Death Wobble
Mossrider
Posts: 226
Recently bought a new Look 566 which has proved an excellent bike. Yesterday i did the Etape du Dales and arrived back still feeling pretty good: except for one thing...
Of all the places to experience a "Death Wobble" the start of the descent off Fleet Moss on is not the one I'd choose. THe handle bars started to shake then rapidly the whole bike started to vibrate. Scared witless (that descent is scary without such a phenonomen), I gripped hard and released the front brake, braking hard with the rear, bringing the bike virtually to a halt. Setting off again it started again, so this time I brought it to a complete halt. The rest of the descent was fine (albeit very cautious!) and all the other fast descents went well.
The initial cause was probably poor surfacing at the top of the road, starting the wobble.
The steering on the bike is much more sensitive than on my old Bianchi. I'll whip it in to the LBS for a quick check over. I know this can happen on any bike, but could there be any other issues. On a relatively new bike I need to rebuild my confidence; can there be anything else wrong other than the road surface?
Apparently the best solution is to grip the cross bar tightly with both knees...alternaivel stand up on the pedals (gripping hard with both knees sounds easier and more natural in the circumstances!)
Of all the places to experience a "Death Wobble" the start of the descent off Fleet Moss on is not the one I'd choose. THe handle bars started to shake then rapidly the whole bike started to vibrate. Scared witless (that descent is scary without such a phenonomen), I gripped hard and released the front brake, braking hard with the rear, bringing the bike virtually to a halt. Setting off again it started again, so this time I brought it to a complete halt. The rest of the descent was fine (albeit very cautious!) and all the other fast descents went well.
The initial cause was probably poor surfacing at the top of the road, starting the wobble.
The steering on the bike is much more sensitive than on my old Bianchi. I'll whip it in to the LBS for a quick check over. I know this can happen on any bike, but could there be any other issues. On a relatively new bike I need to rebuild my confidence; can there be anything else wrong other than the road surface?
Apparently the best solution is to grip the cross bar tightly with both knees...alternaivel stand up on the pedals (gripping hard with both knees sounds easier and more natural in the circumstances!)
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Comments
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I think bristolpete had the same problem with a new bike lately. Might be worth doing a quick search on that since there was some useful tips in the thread.0
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There was also a pretty severe cross wind which apparently has some effect...0
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poor geometry?0
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poor geometry?
It is a Look; who should know a thing or two about fast descents. The wheels it is twinned with are campag neutrons which might give a little flex.0 -
You think you've got problems? In aviation they call it "pilot induced oscillation" and it looks like this:
http://www.alexisparkinn.com/photogallery/Videos/SaabCrash3.mov0 -
Exactly same thing happened to me, in exactly the same spot, however I was chainless after snapping my chain on the last switchback up Fleet Moss. Not sure if having no chain made a difference but I think it was cross wind induced though. Not a nice feeling, unfortunately I did not know how to stop it, just hung on for grim death and braked as hard as I could. Managed to stop totally but was shaking like a sh1ting dog from the experience. Got going again and it happened again, probably due to me clinging on like the world was going to end.0
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wickipedia
Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[2] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, they can be fatal if left uncontrolled.[4]
Since shimmy frequency is independent of bike speed, gyroscopic effects "are clearly not essential to the phenomenon."[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.[3]
An academic paper that investigated wobble through physical experimentation and computer modeling, concludes "the influence on wobble mode of front tyre characteristics, front frame inertia and chassis stiffness were shown. In particular, it shows that increasing front tire inflation, stiffness chassis, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability."[5]
HTH
Lee0 -
I'm surprised that nobody has mentioned the tightness of the QR skewers. I only noticed this since getting carbon forks. On my old steel front forks I'd just tighten the front or back QR willy nilly. However, I've noticed on my carbon fork (and possibly back wheel) it needs to be "just" tight otherwise the brakes go out of sync and a wallow can develop as speed increases.
It might be worth testing different tightnesses coming down the same hill.CAAD9
Kona Jake the Snake
Merlin Malt 40 -
grip TT with knees and relax your grip on the bars...............sorted
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