New shimmy

Discussion in 'Cycling Equipment' started by Paul D., Apr 3, 2003.

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  1. Paul D.

    Paul D. Guest

    I have a Trek 520, and have been riding it 11 years now. It has rather suddenly exhibited a rather
    severe shimmy at around 25 mph. There was no indication of a shimmy problem before the snow, and
    during the snowy weather, I replaced the drive train. Any ideas out there as to what could be
    causing the shimmy? Thanks! -Paul D.
     
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  2. "Paul D." <[email protected]> wrote in message news:<[email protected]>...
    > I have a Trek 520, and have been riding it 11 years now. It has rather suddenly exhibited a rather
    > severe shimmy at around 25 mph. There was no indication of a shimmy problem before the snow, and
    > during the snowy weather, I replaced the drive train. Any ideas out there as to what could be
    > causing the shimmy?

    How much of the drive train was replaced? If you bought new hubs, check that the back wheel is
    dished properly - I've certainly had wobbles caused by incorrectly dished (7sp. when it should've
    been 8sp.!) wheel. Very easy to judge, mind, if you ride a frame with vertical dropouts, as it's
    then near-impossible to fudge the fitting of a badly-dished wheel so that it *appears* to sit
    properly relative to chain & seat stays (where as it's not so obvious with horizontal ends).

    David E. Belcher

    Dept. of Chemistry, University of York
     
  3. Mark Wolfe

    Mark Wolfe Guest

    Lots of things, check the following:

    headset wheels for true/loose spokes

    If a complete mechanical inspection doesn't reveal anything, then just keep your speed above 25,
    your obviously going too slow. :)

    Paul D. wrote:

    > I have a Trek 520, and have been riding it 11 years now. It has rather suddenly exhibited a rather
    > severe shimmy at around 25 mph. There was no indication of a shimmy problem before the snow, and
    > during the snowy weather, I replaced the drive train. Any ideas out there as to what could be
    > causing the shimmy? Thanks! -Paul D.

    --
    Mark Wolfe http://www.wolfenet.org gpg fingerprint = 42B6 EFEB 5414 AA18 01B7 64AC EF46 F7E6 82F6
    8C71 MISTAKES: It Could Be That The Purpose Of Your Life Is Only To Serve As A Warning To Others

    http://www.despair.com
     
  4. Todd Kuzma

    Todd Kuzma Guest

    Mark Wolfe wrote:

    > Lots of things, check the following:
    >
    > headset wheels for true/loose spokes

    Read the FAQ <http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html>:

    Subject: 8h.5 Shimmy or Speed Wobble From: Jobst Brandt <[email protected]>

    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.

    Todd Kuzma Heron Bicycles Tullio's Big Dog Cyclery LaSalle, IL 815-223-1776
    http://www.heronbicycles.com http://www.tullios.com
     
  5. Andymorris

    Andymorris Guest

    Paul D. wrote:
    > I have a Trek 520, and have been riding it 11 years now. It has rather suddenly exhibited a rather
    > severe shimmy at around 25 mph. There was no indication of a shimmy problem before the snow, and
    > during the snowy weather, I replaced the drive train. Any ideas out there as to what could be
    > causing the shimmy? Thanks! -Paul D.

    I'd check your forks for cracks around the fork crown. I has a road bike develop a very floaty front
    end and when I checked a crack had spread almost across one fork blade.

    --
    Andy Morris

    AndyAtJinkasDotFreeserve.Co.UK

    Love this: Put an end to Outlook Express's messy quotes
    http://home.in.tum.de/~jain/software/oe-quotefix/
     
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