speed wobble, geometry, and dampening

Discussion in 'Cycling Equipment' started by Invisionsf, Mar 21, 2003.

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  1. Invisionsf

    Invisionsf Guest

    I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
    website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
    one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
    descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.

    The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
    My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is the
    trail calculation?) Will the result be within "normal" for a road bike? And is it possible that this
    change in trail with the added dampening of carbon will change the parameters of this speed wobble?

    Thanks for anyone's insight.

    Clifford Lee
     
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  2. Richard Ney

    Richard Ney Guest

    Invisionsf writes:

    > I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
    > Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
    > no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
    > often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
    > one bike of mine.
    >
    > The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
    > My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
    > the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
    > that this change in trail with the added dampening of carbon will change the parameters of this
    > speed wobble?

    I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
    bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
    practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.
     
  3. Clifford Lee wrote:
    > I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
    > Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
    > no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
    > often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
    > one bike of mine.

    Many cases of shimmy result from insufficient spoke tension in the rear=20 wheel. Because you
    notice it in the front, most people don't think to=20 look at the rear end for the cause, but that
    is very common.

    Sheldon "'Dampening' Can Be Helped By Installing Fenders" Brown
    +----------------------------------------------------+
    | I=92m not convinced that this was the right time | to attack Iraq, but I would like to express
    | my | support for, and gratitude to U.S. and allied | fighting forces, and to wish them all
    | success. |
    +----------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  4. Dave Lehnen

    Dave Lehnen Guest

    Invisionsf wrote:
    > I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
    > Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
    > no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
    > often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
    > one bike of mine.
    >
    > The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
    > My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
    > the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
    > that this change in trail with the added dampening of carbon will change the parameters of this
    > speed wobble?
    >
    > Thanks for anyone's insight.
    >
    > Clifford Lee

    Trail calculation: rad = wheel radius under normal load off = fork offset ("rake" in bikespeak)
    theta = head tube angle from horizontal

    trail = (rad * cos(theta) - off)/sin(theta)

    I don't know your wheel radius, but if it's a 700-23, 333 mm is fairly typical. Your original
    trail is 54.75 mm, after the change it's 56.84 mm. (I know the four-place precision is meaningless
    here) This assumes the geometry of both forks keeps the true head tube angle at 73 degrees. Trail
    increases by slightly over 2 mm. I'll leave it to others to speculate how much this will change
    the wobble.

    Dave Lehnen
     
  5. Belij3

    Belij3 Guest

    My bike has the shimmies at 30 + mph, esp when going downhill. stem has been changed w/no help. Fork
    has been changed w/no help. Rear wheel has been changed and I cannot tell a difference. FWIW I can
    almost repeat the same type of "feel" of the shimmie by moving one side of the handlebar when going
    slow. Kinda difficult to expain, but I "vibrate" my right hand on the hoods and the front end
    shimmies very similar to what it does/feels at high speed. B
     
  6. Tom Paterson

    Tom Paterson Guest

  7. Jobst Brandt

    Jobst Brandt Guest

    Sheldon Brown writes:

    > Many cases of shimmy result from insufficient spoke tension in the rear wheel. Because you
    > notice it in the front, most people don't think to look at the rear end for the cause, but that
    > is very common.

    As long as no spoke becomes slack under wheel loading, spoke tension has no affect on wheel
    elasticity, the modulus of elasticity of steel (spokes) is independent of preload. On the other
    hand, tire inflation has a large effect on lateral elasticity, varying linearly with inflation
    pressure. I don't believe we are talking about a wheels with rattling spokes.

    Jobst Brandt [email protected] Palo Alto CA
     
  8. I observed:

    >>Many cases of shimmy result from insufficient spoke tension in the rear wheel. Because you
    >>notice it in the front, most people don't think to look at the rear end for the cause, but that
    >>is very common.

    Jobst demurred:

    > As long as no spoke becomes slack under wheel loading, spoke tension has no affect on wheel
    > elasticity, the modulus of elasticity of steel (spokes) is independent of preload. On the other
    > hand, tire inflation has a large effect on lateral elasticity, varying linearly with inflation
    > pressure. I don't believe we are talking about a wheels with rattling spokes.

