Wobble at Spped !

datpat64
datpat64 Posts: 85
edited June 2010 in Road beginners
Wonder if anyone can suggest a fix for this ?

I've had a Specialized Tarmac Expert for about 6 months (got it second hand from a quality bike shop) but have only recently started getting out in the Peak as the weather has got better.

Got a frightening front wheel wobble coming down the Cat and Fiddle about five weeks ago (not the place for a front wheel wobble) but assumed I'd just not tightened front wheel properly. Took wheel off, re-seated and seemed OK (although I got the wobble on fastest part and didn' travel that fast again).

Got another wobble round about 35 mph last week coming down into Ashbourne, which nearly caused me to come off. Wheel is straight, spokes are tight and fitting is fine. Could it be the hub ? Could it be the headset ? Onyl happens at very high speed but last ine really quite scared me!

Any advice appreciated !

Comments

  • saltpeter
    saltpeter Posts: 1
    Having had two frightening instances of high speed wheel wobble recently, firstly on my Trek 5500 then yesterday on my Giant TCR Advanced, Zero I felt the need to join the debate on wheel balancing. This is frustrating for a good descender. The Ksyrium SL rear wheel on my Trek was very worn on the rim so I condemned it. The DT RR1.1 rear wheel on my Giant was only slightly worn with no bearing play and running true in both planes, but it would always stop at the same point when spun in the truing jig. The tyre is new and also running true. I counterbalanced this by adding weight to one spoke on the opposite side until the wheel stopped at random points but when spinning the wheel at high speed on the bike in the repair stand the wobble was every bit as bad as before. So I removed the single weight and replaced it with several pieces of lead wire wrapped around 5 adjacent spokes until static balance was achieved. Now spinning very fast on the stand produces very little wobble but of course a road test is the final proof. I've read all the arguments about headsets, crowns, cracks, frame design road surface, tyre distortion, self-centring effect but answer me this: how else, other than wheel misbalance, can a bike wobble [badly] in a repair stand when the only thing moving is the wheel? It's clear that this MUST contribute to wheel wobble whatever the pundits might say. I realise that different tyres, or tyre wear, can put a wheel out of balance and race-service mechanics can't spend time balancing each time they change a tyre but that doesn't mean it's not the right thing to do. I've seen cases of wheel wobble in top races so it happens. I also think there's a case for dynamic wheel balancing for bicycles.
  • balthazar
    balthazar Posts: 1,565
    saltpeter wrote:
    It's clear that this MUST contribute to wheel wobble whatever the pundits might say. I realise that different tyres, or tyre wear, can put a wheel out of balance and race-service mechanics can't spend time balancing each time they change a tyre but that doesn't mean it's not the right thing to do. I've seen cases of wheel wobble in top races so it happens. I also think there's a case for dynamic wheel balancing for bicycles.
    Wheel balancing has no consequence for bicycles. If it did, then those riding unbalanced wheels (nearly all of us) would experience speed wobble whilst those few who bothered to tape weights here and there would not.

    I think this is the definitive article on bicycle shimmy (which is why I cite it in every related thread here):

    http://www.sheldonbrown.com/brandt/shimmy.html
  • sungod
    sungod Posts: 16,863
    if it's traditional wobble then the bike could be perfect, but you'll still get wobble

    the bike+rider is a dynamic system, at some point it can resonate

    try pushing your knee against the top tube on descents, the extra damping should stop the resonance

    don't just try gripping the bars harder, it'll probably make it worse, relaxed grip is better
    my bike - faster than god's and twice as shiny
  • Hals1967
    Hals1967 Posts: 231
    sungod wrote:
    if it's traditional wobble then the bike could be perfect, but you'll still get wobble

    the bike+rider is a dynamic system, at some point it can resonate

    try pushing your knee against the top tube on descents, the extra damping should stop the resonance

    don't just try gripping the bars harder, it'll probably make it worse, relaxed grip is better

    +1

    I do a regular descent of 40 - 42mph and don't have any wobble or shimmy whatsoever. Relaxed on the bike and slightly raised off saddle to minimse and help damp out any "bounce" in the road. I've done the very same descent on a different bike with the same results. IMHO a lot is down to technique with the caveat you might just luck out and have a bike that's prone to wobble at a certain speed. Natural resonation (if that's the right term ?) is definately a factor so cannot be discounted either.


    1967 Engine
  • datpat64
    datpat64 Posts: 85
    Some good stuff on here and many thanks to all of you who took the time to post ! I'm actually hoping that it's my tecnique rather then the bike (once again haveing checked all componenets everything seems fine).

    So fingers crossed chaps, I'm off up Cat and Fiddle to see if the various advice around slight off setting and having knee in contact with top tube helps.....if not, it's a long way down ! :x
  • Mossrider
    Mossrider Posts: 226
    I had the same a few weeks ago on Fleet Moss, but no repeat. I've been riding for quite a few years and never experienced this on my older bikes. It will simply be the mix of bike geometry, your stance, road surface and wind conditions all combining to give you the fright of your life at a particular combination. I've not had a repeat in the last few weeks, though I am a tad wary and tend to rest my knee on the cross bar if in doubt.