The dreaded shimmy

[Yellow Saddle]

Example:

The real thing:

Two brilliant videos. Thanks for posting.

In the first one you can see how quickly it is arrested when you put your hands back on the bars.
In the second one you can see from about 1:50 onwards in the slow-mo section, how the frame and wheel's flexibility snaps the movement back and forth. That's why I say loose spokes will arrest the movement rather than twang it back and forth.
Pity about the music in the second video, the raw sphincter-snapping sound would have been interesting.

 

[Yellow Saddle]

My understanding, although I'm open to correction, is that shimmy/speed wobble/tank slapper is a resonant vibration in the bike & rider system. Seeing as the bike hasn't changed, maybe the rider (or the mass of the rider) has and can affect it.

Comments?

That's exactly what was said two posts back from your question, albeit with explanation and avoidance of resonant. I avoid using vibration because that implies something with a smaller amplitude and higher frequency.

 

[Tim Hall]

That's exactly what was said two posts back from your question, albeit with explanation and avoidance of resonant. I avoid using vibration because that implies something with a smaller amplitude and higher frequency.

Right, in that case seeing as the OP has "still fighting to regain fitness after TB so I had two years off", could it be that he's changed in weight (mass) and that's what's causing it?

 

[Yellow Saddle]

Right, in that case seeing as the OP has "still fighting to regain fitness after TB so I had two years off", could it be that he's changed in weight (mass) and that's what's causing it?

Absolutely.

In fact, even he had maintained his weight and he say, lost some muscle but gained some fat elsewhere, that could also trigger it.

The bike and rider and his/her weigh distribution (be that as per above example or different stem or saddle position) is one critical system that only resonates under specific conditions that also include speed, tense/relaxed hold on the bars and other things that affect elasticity of the bike and inertia of the load.

 

[Tim Hall]

Absolutely.

In fact, even he had maintained his weight and he say, lost some muscle but gained some fat elsewhere, that could also trigger it.

The bike and rider and his/her weigh distribution (be that as per above example or different stem or saddle position) is one critical system that only resonates under specific conditions that also include speed, tense/relaxed hold on the bars and other things that affect elasticity of the bike and inertia of the load.

@gbb the answer is More Beer and Pies.

 

[Yellow Saddle]

This is how you recover. Notice the guy at 0:47 standing up to arrest it.

 

[Gravity Aided]

I still don't know what a speed oscillation is but that aside, for that matter any loose component will induce a wobble at any speed. I can imagine someone wobbling because their saddle just came loose or...someone wobbling because the stem just moved on the steerer etc etc.

Let's focus on loose spokes. One or two or three loose spokes cause the wheel to buckle and hit the brakes or chainstay. However, as long as the wheel can turn, it doesn't introduce a wobble/shimmy at all. It just carries on, as dozens of people here will testify who have ridden with a broken spoke or two. Most people don't know that the wheel wobbles unless it hits the brakes. Then only do they notice but it's not like the bike goes zig-zag down the road like a drunk figure from an Andy Capp cartoon.

Let's look at a wheel with loose spokes. The next step up from a few loose/broken spokes is a wheel with loose spokes. Spokes so devoid of tension that the hub and rim can be moved independently by hand. That's not a wheel and I doubt anyone will attempt to get up to speed with that. You will have to define "loose spokes" for me if you feel it lies anywhere between the two parameters I've outlined.

Let's look at weak fork and weak top tube. What is weak? Flexible? Rubbery?

A bent fork makes the bike ride like a crab but is really only noticeable if you ride towards an assistant and he/she eyeballs the wheel alignment during the one or two split seconds that you actually ride straight ahead towards them. A very bent fork causes the brakes to catch and cannot be ridden.

Now for the "oscillate around a point centered on the head tube" bit. You'll have to clarify that. I assume you are referring to an oscillation made possible by flex in the frame - like a wobble boards. But where is this centre? Let's call it a node - is it along the axis of the fork or is the amplitude of the oscillation biggest around the axis of the fork?

Oscillation could be centered along the axis of the steering tube or the top tube. Fork weakness would imply vibration leading to oscillation, but this could also be affected by tire composition or road surface. I was having a short oscillation like effect at one spot in the road. I later saw that the slip form paver had made a jog in the roads surface, causing this effect.

 

[Gravity Aided]

BTW @gbb best wishes on a full recovery.

 

[KnackeredBike]

This is how you recover. Notice the guy at 0:47 standing up to arrest it.

Indeed, although somewhat like skidding (releasing the brakes = regain grip) standing and relaxing are difficult to do instinctively when your bike has become a bucking bronco.

Last time it happened to me was when I noticed poor road surface ahead and so tried to brake and steer away, inevitably tensed up and was lucky not to be thrown off my bike.

 

[DaveReading]

Isn't the conventional wisdom that pressing one or both knees against the top tube helps to stabilise the shimmy, or am I thinking of something else ?

 

[Yellow Saddle]

Isn't the conventional wisdom that pressing one or both knees against the top tube helps to stabilise the shimmy, or am I thinking of something else ?

No, squeezing your legs together is the conventional wisdom for birth control.

 

[Smokin Joe]

OK, fair question.

First my view on counter steering. At "advanced" riding schools they tell you that you have to counter-steer in order to turn. They tell you to push on the handlebar on the side that you want to turn to. This is told as some sort of revelation and everyone is amazed, forgetting that they've been turning since they first got on a bicycle back in 1964 without ever actively counter steering. Fact is, we don't need to be told to counter-steer. We just steer and it works. Behind the scenes there is counter-steering going on but we can corner as well anyone else without even knowing it. They tell you that "this is how you should steer."

Truth is, counter-steering happens automatically. This is easily proven by riding no-hands. Yes, when you lean into a corner, the steering briefly swivels to the opposite direction, the bike leans, the steering returns and overshoots (the centre) and leans in towards the corner. This all happens without you knowing that it even happens or without you even thinking about it.

That's why I say the whole lore of counter-steering is BS.

That's what I've been saying both here and on motorcycling forums for years. People who "Teach" counter-steering don't really understand it.

 

[gbb]

Right, in that case seeing as the OP has "still fighting to regain fitness after TB so I had two years off", could it be that he's changed in weight (mass) and that's what's causing it?

Absolutely, I was always 10.5 stone, always, whatever I did. I find myself now at 11.5 stone, more upper body mass, a bit skinnier legged with muscle loss so yes, this may well be having an effect.
Grip on the bars is often mentioned. First time I was deliberately light on the bars, yet the shimmy stated at quite low speed as I started a down slope, actually I do remember hitting a rough patch in the road which unbalanced me no doubt. Seconds before the shimmy hit.
This last time, I had a firm grip but not hard, I'd accelerated downhill considerably faster before it started.
One time, it had JUST started and I pressed forward and it seemed to quickly resolve itself, but I wasn't going the fast then either..

Frankly I think it's down to me, the bike is in good mechanical order, meh a couple minor adjustments that may help, I'll find out next ride perhaps.

Getting it under control seems to be the answer and doing it promptly before it accelerates in frequency.

Thanks for the input, keep em coming,.

 

[DaveReading]

No, squeezing your legs together is the conventional wisdom for birth control.

Ah, that explains why no-one has ever been able to conceive on a shimmying bicycle.

 

[Fab Foodie]

Worth a read boys and girls:

https://www.sheldonbrown.com/brandt/shimmy.html