
>> than 23".  I also believe that removing the mass of stem and
>> handlebar, shimmy would be rare because the only mass to induce
>> steering would be too small to cause noticeable excursions.

> Rare?  What do you mean by rare?  Am I to assume that the mass of
> the handlebars and stem is an order of magnitude more important than
> the mass of the wheel and forks?

Rare only because I don't believe it cannot occur.  Trail and fork
flexibility could be such that in spite of no mass at the handlebar,
it could shimmy.  However, for the average road bicycle, I think it
would not occur.

My bicycle probably shimmies at a lower frequency and speed than most,
because it is tall.  I use a 120mm stem and old steel Cinelli bars.
The speed is between 23 and 28 mph and I never gave it a thought, all
my previous bicycles having had the characteristic.

>> The FAQ item does not exclude hands-on shimmy, it even mentions
>> when shimmy is unavoidable at any speed with hands on, even on a
>> bicycle that has no propensity to shimmy.

> I've read the FAQ a few times.  The emphasis has always seemed to me
> to be towards hands-off shimmy.  There are also one or two
> observations with regard to hands-on speed-wobble that I and others
> have experienced which don't totally concur with the FAQ.  I put
> this down to the FAQ concentrating on the hands-off phenomenon.

The term shimmy describes both phenomena.  There is no difference
between it and speed wobble, semantically or otherwise.  Hands off is
mainly a diagnostic of whether the bicycle has the tendency without
rider input.  I think any bicycle can be made to shimmy with shivering
like input.  This has always been a problem when descending in cold
weather.  That is where I experienced the induced hands-on shimmy.
When this occurs, I realize it's me not the bicycle and I make sure to
flex my muscles to avoid oscillation while keeping my leg against the
top tube.

> If they are one and the same thing, then:

> 1) Why does hands-on speed-wobble invariably happen on descents?
> This has nothing to do with speed as I and many others have ridden
> at similar speeds on the flat with no problems on the same bike.

That is where the bicycle rolls undisturbed by pedaling side forces
and no forceful input from the hands.  The hands add mass to the
handlebars and this increases the shimmy moment.  Once it starts, it
picks up sympathetic input from the riders arms, this coincidentally
being a natural frequency for involuntary contractions.  I also think
it is less common in warm or hot weather but that is only a guess
based on my inverse experience.

> 2) Why is hands-on speed-wobble so sensitive to weight distribution?
> Hands on the drops tends to make matters worse.

That seems logical because the mass of the hands is farther forward,
enhancing the mass of the bars while grasping them more tightly.

> 3) Why does descending on rough roads make hands-on speed-wobble
> more likely?  My personal experience on this is that rough roads can
> cause speed-wobble.

My experience is that no-hands it is less with road roughness just as
it is with no-hands forceful pedaling.  That a bumpy road makes it
occur with hands on goes back to the previous explanation in that it
excites natural involuntary muscular response.

> Funny you should mention Cannondales. I had my worst ever speed-wobble
> on a 2.8i multisport frame. I changed the entire front-end, shortened
> the stem, removed the STIs in favour of down-tube levers, and it would
> still try to throw me off when the conditions were right. I have an
> observation to make with regard to frame stiffness - when a stiffer
> frame goes, the wobble is much more violent, and the onset much
> quicker.

This is interesting because it narrows the culprit to the forks and
wheel.  I haven't had that opportunity and that's why I proposed it.
In any case, there is a spring mass system at work here together with
gyroscopic forces.  Old cars did it and I think their spring was
mainly the tires.  Cars have vertical steering axes and almost no
trail.

>>> There may not be such a thing as a bike that "can not" wobble, but
>>> there may be such a thing as a bike that will not wobble under
>>> reasonable conditions.  There certainly should be!

>> I think that is where most sports and racing bicycles are.

> If that were true, then why do we continue to get periodical
> postings on this matter?

I think that is primarily because some riders are less attuned to the
remedies and are caught of guard when it occurs.  As I said, I have
always had a shimmying bicycle and it has never bothered me because I
know how to stop it and if I can't it's probably because I am cold.

>> My bicycle has served me well for many years and it will shimmy
>> coasting no-hands and when I'm cold to the extent of shivering.

> My experience of hands-on speed wobble and the experience of others is
> that it is a much more violent phenomenon than this.

It all depends on how much you let it get out of control.  I am
certain that it would go away if you unloaded the saddle or laid you
leg against the top tube hear the head tube.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <qhfc8.1554$hb6.9624@typhoon.sonic.net>
Date: Mon, 18 Feb 2002 22:31:50 GMT

Theodore Heise writes:

>> It all depends on how much you let it get out of control.  I am
>> certain that it would go away if you unloaded the saddle or laid
>> you leg against the top tube hear the head tube.

