Klaas & Maryanne ? writes:
> Can anybody tell me why my silktour cdale goes in to a
> front wheel wobble. It's happens every time I go on a fast
> down hill above 30 mph. It's with rear pannier one side
> 40lbs , bar bag 6lbs and ruff road on the hill. Can
> anybody help.
This subject comes up often so there is an FAQ item:
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Subject: 8h.5 Shimmy or Speed Wobble From: Jobst Brandt
<
[email protected]> 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.
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Jobst Brandt
[email protected]
