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PostPosted: Wed Jan 19, 2011 10:52 am 
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one-eyed_jim wrote:
Anthony wrote:
The bike stays upright because of the gyroscopic effect of the wheels turning. The faster the wheels are rotating, the greater the gyroscopic effect. That's why it doesn't fall over when you're moving, but it does want to fall over when you stop. You can keep it upright when stationary using balance, but you only need to use balance when there is little or no gyroscopic effect.

I don't think that's quite right. There is a gyroscopic component, and it does increase with speed, but it's not the most important factor.

Depends on your view of most important?

Think about when we learn to ride a bike, or teach a child to ride a bike. One of the key things is them cycling at a good enough speed for the gyroscopic effect to make it all easy for them, rather than the whole struggling for balance and trying to correct with steering.

I think Anthony got it just about right in that paragraph, in terms of the reality of cycling. Largely, the gyroscopic effect is the most important factor - since when we're teaching kids to cycle, or learning, it's the major thing we try to exploit to make it all simple and work.
one-eyed_jim wrote:
A cyclist in motion is constantly falling to one side or the other, and steering to correct his fall. It's this steering that keeps the bike upright.

I'm not fully buying that.

Sure, as you say in the bits I've snipped, gyroscopic effect increases with speed, but when cycling at a normal pace, there's not normally a concept of falling to one side or t'other. I accept and recognise there maybe some steering correction, and still some balance input from the cyclist, but some of that is due to changing direction. True enough, at low enough speed, without much in the way of gyroscopic effect, bikes do feel like their constantly falling to one or the other side - but that's merely because there's little gyroscopic effect going on.

Once there is sufficient speed, and a fair degree of gyroscopice effect going on, I'd actually contend the bike really wants to stay upright, and in "perfect" conditions, like the world of physics normally suggests to initially understand something, a bike at a certain speed would keep itself upright, without rider to correct (assuming no friction, perfectly flat surface... etc...etc)

People can cycle at a reasonable pace with no hands on the handlebars - now sure, I get you can still correct steering by leaning, and at lower speeds you'd need to - but at normal pace, any steering correction would be minimal - perhaps similar to minor steering correction when you're driving.

Don't get me wrong - I recognise your point that there is still a balance thing going on, even at speed. But the gyroscopic effect is significant. At a certain speed, a bike rolling along without a rider to correct would probably largely stay upright (given a reasonably flat surface and no other significant affecting factors) and carry on in it's current trajectory, until it's speed dropped below a point where steering correction to balance becomes more significant that the gyroscopic effect of the bike / wheels at speed.

That whole rider-less bike thing, at speed, plus consideration of, say, the higher speeds of motorcycles, does show the significance of the gyroscopic effect.


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PostPosted: Wed Jan 19, 2011 11:31 am 
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I would say, Neil, you are largely right there. I say 'largely' because no-one knows, or agrees (not even the world's top experts) on quite how a bicycle stays upright when travelling at 'normal' speed. Google the topic and you'll find a paper by a renowned scientist who fitted a secondary wheel right next to his front wheel (but not touching the ground) which was spun in reverse, thus countering any gyroscopic effect. The bicycle rode as normal; this, he claimed, proved that GE has no effect on balancing a bicycle. To me, this doesn't 'feel' right, and I would say here that nearly all of my own theories about the matter are intuitive. I'm not alone, judging by the response to a question on BBC Radio 4's Home Planet a couple of weeks ago (available as a podcast from the BBC website) where the experts trotted out as much garbage as practical information.

The basis of this thread is the design of the Cleland AventuraTT, which is designed to be easy to balance at speeds well below 'normal'. This is because it is intended to traverse all kinds of terrain, often where no path exists, and thus has to balance efficiently at slower than walking pace when GE has no effect. It just happens that this ability seems also to work very well at higher speeds.

So, this discussion is not about 'normal' cycling at all...

... and just a reminder about it: www.clelandcycles.wordpress.com


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PostPosted: Wed Jan 19, 2011 11:40 am 
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GeoffApps wrote:
So, this discussion is not about 'normal' cycling at all...

Geoff - the thread may well be about a specific bike, true enough - but the comments that I responded to were general.

As to the importance of the gyroscopic effect, well as I said, one of the revealling things is consideration of how we first learned to ride a bike. That is normally done by fully exploiting the gyroscopic effect, because it's a lot easier for a child to deal with, than more complex, lower speed balancing that comes with experience, learning and conditioning.

Also, I can't help but remember the odd comical clip of riderless bicyles carrying on at pace, and only falling over when either something interfered with them, or they slowed enough.


