Cycling UK Forum

i have a new 2008 Roubaix and had a terrifying experience whilst cycling in France recently and wanted to know if others have had similar experiences.
the first time was descending from the top of the St.Nazaire Bridge over the Loire into Brittany. I left just before the rest of the group and was accelerating downhill when the bike seemed suddenly take on a life of it's own. i was doing 33 mph in a high side/head wind when the bike started shaking and vibrating so much that I thought I was ging to come and fall into the overtaking motor traffic. I had to use all my strength to stay on and bring it back under control afte about 200 metres.
It left me really shaken but I put it down to the special conditions - the height, the exposed nature of the bridge and the wind.

But it happened again two days later on the road from Rosporden to Quimper. Again I was accelerating down after a long uphill stretch and it was windy with open fields either side of the road. Though only going 23mph the bike started behaving in the same way leaving me fighting to stay on. My two neices behind me thought I was having a fit as I struggled with the bike.
These two incidents have left me nervous about how the bike will perform in future. I had done 1,500 miles on it by the first time it happened and really enjoy the bike.
I have taken it into the shop where I bought it to have it checked but would welcome feedback from anyone with similar experiences.

Look up Shimmy - plenty of threads about it here.

This is indeed quite common.
Do look at the threads on the subject.There will be good scentific explainations.
When it happens to me, I stop peddling and gently touch one knee on the top tube and it stops at once, then gently brake a little
to get the bike out of that particular vibration speed.
I agree it is scarey.

Is that what I called "speed wobble" as a kid? Usually it was a loose headset. Had it once trying to do an emergency stop at 100mph on a Honda CBX 750. I never did stop but I made it through a rapidly closing gap in traffic ahead Much more sensible these days. I'd be upset if a nice expensive bike started doing that though.

What you have experienced is 'shimmy' it arises when a set of variables encompassing: tyre pressure, centre of gravity, rider position, pannier loading amongst others come together to create a set set of undamped oscillations at the front wheel.

The cure is, as previous mentioned, to move your knee towards the crossbar until it makes contact. This provides the necessary damping.

I'd put it down to bad design.
A proper racing bike should never wobble. Problem is that too many bikes are designed on computers to look good. Ride quality is secondary. In my opinion a mistake many usa companies make.

That said if it is a touring bike then many factors can cause it. Weight distribution and so on.