    Perhaps we are. Then again, a highly dished wheel with sub-optimal tension may well have left spokes
    going slack due to dynamic stresses.

    I've seen this problem in practice on customers' bikes, and have corrected it by tensioning and
    truing the offending wheel.

    Sheldon "Empiricist" Brown +----------------------------------------+
    | All theory, dear friend, is grey, | but the golden tree of actual life | springs ever green.
    | --Goethe |
    +----------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
    617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  9. Jay Hill

    Jay Hill Guest

    Invisionsf wrote:
    >the added dampening of carbon will change the parameters of this speed wobble?

    I think you can dismiss this one.
     
  10. Dax

    Dax Guest

    On 21 Mar 2003 22:35:24 GMT, [email protected] (Invisionsf) wrote:

    >I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
    >website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
    >one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
    >descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.
    >
    >The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
    >My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
    >the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
    >that this change in trail with the added dampening of carbon will change the parameters of this
    >speed wobble?
    >
    >Thanks for anyone's insight.
    >
    >Clifford Lee

    check the dish of your front wheel - Ъ×
     
  11. > I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
    > bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
    > practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.

    Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
    have a very significant effect. :>)

    --Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com
     
  12. Jobst Brandt

    Jobst Brandt Guest

    Mike Jacoubowsky writes:

    >> I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
    >> bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
    >> practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.

    > Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
    > have a very significant effect.

    With all the tests we've seen on this subject, wheel balancing, wheel unbalancing, filling the tire
    entirely with water and other things, I don't think there is a clear solution. Greater frame and
    fork stiffness will increase the resonant frequency to a level where it cannot initiate. I have seen
    more effect without changing anything other than inflation pressure.

    My bicycle has always shimmied, if I let it, and it hasn't impaired my riding at any speed. I coast
    down hills no-handed at maximum speed and, when I tuck in, I have both hands on the stem.
    Considering that shopping carts occasionally shimmy, should make apparent that it is an instability
    common to wheels operating with trail, something older cars with king-pin steering knuckles suffered
    from often.

    Jobst Brandt [email protected] Palo Alto CA
     
  13. On Sun, 23 Mar 2003 23:13:57 -0500, Mike Jacoubowsky wrote:

    > Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
    > have a very significant effect. :>)

    However, a shimmy is a resonance response to the bike as a spring-mass system driven (sideways) by
    the rider's balancing motions and the geometry's response. The shimmy frequency is very acutely
    dependent on a whole range of factors, from tire pressure to riding style, rider weight, position,
    etc., etc.

    Changing _anything_ might very well stop a shimmy. Or start it. And if you have the nerve to unload
    the saddle during a shimmy (say a downhill, fast speed shimmy rather than a no-hands slower one), it
    will stop instantly.

    A bad downhill shimmy may feel like you are losing control, but if you get out of the saddle, or
    even ease up that death-grip on the bars, it will go away and not cause you to crash.

    As Jobst suggests, it is not that big a deal. My old road bike did it all the time, the new one,
    not at all.

    If it concern you, then change anything you wanted to replace anyway. It'll probably fix it, and if
    not, then at least you got your new part.

    You might be able to parlay this into a whole new bike if you play it right with your
    significant other.

    --

    David L. Johnson

    __o | "What am I on? I'm on my bike, six hours a day, busting my ass. _`\(,_ | What are you on?"
    --Lance Armstrong (_)/ (_) |
     
  14. Matt Locker

    Matt Locker Guest

    Clifford:

    I believe that increasing trail (reducing rake) can magnify shimmy. The increased trail increases
    the lever arm between the steering axis & the tire contact point. Any force input through the
    contact patch will be transmitted as a greater torque around the steering axis.

    MOO, Matt

    Invisionsf wrote:

    >I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
    >website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
    >one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
    >descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.
    >
    >The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
    >My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
    >the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
    >that this change in trail with the added dampening of carbon will change the parameters of this
    >speed wobble?
    >
    >Thanks for anyone's insight.
    >
    >Clifford Lee
     
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