Bike trail - how critical is it ?

somerandombloke
somerandombloke Posts: 3
edited November 2008 in Commuting chat
I started cycling to work last summer. Usually I follow a flat course but live in a hilly area so sometimes take in a few hills that are 10-15% gradient.

When I started I was persuaded that a Giant would be a good starting point, but did not get on with it - huge high speed "shimmies" 3-4 separate occasions on downhills. I then changed to a Colnago Primavera which feels better planted generally. All has been great for several months until last week.
On a steep downhill section strewn with gravel and with a Transit van behind me I experienced a huge shimmy at around 30mph.
I used to ride a race bike all the time when younger and never had this before, so can't believe it is all down to cruddy technique.

Is shimmy due to tensing up ("death grip" on the bars) or is it purely a function of geometry / trail ??
- I am trying to understand whether I would be better off with daily Vallium or a replacement frame ! :lol:

Comments

  • always_tyred
    always_tyred Posts: 4,965
    It can be any number of things. I doubt its anything inherent to the bikes you've tried though.

    Most bikes can be induced to shimmy, although 30mph isn't very fast, I'll grant you. The best thing to do for any shimmying situation is to brace the top tube with one or both knees to damp the oscillations before they become trouser filling.
  • salsarider79
    salsarider79 Posts: 828
    Also check that everythings tight. Sounds stupid and simple, but start with that. :wink:
    jedster wrote:
    Just off to contemplate my own mortality and inevitable descent into decrepedness.
    FCN 3 or 4 on road depending on clothing
    FCN 8 off road because I'm too old to go racing around.
  • ansbaradigeidfran
    ansbaradigeidfran Posts: 526
    I know from looking at it that it's a highly technical term, but what exactly is shimmy?
  • biondino
    biondino Posts: 5,990
    I've only experienced it on a poor road surface at approx 39mph. Since I have been as fast and faster on the bike on other surfaces without shimmying I suspect that had a lot to do with it.
    My Focus / My Bob Jackson
    Blog (incl. bikes)
  • jonginge
    jonginge Posts: 5,945
    ansbaradigeidfran wrote:
    I know from looking at it that it's a highly technical term, but what exactly is shimmy?
    Over to sheldon:
    http://www.sheldonbrown.com/brandt/shimmy.html
    FCN 2-4 "Shut up legs", Jens Voigt
    Planet-x Scott
    Rides
  • ansbaradigeidfran
    ansbaradigeidfran Posts: 526
    Ta. :) I know that kind of behaviour by another name, but I can't recall what...
  • spursn17
    spursn17 Posts: 284
    In motorcycle parlance = Tankslapper
  • somerandombloke
    somerandombloke Posts: 3
    salsarider79 wrote:
    Also check that everythings tight. Sounds stupid and simple, but start with that. :wink:

    Thanks for that, it was the next step for a free evening.
    Tore the whole bike apart last night and found several things (wheel run-outs of, spokes tensions all over the place, bottom headset bearing dry, crank bolts and bearing cups loose, etc :roll: ) - all put right now.
    Hopefully one or more of these will have it sorted .

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