Have I lost my bottle???

[Tonyf33]

I had a horrendous high speed wobble at 40mph a few weeks back down a local road that I'd not being down since they relaid it last year/early this with that cruddy chip n tar rubbish (making it really bumpy/uneven)
The wobble was so bad it lasted for about 200metres (& with a car trying to nose past me at about 42mph ) that I honestly thought I was going to crash, only for the fact I have very strong arms I managed to get it under control.
I went out the next day on my normal everyday flat bar bike and the wobble started at 20mph as I took it gentler than on my titanium racer and just braked all the way down the hill. I've been down this hill loads in the past and at higher speed, there must be something in the undulations early in the descent that sets it off..I think.

Anyhow, I've being down the short sharp hill at the back of the estate (on the ti bike I've had for a while) since without any problem hitting 36 ish and that is actually bumpier in places but not a problem control wise:? but out this Saturday down another stretch of urban road with a small descent that I've gone down not a few weeks ago and I got what felt like the murmurings of a wobble and just bottled it and hit the brakes.
It quite unnerved me tbh which for me isn't something I've ever felt in any cycling situation I've being hit n run and got back on the back the next day, crashed at high speed before, had close shaves with HGVs and tipper wagons all in the last few years so not just incidents from years and years ago, I don't tend to have much feeling past the first few minutes and a rant on a forum if I can be bothered to mention it.


Just to say I'm kinda just getting back to using drops a lot more after a couple of years of not really riding in anger at higher speeds after just pootling on leisure rides for the most part but don't feel to have lost the feeling for control etc. As mentioned I have been at a reasonable lick on the drop bike this year already so I'm not sure if it's the bike, the surface, my confidence or a bit of all of it .

[mercalia]

40 mph on a push bike hmm mad I think.

[Tonyf33]

40 mph on a push bike hmm mad I think.

thanks for the advice/in depth thoughts

[al_yrpal]

I have done 45mph many times downhill quite comfortably. But, I have had speed wobble on a flimsy racer type bike when it was heavily loaded with panniers on the back and a heavy bar bag on the front. Some bikes become unstable like this, I think rider weight is also a factor. I have also heard that a loose headset can make things worse. Do any of these things apply to you?

Al

[TonyR]

The curious aspect, if I've understood it correctly, is that the shimmy has appeared on two different bikes both of which have been fine in the past. That points towards something you are doing rather than a sudden simultaneous change in two bikes. Its difficult also to think of what you might be doing to make it happen other than being more sensitive to it after the first episode but one that come to mind is are you gripping the bars harder and trying to stop them moving? That may be making the situation worse so try relaxing your grip and allow the bike sort itself out. The other thing you could try to try and stop it is to rest one leg gently against the side of the top tube. That seems to damp shimmy out in many cases.

[pwa]

Carrying a seat post mounted bag (Carradice SQR) that was too heavy seemed to cause speed wobbles for me. 40mph+ should be possible, so I wouldn't blame the speed itself.

[Vorpal]

Experiment a little? See if you can repeat it on other hills, if not, maybe it's the surface on that one hill. Then try going down at lower speeds and gradually increasing? Or modifying the weight distribution?

[Graham]

Following a rather similar incident some years ago, please find some history and related reading here . . . . . .
viewtopic.php?f=5&t=6907&hilit=ranmore&start=15

The damper knee-on-crossbar is the vital piece of advice that I lacked at the time of the incident.

Years later the psychological component has fixed itself. However, I remain a cautious descender, especially on uneven road surfaces.

EDIT : more reading here . . . .
viewtopic.php?f=1&t=9855&hilit=headset

[Vorpal]

I had bike when I was teenager that would shimmy at some speeds. I was told to grip the top tube between my knees to stop it. There have been a couple of other threads about that sort of shimmy.

viewtopic.php?f=1&t=66685
viewtopic.php?f=5&t=65802

[Audax67]

It's not a matter of weight but resonance. Virtually every undamped rigid or semi-rigid mechanical system has a resonance frequency, the resonance usually being triggered at a certain speed. Wheels are usually unbalanced (valve, maybe spoke reflectors, etc) and this sets up a periodic force whose frequency varies with speed. When the frequency hits your bike's resonance frequency you can get shimmy.

Shimmy is oscillation of the fork/frame combination, so that the HB pivot moves side-to-side, causing the front wheel & bars to turn to & fro. You can get rid of it by changing your fork but all you'll probably do is change its frequency.

If I put a saddlebag on my Audax bike I get a shimmy at around 25 kph. If I add an HB bag the extra mass damps out the shimmy. On motorbikes you'll even see little dampers like mini shock absorbers between steering gear & frame for the same reason.

You can also get shimmy by clamping the bars hard in your hands, thus turning your body into part of the system. Loosening your grip will often get rid of it, trying to stop it will often make it worse.

I would recommend getting back into a shimmy situation and learning to control it, then changing your weight distribution until it disappears. My Audax bike has done tens of thousands of km by now, shimmy-free.

[pwa]

It might be worth trying to balance your wheels. I always put my speedo magnet at the top of the wheel when it has finished spinning. Other small additions (reflectors?) could be used to even out any imbalance. Have you changed the inner tubes you use recently, ending up with a longer or shorter valve?

[mercalia]

40 mph on a push bike hmm mad I think.

thanks for the advice/in depth thoughts

madness dont need much comment just DONT do it! Like those silly so and sos on motor bikes doing 100+ ( and end up creamed)

[661-Pete]

I've occasionally found myself 'fishtailing' on a gentle downslope - going a lot less than 40mph, I doubt if I was even doing 20mph. It's not always down to speed. I think the reason in my case was a greasy road. It's certainly a scary experience. You could try experimenting with different makes of tyre. I normally have Armadillos, which are excellent on p*ncture resistance but not so good on road grip.

[Tonyf33]

thanks for replies
Loose grip on bars so I'm not holding the bars too tight, that's never being my style to do otherwise.
I tried the top tube trick on the 40mph incident, didn't help, a combination of v.light back braking and a gorilla grip on the bars managed to get the violent wobble under control as the hill flattened out, this wasn't a small shimmy, this was a full on tankslapper style.

I recently fitted Conti 4 seasons but those and the tubes I've ridden at 36mph on other small descents without issue and 32mph on a smooth 1%er whilst pedalling hard so it's not the tyres or tubes.

As I repeated this with my other bike and the wobble started straight away I was more convinced it was the road surface that was the culprit, the other incident was also on a recently relaid chip and tar road but that 'wobble' was less pronounced, I just backed right out of letting the bike go at that first wobble hence I questioned my nerve.

I'm off out later this evening down a 40mph d/c, will see how I feel

[beardy]

I'm off out later this evening down a 40mph d/c, will see how I feel

Dont forget your helmet.