front wheel wobble

Discussion in 'Cycling Equipment' started by Kma, Mar 16, 2004.

  1. Kma

    Kma Guest

    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 . Klaas
     
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  2. Jobst Brandt

    Jobst Brandt Guest

    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:

    ----------------------------------------------------------
    ----------
    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.

    ------------------------------

    Jobst Brandt [email protected]
     
  3. kma wrote:

    > 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 .

    You can try getting the front luggage lower by using
    lowriders. Eliminating shimmy is a bit trial-and-error, I'm
    afraid. I had a bike that did it at a similar speed, but
    that was years ago. All the ones I have now (even the MTB)
    are good at over 50mph.
     
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