Speed wobble

artika
artika Posts: 46
edited February 2015 in Road general
Terrible speed wobble yesterday going down steep hill that i use all the time, so bit shocked really. Apart from holding too tight, which is not the answer - any ideas as to what I need to check?

Comments

  • munkster
    munkster Posts: 819
    any ideas as to what I need to check?

    Apart from your pants?? :D

    I've never experienced it but have seen the face of someone who has just experienced it at the bottom of a descent and they were ashen and barely able to speak so I can imagine it was no fun. IANAE but personally if I'd experienced it the first thing I would check would be radial trueness of my wheels to make sure they were round but I could be totally wide of the mark there.
  • team47b
    team47b Posts: 6,425
    Assuming you haven't just rebuilt the rear wheel to a point where the wheels don't track exactly the same or you have not changed the headset/forks then speed wobble happens because of balance.

    Too much weight over the saddle (not a comment on your weight :D ) too little on the bars, pedals at quarter to three during descent, this happened to me it was started by a small sudden movement of the bars caused by a change in road surface, moved forward, gripped the top tube with knees, pedals moved to half past twelve, change shorts and all was well, I now cycle differently on this hill.

    Tank slap on a motor bike is still scarier though :shock:
    my isetta is a 300cc bike
  • meanredspider
    meanredspider Posts: 12,337
    I've only had this on a really fast descent (Garmin recorded 58mph) - had I realised how fast I was going, I'd have probably been more worried. My understanding is that an unbalanced wheel/tyre set up a resonance in the frame and that pros damp it down by putting a knee against the crossbar. Balancing your wheels will obviously help.
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • artika
    artika Posts: 46
    I am going to try this descent again today - cant check the wheels are true but will Monday, I will try knees on the frame and taking weight off the saddle and putting it more on pedals and the bars, to see if this changes anything. Anyway Ill be ready this time....
  • markynulty
    markynulty Posts: 409
    Had this a couple of years ago coming down the snake pass at very high speed. Possibly the scariest moment to date. I found that the knee touching the cross bar worked as did loosing the grip on the drops slightly; mentally hard to do that at speed though!
  • MountainMonster
    MountainMonster Posts: 7,423
    markynulty wrote:
    Had this a couple of years ago coming down the snake pass at very high speed. Possibly the scariest moment to date. I found that the knee touching the cross bar worked as did loosing the grip on the drops slightly; mentally hard to do that at speed though!

    The first time I had speed wobble I made it much worse by tightening my grip to try to control things. Only did that once though!
    My London Road
  • andrewjoseph
    andrewjoseph Posts: 2,165
    you could also check wheel bearings F+R, headset bearings/tightness.
    --
    Burls Ti Tourer for Tarmac, Saracen aluminium full suss for trails
  • team47b
    team47b Posts: 6,425
    artika wrote:
    I am going to try this descent again today...

    Most people only want to experience this...once :shock:

    If you want to repeat a speed wobble, and I honestly don't recommend it, descend, then slap the end of the bars, this will start the wobble!

    PLEASE DO NOT TRY THIS TO SEE IF I AM RIGHT!
    my isetta is a 300cc bike
  • tourdriving
    tourdriving Posts: 228
    Check your tires for damages
    https://tourdriving.wordpress.com bike hire and travel links, in 47 countries http://ukbikerentals.com UK Bike & Motorcycle hire in Counties after ABC
  • iron-clover
    iron-clover Posts: 737
    Yup, they certainly are terrifying, mostly because of how dangerous they are.
    The last time I had a proper speed wobble was at some point on my old steel 'touring' bike which would normally suffer when loaded, and could sometimes resonate at speed- I could make the frame flex the headtube fairly easily even as a young beginner and the forks had been bent back slightly from an old crash (if I knew what I do now, I would have condemned the frame immediately, but it was my first real road bike). I think it used to start getting hairy much above 30mph, and be in the danger zone by 40.

    I tend to try and loosten up, look a really long way ahead to make the bike aim that way, and very gently apply the rear brake to reduce the speed. Trying to brake hard isn't a good idea as it throws the weight forward making things worse.

