Ok this has been the second time I have had the wobbles while descending it is bloody scary to say the least, life and death would not be an understatement.

What am I doing wrong and what actions have you undertaken to stop this while your bike literally wants to throw you off. 

The video shows how severe the wobbles were at 2.44sec doing 60-65kmh have done 85kmh no problem so why now?

I have an Avanti corsa SL, lovely bike but would agree with a review I read about them, great bike but they don't like rough surfaces. On a smooth surface predictable and fast otherwise ?

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I've never had a speed wobble so have never tried to stop one or diagnose the issue, but I believe the basic rule of thumb is to grip the top tube with your knees and not fight the handle bars too much.

As for what causes it, could just be bad bike geometry, loose headset, your position on the bike (weight distribution?), maybe you get a side gust and your response sets it off (death grip on the bars etc).
Hard to say, has it only ever happened to you on this bike? Have you changed something recently that might explain it?

Something I read also suggested just taking a bit of weight of the seat. Not standing up.  Lots of discussion on the internet 

http://www.sheldonbrown.com/brandt/shimmy.html

http://cyclingtips.com/2011/03/speed-wobble-when-the-bike-shakes-it...

Judging what a friend of mine went through with two custom touring bike frames in succession and the speed wobbles, I'd be concentrating on the forks. Try some completely different forks and see how the bike changes. Increased stiffness and slight rake changes might do it.

most times speed wobbles are caused by weight distrubition on bike ,the avanti has fairly standard geometry on rake and trail,head angle you will find that if weight on front end on bike is wrong ,you may get away with it on smooth surface as you say but once a sde wind or rough surface unsettles bike it starts the oscillation ,a bit like a shopping trolley wheel going berserk and thats caused by no weight on that wheel .changing forks with different rake is fraught with problems , you will change the way bike handles ,you may make problem worse maybe better ,toe overlap will change as well .the answer is that the weight distibution should be minimum 40% front wheel 60% rear with your  hand position in the drops where you should be when descending keeping your centre of gravity low and weight distribution in the 60 /40 range

Cheers would agree with that. Not sure how my legs were positioned they may not have been tucked in at that time. Usually I'm pretty good but was not conscious of this at the time. Applied the rear brakes heavily while violently shaking knowing it would have been disaster to use the front. My understanding was this would take weight off the front wheel.

Think I may have found the problem, will be dropping my handle bars, not the height or reach just bringing them towards me a bit more.

Did a bit of experimentation today and if I put my hands at the end of the drops it seemed to be not as stable.

Thanks all for your input, have heard a lot of different opinions in the past but never sure what is fact or fiction the worst one I heard was take your hands off the bars and it should sort itself out.

This happened to me about 20 odd years ago on a steel framed Centurion coming down Mcintyre Rd. Para Hills doing just over 70km. Scared the absolute s..t out off me. The driver of a car coming up behind me must of seen what was happening as I remember hearing the V8 engine come of the gas.
It happened one more time after that riding in the hills, cant remember where now, but as soon as it started, it stopped.
I've always been rather nervous while descending since then.  

If you google Speed Wobbles, you will see there's lots of theories as to the cause but not 1 specific reason.
I think it just comes down to a number of things, the bikes geo, road surface, weight distribution & a riders style. If all the planets just happen to line up, then then you run the risk of getting the wobbles.

Now having said that, you could have 2 riders ride the same bike, one goes down like a bat out of hell with a great big smile on his face while the other gets the wobbles so weight distribution & style probably play a larger role.
One remedy floating around is to put your knees hard up against the top tube as that, supposedly, dampens the vibrations and stops the wobble.
Haven't tried it, and not interested proving it works..:)

With my current bike, on a smooth road, bike feels good & I just tuck down & go for it but if the road is just little rough, the front just doesn't feel planted, but only while coasting. On smaller descents, no matter how steep, if i can keep pedalling and provide even a light amount of forward thrust, the bike feels good.
Some will say I need more weight at the back while others will say I need more over the front. I've tried both but neither seems to work each & every time so I've just learnt to back of & take my time.

Glad to hear it happens on steel framed as well as carbon thought it was a carbon thing only. Will try the knees next time. Hopefully there won't be a next time.

What tyre pressure do you run Fred? Overinflation on rough surfaces will cause speed wobbles.

Gary i have 100psi on front and 110psi on rear if wet I drop down to 90psi all round

I have have speed wobbles a couple of times and advice from a post on AC sorted that out in my case
- stay out of the drops (probably a weight distribution thing)
- put some forward pressure on the hoods
- always have one foot down so cranks are vertical for stability as you would when cornering
I haven't had any more wobbles but I'm still nervous if speed gets over 60-65.

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