Jobst Brandt will disagree:
none of the items listed cause shimmy. We have had discussions in this newsgroup at length and theories tested, none of which had any effect on shimmy.
All of the following is irrelevant as the claim of Rolf Dietrich that the wheels prevent shimmy. That claim thoroughly failed a reality test by independent testers.
I think you should read the FAQ before hypothesizing on causes of shimmy:
http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html (update no yet posted)
# Subject: 8h.5 Shimmy or Speed Wobble
# From: Jobst Brandt <jbrandt@hpl.hp.com>
# Date: Mon, 25 June 2004 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.
# The shimmy that concerns riders the most occurs with hands firmly on
# the bars. This 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.
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