> I had an extremely unsettling experience a few years ago.  My
> bicycle is a 60 cm Waterford 2200 with a 120 cm stem, Cinelli bars,
> 32 spoke Mavic Open Pro wheels, and Conti 23 mm tires.  I'm 6'1" and
> weighed about 190 lbs at the time.

> I was descending a pass in the Rockies and at just over 40 mph the
> shaking started.  Thomas's description of the bike trying to "throw
> him off" was apt.  A rider following behind told me he was surprised
> I kept the bike upright.  The onset was very sudden, I don't recall
> any warning signs.  It was most definitely violent--my thought was I
> had blown out the tire.  I don't recall it being cold that day, but
> it was high enough elevation that it probably was.

Although my bicycle will always shimmy when coasting on a smooth road
in the critical speed (no-hands), I achieve my maximum speeds,
essentially no-hands in a crouch with my hands on the stem.  This is
the most stable condition I can imagine.  I suspect that if I were to
ride next to someone experiencing shimmy, I believe I would see a
different scenario than what the rider gives after the experience.

> The thing I find curious is that I have descended with that bike
> many times at speeds well over 50 mph with no problems whatsoever.
> Many of those times it was cold, too.  It was a bit gusty though, on
> the day in question.

I don't have the terrain to reach 50mph without a good tailwind, one
that is not also a cross wind due to turbulence.  Most steep roads I
encounter are too crooked, and the straight ones aren't steep enough.
However, your bicycle makes it spooky, not knowing when it might
occur.  I think getting to the bottom of the phenomenon and getting it
under control is important.  As has been brought out in this thread,
most riders seem not to have the problem.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <NbTc8.2444$hb6.15436@typhoon.sonic.net>
Date: Wed, 20 Feb 2002 19:56:29 GMT

Thomas Harrigan writes:

> Jobst says he has never experienced nor seen the type of violent
> hands-on-out-of-control-speed-wobble that I and several others
> report.  Almost to a man, everyone I have personally spoken to who
> has experienced this is 6'4"+, rides 60+(C-C)cm frames, and of a
> heavier build than your classic cyclist. My first experience
> occurred when I was about 250lbs at 70kph.

I think I fit the description and I have descended many hills, some at
unusually high speeds on occasion and this is not a problem or I
wouldn't go fast.  My bicycle shimmies every time I let it do so at
the same speed.

http://www-math.science.unitn.it/Bike/Countries/Europe/Tour_Reports/Tour_of_the_Al

The cornering picture is at about 40mph.  Note that there is no
significant steering angle, indication that this is a fairly broad
turn.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <ly%e8.340$en5.2765@typhoon.sonic.net>
Date: Wed, 27 Feb 2002 07:04:49 GMT

Tim McNamara writes:

>> I can't visualize where a rider would even consider yanking the
>> bars to one side at the rate of a shimmy, much less be capable of
>> such a move.

> I think what I was wondering about is what type of response the
> bicycle makes.  I certainly can understand that human reaction times
> are slower than the "event time" of hitting a bump.

I mean acceleration of shimmy steering excursions and velocity are
greater than those produced by a rider.  Besides, hitting a bump does
not generate steering motions, so that has no effect.

>> If you watch the head tube in a shimmy while riding no-hands,
>> you'll see that the top of the stem makes the largest excursions as
>> the tire follows a nearly straight path.  The brake levers also
>> remain nearly at a node.  I believe this is the essence of the
>> effect because that leans the wheel from side to side and steers it
>> from gyroscopic forces.

> I'd noticed the movement of the stem and the fact that the tire goes
> in a straight line with minor (if any) excursion.  The tire has to
> do that or the rider would probably crash when riding no-handed and
> shimmying.  So in retrospect the movement you'd be able to trace
> from above (say, the path of the binder bolt head) would be a sine
> wave intersecting the almost straight path of the tire?

I performed an experiment today filling my front tire with 930g of
water, making my 1370g wheel weight 2300g, a substantial increase in
peripheral weight.  I made sure there was no air in the tire by
releasing air/water with the valve stem at the top.

The result was that spontaneous shimmy did not occur when coasting
no-hands between 30 and 20mph, the normal shimmy range being 23-27mph.
I tried this with pedals horizontal and vertical and noted that
slapping the top of the stem sideways induced shimmy at 20mph that
died down in 2-3seconds but had the usual excursion but at a lightly
lower frequency.  At higher speeds, shimmy would die out in less than
a second by the same method but also produced a smaller amplitude.

> Now, the brake levers is an interesting thing that I had not
> noticed, and now I'll have to go out and make some bikes shimmy to
> watch.  I've never ridden a bike with no handlebars- can it be
> steered no-handed in the usual manner or is the mass of the
> bars/stem important?

