Speed wobble (not sure if this is the right place)

irezumi

Was going down a very steep, and is it turned out very poorly surfaced, hill the other day. Which developed in to a monumental speed wobble (I wont use the term 'tank slapper' as that for motorbikes only!). Anyway, the bike is all in good working order, head bearings good and correctly tightened etc.

Any suggestions on how this could be caused (I realise lots of speed and poor surface are the contributing factors) but what about body positiong, weight distribution etc, and also potential preventative remedies? I tried dragging the rear brake which did nothing and there was no chance I could grip the top tube with my legs due to the violent nature of it.

I am particularly light, weighing in at 57kg, is this likely to be the primary cause?

Frame is an older Giant TCR composite if it makes any difference.

GiantMike

Sounds to me like you did all the right things to stop it, so I wonder why it didn't.

[amateur scientist geeky mode]

I think speed wobbles are a combination of:

Speed
Natural resonant frequency of your bike
Outside impulse to start the wobble

As you ride you get a lot of impulses to the bike (road surface, steering by you, gusts of wind etc). Almost all the time these are just irrelevant as the bike is not at its resonant frequency speed, Most bikes will have a natural frequency at which they will wobble at a certain speed, much higher than normal speeds (I think it's a result of the speed the wheels rotate at. Why don't bikes have their wheels balanced? They don't go fast enough to need it, until they do and then start to wobble???).

So, what happens is, you accelerate and at some point reach your bike's resonant frequency speed. Then, an external impulse (such as a road bump) causes a disturbance to the previously balanced system of moving parts. The bike then starts to oscillate around its centre of gravity, and continues to do so until either:

Your speed decreases to below the resonant frequency (or increases above it) or
A perfect impulse rebalances the 'system' (very unlikely) or
You end the experiment by falling off.

Therefore, possible solutions are:

1. avoid the speed at which you bike wobbles, or get away from it as soon as you realise (back brake best I'd think)
2. change something on your bike to damp out the oscillations (different stem length, better balanced wheels, change seat height, change position of water bottle etc)
3. ride only on perfect days when there are no external impulses to upset the system (impossible)
4. fall/jump off as soon as the wobble starts, thus ending the wobble (stupid)

[/amateur scientist geeky mode]

Hope that helps. If not, don't sue me.

Tom Butcher

Some of the older Giant TCRs had a reputation as being prone to speed wobble - whether it was deserved or not is another matter but it was something often discussed on this kind of forum

I am the god of hell fire

This happened to me once and it was jolly scary

It was a descent I have ridden a squillion times

At the time I could see no reason for it but with reflection the signs were possibly there

When it happened I was a relatively novice roadie
Ive ridden MTB for years and had adopted the arse over back wheel MTB descent position
I had also been having back ache and had flipped my stem
In addition I was using aero wheels on a windy day
The road in question has a sharp bend with lots of subsidence in the braking zone
Plus there is a gate in a stone wall allowing the strong cross wind to get to you

I had a tank slapper no question
Ive ridden motorbikes and know full well what this means
I also know what its like to ride a TL1000S if it helps

My steering wobble was awful and shook me up big style
2y later I am riding that stretch of road faster than I have ever but only when conditions are good

Best of luck!

bristolpete

I had terrible speed wobble on a Cervelo last April. Literally all over the road and how I survived is beyond me even now. However, since then nothing but yesterday, almost a year to the day, I had another one, this time on my SL3 but I dealt with it much better, relaxed, loosened my grip, clamped the top tube with my knees and ended the wobble within 10 seconds.

Personally, like others, I think it is like 88mph in Back to the Future. When certain things come together it kicks in. Side wind, speed, deep section wheel and road frequency. It could even be seasonal based on my timeline.

What ever the cause, it is very scary and I hope others dont feel it.

Edit* Just found this. 366 days later - speed wobble. How mad is that -

http://www.bikeradar.com/forums/viewtop ... highlight=

holmeboy

Sidewind Me Thinks!

paddlemyowncanoe

I get speed wobbles when I am cold and tired, usually after a break. A mixture of shivering plus tightening grip plus a little wobble leads to more tensing up and the vicious spiral takes off. I can spot it developing now and know to put on another layer or get a coffee.

freehub

paddlemyowncanoe wrote:

I get speed wobbles when I am cold and tired, usually after a break. A mixture of shivering plus tightening grip plus a little wobble leads to more tensing up and the vicious spiral takes off. I can spot it developing now and know to put on another layer or get a coffee.

Speed wobble is to do with some sort of frequency resonance I thought?

borisvanhalen

had one the other day, had a slight bulge in the tyre where a bit of flint had been... replaced the tyre, result no wobble

Once_In_A_Blue_Moon

I've never experienced one myself, but I've been descending behind another rider who had one, on an alloy Cervelo Soloist. I was scared watching it, how he stayed on I will never know, but if it was me it would've probably get very messy, I would have no idea what to do in said situation. No obvious signs as to what caused it either, perhaps being a very stiff frame on a fast road surface on a windy day. Amongst other probabilities...

rnath

Going down Reigate hill today and had what may have been a speed wobble. Started off feeling like I had a flat in the back tyre then wobble quickly grew more exaggerated - felt like I was going to lose control of the bike so had to slam on the brakes. Bike seems fine - maybe a tiny bit of play in the hub. Is this likely to be a freak one-off, or a characteristic of the bike? I'm not the most confident descender at the best of times :-(

elmsallmissile

Not push bike, but oo er!

http://www.visordown.com/snippets/dan-k ... 18273.html

TheBullet

I used to get speed wobbles all the time on my old Trek 5200. What fixed it for me was tightening the headset as it felt a bit loose.

Thinking about it maybe this concurs with what other people have been saying about resonance and tightening the headset changed the 'speed wobble' resonance.

Brommers76

Had one once coming down Alpe d'huez. Certainly not the bike but was caused by being cold, gripping the bars too tight and braking too much.

SteveR_100Milers

I can honestly say I have never had any kind of obvious unintentional bike wobble. Not managed to get much abov 50mph so maybe it's a speed thing

That said, stem length can make a difference to front end twitchiness especially if you only have one hand on the bars on a bumpy surface for example. Bike frame / geometry obviously has a strong contribution - this new izalco rides like its on rails, even one handed / no handed it just tracks beautifully predictably.