The dreaded shimmy

[gbb]

So it's getting boring now, today's ride I had another shimmy at between 20 and 25mph I guess, always as I accelerate downhill.
What's changed ?
I used to be a fanatic on the bike, confident, aggressive, pushing myself hard in my own little way. Yeah it's happened before but usually at 30mph plus.
Now I'm still fighting to regain fitness after TB so I had two years off so no doubt my style or confidence may well have changed.
Bike and wheels are the same. Tyres I did change earlier this year from beaded Gators to folding GP4000S, this may be worth investigating, going to have them off and reseat them.
Inner tube has a very long valve as well, might change that to something more appropriate.
No wheel magnet on the spokes.
I have noticed my handlebars are a few millimetres offset for some reason, will rectify that today.

Riding style ? It's happened when I've had a relaxed grip at quite low speed , I did once seem to get it under control by leaning forward and pressing into the drops, it's happened when I've had a firm grip, pressing as before didn't really have any effect.

Why ?.....any thoughts appreciated.

 

[Drago]

Wheel bearings and headset correctly tightened? Wheels aligned in the drop outs properly?

 

[Gravity Aided]

You might also check spoke tension. That can cause quite a tank slapper moment at times as well.

 

[Yellow Saddle]

You might also check spoke tension. That can cause quite a tank slapper moment at times as well.

Explain?

 

[Gravity Aided]

A loose or broken or uneven spoke tension could also cause this phenomenon. Although hubs, or a bent fork, would be my first culprit.

 

[Yellow Saddle]

A loose or broken or uneven spoke tension could also cause this phenomenon. Although hubs, or a bent fork, would be my first culprit.

None of the above causes shimmy.

 

[gbb]

Tyres reseated...not that I think there was anything amiss there anyway.
Inner tube with extra long valve changed to something a bit shorter.
Handlebars centred....
Headset seems OK. Forks are full carbon with no detectable faults or problems. Interestingly, when the bike was new, someone's review did remark on the lack of rigidity of the forks, Ribble own, specific to the bike, but I never really detected anything amiss before.
Stem is short, but always was. Weight too far back perhaps but then I've already said it wasn't a problem before.

I'm probably over analysing, it's possibly more me than the bike .

 

[fullcycle]

Forgive me for sounding inexperienced and newbish but what is shimmy it's a term I've not come across before

From reading the above it definitely sounds like something I have experienced though

 

[Smokin Joe]

Forgive me for sounding inexperienced and newbish but what is shimmy it's a term I've not come across before

From reading the above it definitely sounds like something I have experienced though

Motorcyclists call it a tank slapper. It is when the front fork shimmy's violently left and right of it's own accord. Short wheelbase bikes with steep steering angles are more prone to it, you'd never see it happen on a Harley Davidson (Not that I'd be seen dead on a Harley Davidson, mind).

 

[fullcycle]

Motorcyclists call it a tank slapper. It is when the front fork shimmy's violently left and right of it's own accord. Short wheelbase bikes with steep steering angles are more prone to it, you'd never see it happen on a Harley Davidson (Not that I'd be seen dead on a Harley Davidson, mind).

I see thank you for explaining all down to physics I suppose

 

[nickyboy]

Tyres reseated...not that I think there was anything amiss there anyway.
Inner tube with extra long valve changed to something a bit shorter.
Handlebars centred....
Headset seems OK. Forks are full carbon with no detectable faults or problems. Interestingly, when the bike was new, someone's review did remark on the lack of rigidity of the forks, Ribble own, specific to the bike, but I never really detected anything amiss before.
Stem is short, but always was. Weight too far back perhaps but then I've already said it wasn't a problem before.

I'm probably over analysing, it's possibly more me than the bike .

Whilst there are certain configurations of wind direction, centre of gravity etc that seem more prone to this than others, I think you hit the nail on the head when you say it's more about the person on the bike than the bike itself

I've only had it happen to me on one descent. I'd done hundreds before that day on the same bike and never experienced anything at all. I was descending at speed and I cut a bend a bit more than I should have. There was a car coming up, far enough away for me to get out of his way. As I braked and changed direction I got a severe shimmy. It was frightening so I stopped, had a breather and then set off down the hill again. After about another mile it happened again, this time with no other traffic around

Since then I've done hundreds of descents and it's never happened again.

This has lead me to think that my tensing up on descent is probably the main contributing factor. It's hard to explain particularly the second shimmy in any other way. Tense braking and change of direction = first shimmy = massively tense = second shimmy

 

[ColinJ]

This has lead me to think that my tensing up on descent is probably the main contributing factor. It's hard to explain particularly the second shimmy in any other way. Tense braking and change of direction = first shimmy = massively tense = second shimmy

I agree. I have mentioned it before in similar threads but it is worth repeating ...

I chased the same rider down the descent of Holme Moss towards Woodhead on two occasions.

The first time, he was riding a heavy touring bike and the second time he was on his lovely new lightweight carbon fibre racing bike. He experienced violent shimmy on the right hand bend at the bottom of the descent on both occasions!

Since the weather conditions were different on the two days and the bikes were very different, I conclude that it was due to the way he was riding the bikes. I had no problems either time. I think he carried too much speed into the bend, scared himself, tensed up and got into panic braking. I knew what the descent is like and probably began braking before he did so I was able to do it in a much more controlled fashion.

 

[Gravity Aided]

None of the above causes shimmy.

I'll have to disagree with you on that one, although speed oscillation itself may be attributable to incorrect distribution of weight or a weak top tube, allowing oscillation around a point centered on the head tube, other situations can also closely mimic this effect, including bearing problems, poor spokesmanship, and fork weakness or misalignment. I've had all of these situations happen to me on test rides, and I think they could, possibly, be a contributing factor.

 

[nickyboy]

I agree. I have mentioned it before in similar threads but it is worth repeating ...

I chased the same rider down the descent of Holme Moss towards Woodhead on two occasions.

The first time, he was riding a heavy touring bike and the second time he was on his lovely new lightweight carbon fibre racing bike. He experienced violent shimmy on the right hand bend at the bottom of the descent on both occasions!

Since the weather conditions were different on the two days and the bikes were very different, I conclude that it was due to the way he was riding the bikes. I had no problems either time. I think he carried too much speed into the bend, scared himself, tensed up and got into panic braking. I knew what the descent is like and probably began braking before he did so I was able to do it in a much more controlled fashion.

To be fair, that compression and following right hander at the bottom of Holme Moss is scary. It's easily a 50mph descent and it looks like you're going to end up in a farmer's field. Having done it plenty of times now you just have to try to relax and it's pretty easy to go around the bend without braking

So another vote for tensing up and (probably) trying to manoeuvre the bike in an inappropriate way for the speed

 

[galaxy]

As a advanced Motorcyclist one of the first things you teach people is speed into corners. We have a saying. Slow in,fast out. Fast in, shoot out. Also to keep your head and look as far a head as your going. You generally tend to go where your looking. Most people who come a cropper do so because they look at the ground or focus on a point. You then tense up and it goes wrong. Relax and look where you need to be.