The dreaded shimmy

[kingrollo]

It wouldn't be something as simple as March/April do tend to be pretty windy ?

 

[Yellow Saddle]

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.

However, that advice does not translate to bicycles in the least. We cannot power out of corners because we can't pedal in corners. That changes the apex for cyclists as well.

 

[Norry1]

However, that advice does not translate to bicycles in the least. We cannot power out of corners because we can't pedal in corners. That changes the apex for cyclists as well.

Well that bit may not - but the "you go where you look" bit does.

 

[Yellow Saddle]

Well that bit may not - but the "you go where you look" bit does.

Yes, I know that's the type of stuff they tell you at these advanced places but that's hardly advanced advice. Think about it next time someone accidentally walks into you or rides into you for that matter. They weren't looking where they're going and certainly not going where they were looking. If they were going where they'd been looking they would not have hit you. I just don't get this type of advice, it's a bit like those gym instructors telling you to not forget to breathe.
This is a discussion on shimmy.

 

[Yellow Saddle]

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.

I have no idea what "speed oscillation" is. But I'm struggling to figure out how a misaligned/weak fork will induce shimmy. I also cannot think of a single example of "poor spokesmanship" that can induce shimmy. Please enlighten me.

 

[Norry1]

Yes, I know that's the type of stuff they tell you at these advanced places but that's hardly advanced advice. Think about it next time someone accidentally walks into you or rides into you for that matter. They weren't looking where they're going and certainly not going where they were looking. If they were going where they'd been looking they would not have hit you. I just don't get this type of advice, it's a bit like those gym instructors telling you to not forget to breathe.
This is a discussion on shimmy.

You are being a bit literal there. At pace when cornering, if you look through the corner rather than at the hedge you are trying to stay out of, you have a much better chance of getting through it smoothly. However, you are correct, this is about Shimmy so I'll stop now

 

[Gravity Aided]

I have no idea what "speed oscillation" is. But I'm struggling to figure out how a misaligned/weak fork will induce shimmy. I also cannot think of a single example of "poor spokesmanship" that can induce shimmy. Please enlighten me.

Spokes, either broken or loose, may induce wobbling at speed, I've had it happen when I did not discover the problem before I rode a bicycle I had bought used. I have also had the same thing happen with loose spokes, and the unevenness of the wheel induced a wobble as the bicycle went down the road. A weak fork or top tube will induce shimmy as it permits the wheel to oscillate around a point centered on the head tube.

 

[Yellow Saddle]

Spokes, either broken or loose, may induce wobbling at speed, I've had it happen when I did not discover the problem before I rode a bicycle I had bought used. I have also had the same thing happen with loose spokes, and the unevenness of the wheel induced a wobble as the bicycle went down the road. A weak fork or top tube will induce shimmy as it permits the wheel to oscillate around a point centered on the head tube.

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?

 

[Yellow Saddle]

You are being a bit literal there. At pace when cornering, if you look through the corner rather than at the hedge you are trying to stay out of, you have a much better chance of getting through it smoothly. However, you are correct, this is about Shimmy so I'll stop now

Let's translate that: If you look where you are going you will not crash into things so often.

Don't forget to breathe.

This is just gratuitous stuff "experts" in classes tell their students to confuse and/or impress them. It smacks of the "counter steer" advice given in those same advanced classes.

 

[Norry1]

OK - I won't deflect this thread but if you don't agree with counter-steering, you are missing a lot.

 

[Yellow Saddle]

OK - I won't deflect this thread but if you don't agree with counter-steering, you are missing a lot.

I assume you are addressing me since I used the words "counter steer". However, you can't possibly know what I think about counter-steering. I said I don't agree with the advice on counter steering, nothing else. Counter steering is not a distraction though, it is at the very heart of shimmy.

 

[Norry1]

I assume you are addressing me since I used the words "counter steer". However, you can't possibly know what I think about counter-steering. I said I don't agree with the advice on counter steering, nothing else. Counter steering is not a distraction though, it is at the very heart of shimmy.

Very true - I inferred what you thought (incorrectly by the look of it). So if it is now back on topic, I'm interested in the linkage between countersteering and shimmy in your view.

 

[Yellow Saddle]

Very true - I inferred what you thought (incorrectly by the look of it). So if it is now back on topic, I'm interested in the linkage between countersteering and shimmy in your view.

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.

Back to shimmy. A shimmy is a severe steering wobble that starts off as an automatic counter-steer and subsequent over-compensation that in itself requires a counter-counter-steer and so on and so on. Bikes which are not torsionally stiff enough to resist the flex and rebound in the front end of the frame do this shimmy at speeds which we can attain. Remember that for each "steer" a whole lot of inertia has to be overcome and it is the stiffness of the frame that has to cope with how the inertia catapults past the centre point to the opposite end of frame flex and bounce back again. The reason the shimmy is speed related is because as you steer, you move sideways and the faster you're going when you steer, the more you move sideways. It is this exact frequency that harmonises with the frame's springiness that brings it on. This flex is more or less about the axis of the top tube but involves the bottom half of the bike moving about your body's anchor on the seat. The plane of the wave is not horizontal though, more like perpendicular to your fork, I guess. Your arse is a node for the wave. Standing up often destroys the node and arrests the shimmy. It also destroys your nerves.

Because intertia and stiffness are involved, we have three variables that makes a shimmy almost impossible to replicate by others going down the same hill at the same speed.

Bikes which are stiffer can also shimmy but at speeds we cannot attain.

Something like a wheel with loose spokes will have a dampening effect o the shimmy, not an accelerating effect. Ditto for loose bearings. A stiff, springy wheel is more likely to contribute than a loose wobby one. A wobbly wheel is highly unlikely to have a wobble that resonates exactly with the critical shimmy frequency. Highly.

Our natural reflexes can induce shimmy. If you have a bike prone to shimmy and you grip the bars tightly and flinch at the onset of a shimmy, you can fuel the fire by your reactions and initiate it strongly.

It is easy to get just about any bike to shimmy. Find a long gentle straight slope where you can cruise downhill and gently pick up speed. Ride no-hands whilst pedaling and sooner or later a shimmy will start. Because you have no hands on the bar, by dropping down and gripping the bar, you can arrest the shimmy. The bike's real hands-on-bar shimmy point is at a speed much faster than that. However, inducing it at low speeds teaches you about onset, control and arrest of this very scary phenomena.

I described my experiments with shimmy here:

https://www.cyclechat.net/threads/speed-wobble.198346/post-4302106

 

[ColinJ]

It is easy to get just about any bike to shimmy. Find a long gentle straight slope where you can cruise downhill and gently pick up speed. Ride no-hands whilst pedaling and sooner or later a shimmy will start. Because you have no hands on the bar, by dropping down and gripping the bar, you can arrest the shimmy. The bike's real hands-on-bar shimmy point is at a speed much faster than that. However, inducing it at low speeds teaches you about onset, control and arrest of this very scary phenomena.

Example:

The real thing:

 

[Tim Hall]

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?