Severe front end vibration

Alastair C

Hi
I am a newbie to this forum but had a big scare yesterday and would really like your advice and guidance.
I have had a Giant SCR 2 since February - my first road bike after a couple of years commuting on a hybrid. Bike is all standard spec and (if it is relevant) I weigh about 13 1/2 stone.
Problem has happened twice now, both when doing about 30+mph. The first time I had a severe shaking I thought that I had hit an unseen pothole as there was a bit of a bang before the handlebars started bucking. But yesterday I was pushing along on a fast downhill stretch when suddenly the handlebars were whipping back and forth and I was sure that I was going to come off. It probably lasted for 2-3 seconds and then I got things back under control.
Is this one of the joys of road bikes that I have just discovered, or is there something I can do to address the problem as it really spooked me yesterday? If it does happen, how can you best control it?
Any advice would be appreciated.

farrell

DO NOT RIDE IT UNTIL IT'S FIXED OR AT LEAST CHECKED OVER!!!!!


If you come off at 30mph + downhill you WILL be seriously hurt. Take it to your LBS tomorrow.

Chris

Monty Dog

It could be two reasons - either the headset assembly is loose, or more lilely due to a condition called 'shimmy' where the conditions or road surface, speed, rider weight causes a resonant harmonic frequency to be reached, where everything starts to vibrate. Certain bikes and combination of parts are more succeptible than others - giant bikes for one are quite well known. Suggest you search for previous posts on the subject. I had a bike once that suffered from this at high speed - a change to a stiffer carbon fork solved it. Others find that pressing their knee against the toptube helps 'damp' the vibration.

fenski

I've had a speed wobble like this only once. Trouble was it was when I was descending the Bealach Na Ba!

There was a fairly hefty cross-wind which I think had something to do with it. I'd also recently tightened (perhaps over-tightened??) the headset, and had inflated my tyres to a higher pressure than normal.

Alastair C

Folks
Thanks for your opinions and advice.
My personal feeling was that it was a flexing of the front wheel that was happening and wondered if I needed to upgrade these. I will speak to LBS where it was bought and get it checked.
Alastair

Lucky Luke

Alastair C wrote:

Folks
Thanks for your opinions and advice.
My personal feeling was that it was a flexing of the front wheel that was happening and wondered if I needed to upgrade these. I will speak to LBS where it was bought and get it checked.
Alastair

Sounds like a shimmy to me too . Would still get the bike checked though .

Lucky Luke

I had a speed wobble once myself at something like 38mph . Very frightening it was too . In the end I put it down to the following factors : a strong crosswind and tension in my body (as I began the decent I felt a cramp in my calf then a car passed me too close for my liking and I tensed up) .

ut_och_cykla

You've had teh bike since FEb - and its only just started wobbling - check headset adjustmetn. Unless you know what you are doing let a LBS that you have confidence in have a look at teh bike. Shimmy due to frame/body mix would probaly have occurred sooner perhaps.

Fab Foodie

I'll bet you a pound to a penny it's shimmy.

Jobst Brandt pretty much sums it all up here:
http://www.sheldonbrown.com/brandt/shimmy.html

Probably nothing wrong with the bike, but no harm in checking everything is connected OK. Loose headsets IMO do not cause shimmy either,

All of my road bikes have shimmied....when I first took them out for a hurtle down a hill. The more relaxed you become, the less they shimmy.

Probably you tense-up as the speed increases, try to relax the arms, sit back in the saddle, hands forward in the bars (or on the hoods), not holding onto the bar ends. Look down the road, the bike will calmly go where you point it if you let it without too much drama.

The early TCR's like mine were meant to be skittish twitchy beasts, but it'll descend steep rough roads even with a heavily loaded saddle-bag (for which it was never designed) without upset. 5 weeks ago I rode down Mt Ventoux averaging 40mph and hitting up to 49 mph on some of the straights....solid as a rock, not a single wobble.

Confidence and Relaxing the "Death-grip" is key

oldwelshman

I get this on my track bike on a tight track evene tho everything is set up ok!!
I had it on a road bike also but I had a very short stem on it and since I put a longer stem on it, it is better now.
I have also had speed wobble when my head set worked a bit loose so it can cause vibrations.

Hugh A

I agree with what has been said about shimmy and other factors not affecting it. I think the weight and balance of the front wheel in conjunction with the flexibility of the forks has a lot to do with it. If you can borrow another front wheel or even just try changing the tyres that may sort it.

Having said that I still think that speed wobbles can be brought on by having a loose headset and it is worth checking that first.

jimwin

As stated by many, get it checked out by your local workshop to make sure it isn't anything loose.

If not, then it's shimmy. I've had this on a steel bike (853 no less) and there's little you can do to fix it. Be careful about changing the forks. In my experience a stiff carbon fork *can* make it worse by feeding more vibration into the frame. Stiff forks need an equally stiff frame IMO. I've never seen a definitive solution to the shimmy problem.

BTW - it also depends on your weight and size. I doubt lighter riders ever suffer this problem. Smaller riders = smaller (i.e. stiffer) frames.

- JImW

Fab Foodie

I don't agree that there is nothing you can do. IMO as long as nothing is out of shape on the bike, then shimmy is all about how you interact with your bike at the 3 contact points, whether you are stiff and part of the resonant system or relaxed to dampen the vibes.