shimmy or speed wobble at 35 MPH

I experienced the dreaded speed wobble tonite on a 38 MPH descent.
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

On 25 Nov 2003 21:42:23 -0800, (Ken Rousslang)
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.

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
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

wrote in message
...
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


At least no-one has suggested fitting a steering damper yet - unlike most
motorcycles suffering from the same problem!
Hugh Fenton

(Ken Rousslang) wrote in message . com...
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

Do you think it was the new carbon stuff, or is this one of those
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.

Jobst --

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.

wrote in message
...
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
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

On Wed, 26 Nov 2003 06:08:41 GMT,
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.


jeverett3ATearthlinkDOTnet http://home.earthlink.net/~jeverett3

Jobst --

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

"Mike Jacoubowsky" wrote in message
. ..

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.