Just tried out my new bike in the hills in wales. Lovely going up, or on the flat. On one decent I hit 55km/hr and the front end of the bike started to ocillate at about 2-3Hz, the whole lot, front wheel forks and handlebar bars ocillating back and forth shimmying me down the road. At one point it was so bad I thought it was going to dump me on the road. managed to hang on and slow down then it stopped. I used to come down this hill on my other race bike at 65km/hr no problem, and my cyclecross bike at 55Km/hr no problem. I once hit 70Km/hr on the race bike no issues. Did a quick examination of my new bike on returning home, can't feel any obvious play in the head set, and nothing other than normal play in the front wheel bearing. The wheel and tyre seem true. I can't feel any obvious flex in the handle bars, fork or wheel which ties up with what it felt like, that the head set bearing are turning. It's like someone grabbed hold of the wheel and was tugging it back and forth.

I am on a Giant OCR with mavic carbone wheels with Michelin race pro tyres.

I have heard of this before but as I never experienced it, I paid little attention.What is going on? Why does this happen? How do I fix my lovely new bike? Any help appreciated.