Hier hab ich die bisher beste Info dazu gefunden, einiges ist zwar fahrradspezifisch, aber vieles passt, langer Rahmen, superdünne gut walkende Reifen mit hohem Druck, ängstlich den Lenker fest umklammern (und dabei das Gefühl haben, es wird noch schlimmer und man muss noch fester klammern...), etc.
Hab das mir am interessanteste Erscheindende mal fett gemacht:
Aus Newsgroup rec.bicycles.tech:
Subject: 8h.5 Shimmy or Speed Wobble
From: Jobst Brandt <
jobst.brandt@stanfordalumni.org>
Shimmy is not related to frame alignment or loose bearings as is often
suggested. Shimmy arises from the dynamics of forward motion and the
elasticity of the frame, fork, and wheels, and the saddle position.
Both perfectly aligned bicycles and ones with wheels out of plane to
one another shimmy nearly equally well. The same is true for bearing
adjustment. In fact shimmy is more likely with properly adjusted
bearings than loose ones. The bearing or alignment concept is usually
offered as a cause of shimmy and each airing perpetuates the idea.
Shimmy, the lateral oscillation at the head tube, depends primarily on
the frame and its geometry. The inflation of the tire and the
gyroscopic effects of the front wheel make it largely speed dependent.
It cannot be fixed by adjustments because it is inherent to the
geometry and elasticity of the components.
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.
In contrast to common knowledge,
a well aligned frame shimmies more
easily than a crooked one because it rides straight and without bias.
The bias force of a crooked frame impedes shimmy slightly. Because
many riders never ride no-hands downhill, or at least not in the
critical speed range, they seldom encounter shimmy. When it occurs
with the hands on the bars it is unusual and especially disconcerting.
There is a preferred speed at which shimmy initiates when coasting
no-hands on a smooth road and it should occur every time when in that
critical speed range. Although it usually does not initiate at higher
speed, it can.
Pedaling or rough road interferes with shimmy on a bicycle that isn't
highly susceptible. When coasting,
laying one leg against the top
tube is the most common way to inhibit it. Interestingly, compliant
tread of
knobby tires give such high lateral damping that most
bicycles equipped with knobbies do not shimmy.
Shimmy is caused by the gyroscopic force of the front wheel that acts
at 90 degrees to the axis of the steering motion. The wheel steers to
the left about a vertical axis when it is leaned to the left about a
horizontal axis. When the wheel leans to the one side, gyroscopic
force steers it toward that side, however, the steering action
immediately reverses the lean of the wheel as the tire contact point
acts on the trail of the fork caster to reverse the steering motion.
The shimmy oscillates at a rate that the rider's mass on the saddle
cannot follow, causing the top and down tubes to act as springs that
store the energy that initiates the return swing. The shimmy will
stop if the rider unloads the saddle, because the mass of the rider is
the anchor about which the oscillation operates. Without this anchor
no energy is stored. The fork and wheels may store some energy,
although it appears the frame acts as the principal spring.
Shimmy can also be initiated with the hands firmly on the bars by
shivering, typically in cold weather. The frequency of human
shivering is about the same as that of a typical bicycle frame.
Und nochmal ähnlich vom gleichen Autor:
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.