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PostPosted: Wed Jan 19, 2011 12:00 pm 
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Blimey! You know how it is when you nip off for a pee, and while the gentle sound of cascading urine massages your intellect, suddenly it occurs to you that you just missed something out of the discussion...

Experiment: we've all done it I suppose; take a bicycle wheel and hold it by one end of the axle, (you'll need to insert the scewer if it a QR hub) spin the wheel and experience the satisfying feel of GE by holding the wheel vertical with only one end of the axle resting on your index finger. Got it? OK.

Note: The wheel will gently 'steer' itself round, on the vertical axis of the place where your finger touches the axle. Counter this effect by grasping the other end of the axle as well.

Keep the wheel spinning, as it would at 'normal' speed. Still holding both ends of the axle, try to 'steer' the wheel. You'll find the GE makes this impossible, the wheel will continue to rotate in the plane it is, and it takes a huge effort to move it out of that plane. Let go one end of the axle, and the wheel will steer in that direction, let go the other and the wheel will drop to the floor, fly across the carpet and into your sleeping dog, who will leap up and startle the cat into rushing up the curtains which you'd been meaning to fix for ages... I digress.

Just try the experiment yourself, and see if you can work out what it's telling you about balancing and steering a bicycle!


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PostPosted: Wed Jan 19, 2011 12:03 pm 
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The riderless bicycles in comic films are achieved by building massive trail into the steering geometry.

The bikes used would be unrideable in normal circumstances.


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PostPosted: Wed Jan 19, 2011 12:08 pm 
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Neil wrote:
Depends on your view of most important?

You can get a fair idea of the importance of the gyroscopic effect by comparing the mass of the front wheel (the rear wheel isn't important) with the mass of the rider.

Quote:
Think about when we learn to ride a bike, or teach a child to ride a bike. One of the key things is them cycling at a good enough speed for the gyroscopic effect to make it all easy for them, rather than the whole struggling for balance and trying to correct with steering.

That's a circular argument...

Speed is important because the steering input required to correct a lean is strongly speed-dependent - because centrifugal force varies with v².

To look at it another way, when the bicycle leans to one side, it will continue to fall to that side unless something happens to put the wheels back under the rider's centre of mass. The faster the bike is moving, the smaller the change in steering angle required to accomplish that lateral correction. If you look at the tracks of a bike ridden through a puddle, you can see that the front wheel is constantly correcting to one side and then the other, even at high speeds.

Quote:
Once there is sufficient speed, and a fair degree of gyroscopice effect going on, I'd actually contend the bike really wants to stay upright, and in "perfect" conditions, like the world of physics normally suggests to initially understand something, a bike at a certain speed would keep itself upright, without rider to correct (assuming no friction, perfectly flat surface... etc...etc)

Ever ridden on ice? A bicycle on a truly frictionless surface can't be ridden, because it can't be leaned.

It's possible to make a self-stable bike, where the steering geometry of the bike coupled with the gyroscopic moment of the front wheel will keep the bike upright above a certain speed. That's largely because the mass of the front wheel is a fairly large fraction of the mass of a riderless bike. Once you put a man-sized lump in the saddle, that mass becomes dominant.


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PostPosted: Wed Jan 19, 2011 12:15 pm 
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GeoffApps wrote:
Note: The wheel will gently 'steer' itself round, on the vertical axis of the place where your finger touches the axle. Counter this effect by grasping the other end of the axle as well.

Geoff, the phenomenon you're describing is known as "gyroscopic precession"...
http://www.google.co.uk/search?aq=f&q=g ... precession

(I knew the long hours spent in 'Dynamics' lectures would come in handy one day... :roll: :) )


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PostPosted: Wed Jan 19, 2011 12:16 pm 
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GeoffApps wrote:
Still holding both ends of the axle, try to 'steer' the wheel. You'll find the GE makes this impossible, the wheel will continue to rotate in the plane it is, and it takes a huge effort to move it out of that plane.

It's a striking experiment, but not least because the effort required to move the axle is magnified by the difference in leverage between the large wheel and the short axle.

The wheel can actually be made to move out of its plane very easily. Try moving the axle ends in circles so that the axle describes a cone. The interesting point is the gyroscopic torque at right angles to the torque applied, and that's a factor in steering correction.


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PostPosted: Wed Jan 19, 2011 12:18 pm 
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Thanks, I couldn't remember what it is called, just as I can't remember the name of the chap who did the counter-GE experiment. I was up all night!


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PostPosted: Wed Jan 19, 2011 12:19 pm 
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I remember watching an Open University programme where a bloke demonstrated how unstable a bike was until the forks were reversed and the rake made the bike stable enough to be pushed down a road on its own.


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