    Since getting my Trek 4 years ago I can't remember ever having a proper wobble again- the frame is much stiffer torsionally which solved the problem, and it does >40 on most hilly rides, and can get to at least 50 without wobbles- at which point I find the tyres and wind buffets are the limiting factors.
  • artika
    artika Posts: 46
    Well I survived the descent, without any wobbling today, shifted my weight to pedals and bars and held the frame with my knees.... Anyway thought you may want to see this, that I found when researching:
    http://youtu.be/xODNzyUbIHo[url][/url]
  • undapressure
    undapressure Posts: 116
    team47b wrote:
    pedals moved to half past twelve

    Unless pedalling when descending, your pedals should be level (quarter to three as you pointed out). The only time you should be in the half past twelve position (unless in some crazy aero position) is when cornering where you drop the outside pedal
  • nweststeyn
    nweststeyn Posts: 1,574
    Had speed wobble once descending Glenshee (too fast). I'm pretty sure it was due to loose front wheel bearings, and was made worse by my panicking and holding tight when it started.

    Definitely thought I was going to die.
  • overlord2
    overlord2 Posts: 339
    I think this is due to imbalanced wheels. Put a bike on a work stand and spin the wheels in the highest gear as fast as you can and see how much the bike shakes. Its not surprising this happens really.
  • ratsbeyfus
    ratsbeyfus Posts: 2,841
    Here's another vid: http://youtu.be/VfngbsIUSj8

    I had one of them red bikes but I don't any more. Sad face.

    @ratsbey
  • meanredspider
    meanredspider Posts: 12,337
    nweststeyn wrote:
    Had speed wobble once descending Glenshee (too fast). I'm pretty sure it was due to loose front wheel bearings, and was made worse by my panicking and holding tight when it started.

    Definitely thought I was going to die.

    That's where I had mine. That's a fast descent
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • telesv650
    telesv650 Posts: 59
    I've had that. Not much you can do about it bar fit a steering damper. It is caused by the natural tendency of the steering to self center due to the gyroscopic couple in the wheels. this can overshoot in certain circumstances and cause a wobble which corrects. In even better circumstances the overshoot can cause another overshoot, and another and another. In motorcycling this can develop into the famous "tank slapper"

    I've had it, the initial wobble caused by my pedaling. the only answer is to hang on and try to slow down.
  • supermurph09
    supermurph09 Posts: 2,471
    There will be many things you can do to stop this, the issue is that when it occurs thinking of them is often the problem! I've had it twice, the worst being on Slack Hill near Matlock, it was pretty terrifying and i've never been that fast again, 40mph is my maximum now, at that speed I feel reasonably happy, any more and the alarm starts going off!!
    Trainer Road Blog: https://hitthesweetspot.home.blog/

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  • jgsi
    jgsi Posts: 5,062
    This all my own work from last run down from Axe Edge and the tops into Leek late 2014.. going past the Roaches... if ever you have done it... you know it gets quick.. quicker... quicker
    http://youtu.be/n9T-FTEzQRU
    The cam is only a cheap one but the wobble is about as pronounced as you can get it at the end.
  • meanredspider
    meanredspider Posts: 12,337
    telesv650 wrote:
    It is caused by the natural tendency of the steering to self center due to the gyroscopic couple in the wheels. .

    Can you explain this some more? Gyroscopic forces tend to resist motion rather than self-centre so that's the bit I don't understand. Grab a bike wheel by its axle and spin it - then try to change its plane of rotation: it resists but doesn't try to return. Any tendency to self-centre is usually related to the caster on the wheel.
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • crikey
    crikey Posts: 362
    It is caused by the natural tendency of the steering to self center due to the gyroscopic couple in the wheels. .

    In reality it's usually caused by the big lump of fat, muscle and bone on the top of the frame. It's to do with gripping the bars, stiffening up as the speed increases and being unable to respond to the movement of the bike. Little wobbles make you grab hold even more and stiffen up even more. Notice in one of the videos that the wobbler is sitting way up on the hoods, wobbles, then carries on sitting up...

    Get down in the drops, use your leg against the top tube, or both knees; the idea is to use your body to damp any resonance rather than wobble like a jelly on a vibrating plate. It also tend to happen on long straights; cornering stops it, bunny hopping the bike stops it too.

    It's all about the rider rather than the bike.

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