That's my next experiment, that of weighting the brake lever hoods.  I
had the pleasure of riding a bicycle without a stem when an aluminum
stem binder bolt broke.  The bars rotated freely from side to side so
I turned them straight and rode no-hands, as it were, to the bicycle
shop.

>> That's true but from the feel of it and that it builds up, it seems
>> to be storing more and more energy somewhat like a pendulum.

> So is this energy stored primarily in the flex of the main tubes of
> the bike frame, the fork blades, both?

I'm not sure but forks, wheel and tire seem more likely to me, if the
testimony of the Cannondale rider is accurate.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Severe speed "wobble"
Newsgroups: rec.bicycles.tech
Message-ID: <Wbzf8.851$en5.6526@typhoon.sonic.net>
Date: Thu, 28 Feb 2002 23:38:30 GMT

More experimentation with shimmy.

With a favorable breeze I coasted down a local expressway that has a
smooth broad shoulder, reaching a little more then 30mph and developed
self generating shimmy at 30mph using the heavier wheel.  The shimmy
frequency was lower (~30%) than with the normal wheel and this made it
feel less threatening than the customary one that occurs at higher
frequency.  No changes other than filling the front tire with 920gm of
water were made to the bicycle that normally shimmies no-hands between
23 and 27mph.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: High-speed shimmy, Speed wobble
Newsgroups: rec.bicycles.tech
Message-ID: <HKseb.26353$dk4.808859@typhoon.sonic.net>
Date: Wed, 01 Oct 2003 04:29:27 GMT

David Mackintosh writes:

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

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

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

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

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: High-speed shimmy, Speed wobble
Newsgroups: rec.bicycles.tech
Message-ID: <0Vlfb.27193$dk4.820414@typhoon.sonic.net>
Date: Fri, 03 Oct 2003 21:31:40 GMT

Bill Bushnell writes:

(2nd edition)

>> The big-frame guys often find their shimmy issues solved, or at
>> least greatly reduced, by going to wider tires.

> Interesting.

> On my 62cm (lugged steel) frame road bike I find low- to
> moderate-speed "no hands" shimmy is greatly reduced by using
> _smaller_, narrower tires (Continental 25mm, vs. Avocet Duro 35mm),
> although I always experienced enough shimmy at speeds >15 mph that I
> never felt comfortable on any of my road bikes riding no hands with
> seated, upright torso for more than a few seconds. In all cases I
> found that leaning forward and/or putting more weight on the front
> wheel, such as putting both hands on or adjacent to the stem, helped
> reduce the shimmy.

To which I can also say "interesting" because that is one of the tests
I performed.  While the bicycle is shimmying (riding no-hands) I
leaned way back, and then leaned forward with my shoulders over the
bars, holding onto the rear of the saddle to keep from sliding off.
Neither position altered shimmy and I believe because the rider's mass
serves only to anchor the saddle so that the shimmy can twist the
frame and bend the fork in its excursions about the rider mass and
rear wheel contact.  I suspect a nearly flat rear tire might have the
biggest effect on this.  I'll have to try that today.

As you may recall from the FAQ, unloading the saddle is one of the
surest ways to stop shimmy because the rider's mass on the saddle is
the anchor for the oscillations.  Without that the frame tubes are
infinitely stiff against the remaining masses involved.  You may have
lightened your contact with the saddle.

I think you'll find that the course the front wheel traces while
shimmying is nearly a straight line.  Looking over the fork at the
tire contact point, only the top of the head tube (and bars) seem to
be moving from side to side as the wheel steers gyroscopically from
left to right, the trail of the wheel limiting its stroke.  I would
like to see a slow motion movie of this event because it is just fast
enough to be seen but not scrutinized while riding.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: shimmy or speed wobble at 35 MPH
Newsgroups: rec.bicycles.tech
Message-ID: <JrXwb.8037$Wy2.148641@typhoon.sonic.net>
Date: Wed, 26 Nov 2003 06:08:41 GMT

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
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 <jbrandt@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.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: shimmy or speed wobble at 35 MPH
Newsgroups: rec.bicycles.tech
Message-ID: <q_dxb.8128$Wy2.152770@typhoon.sonic.net>
Date: Thu, 27 Nov 2003 03:14:30 GMT

Hugh Fenton writes:

>> 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.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: shimmy or speed wobble at 35 MPH
Newsgroups: rec.bicycles.tech
Message-ID: <%iexb.8129$Wy2.152840@typhoon.sonic.net>
Date: Thu, 27 Nov 2003 03:36:27 GMT

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.

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.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: shimmy or speed wobble at 35 MPH
Newsgroups: rec.bicycles.tech
Message-ID: <xVrxb.8182$Wy2.155050@typhoon.sonic.net>
Date: Thu, 27 Nov 2003 19:05:01 GMT

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.
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.brandt@stanfordalumni.org