[Geezer (OP)]

29"? Holy crap! That's bigger than the tires on my truck.

Have you considered getting a penny-farthing? It would probably be the ultimate mountain bike.

[lightarrow]

I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?

When a bicycle is stationary you can only keep it in balance by moving your weight from side to side to keep your CoG over the wheels.  This isn't an ideal solution because you're moving the larger of the two masses instead of the smaller to maintain balance.  Moving the larger of the two masses also requires more force to be used.  When the bicycle is moving though, this is reversed and instead you steer the smaller mass of the bicycle to keep it beneath the larger mass of yourself, with consequently less force being required to do so.

The steering process is also more progressive than than simply moving your body from side to side because the bicycle follows the sum of the forward and steering vectors, instead of just the sideways vector, giving a finer degree of control.

And you cannot steer when the bicycle is not moving?
With *not moving* I intended that you are staionary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?

[lightarrow]

Sir Clive Sinclair developed a portable folding bike with small wheels called the A-Bike and was described by some as a bit "wobbly" because of the size of the wheels.

I remember a science lesson at school when we had several sized bicycle wheels each mounted on a small axle that we could hold in outstretched arms. We sat on a desk type chair (one on wheels) and held the bicycle wheel out in front of us. Someone then spun the wheel and then we were asked to tilt the wheel from the vertical and also simulate it turning like a bikes front wheel.

The outcome was the chair, with you in it, spun round a bit. The different sized wheels produced different sized effects with the larger wheels producing greater effects. Unfortunately that was years ago and I cant remember exactly what that lesson was about but I think it was something to do with the gyroscope effect. [:I]

Yes, and about angular momentum conservation.

[LeeE]

And you cannot steer when the bicycle is not moving?

Well, you can waggle the handlebars about and, because of the castor angle of the front wheel, the front of the frame can pivot around the rear wheel but to keep balance you need to keep your CoG over the wheel/ground contact points.  It doesn't matter where the frame is or whether the frame is tilted, or not.  So if the bicycle is not moving the wheel/ground contact points won't change and you won't have steered anywhere.

[Geezer (OP)]

With *not moving* I intended that you are stationary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?

I think the balancing effect is similar to balancing something like an inverted broom/brush. Put the broom handle on your palm with the brush end vertically above it. It is remarkably easy to balance the broom by only making small adjustments in the position of your hand. The large mass at the other end of the broom handle helps, because it has some inertia, so it takes a large hand movement to cause it to accelerate in any direction.

When a bicycle is moving, it's easy to continuously adjust the steering and make fine adjustments to the lateral position of the tires to maintain equilibrium against the other forces (similar to the hand movements with an inverted broom). When the bicycle is stationary the rider can only maintain equilibrium by transferring weight from side to side. I could never do it, but I knew people who could, and they did it by continually adjusting the steering angle (if I remember correctly).

I suppose they were taking advantage of the castor angle to make very small adjustments to their center of mass relative to the line joining the contact points of the two tires, but that might be another thread entirely  []

EDIT: I think LeeE was perhaps making the same point re. balancing a stationary bike.

[Bored chemist]

I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.

[lightarrow]

To LeeE and Geezer: if you remove unessential things from the physical model of the problem, that is air friction ecc, the only difference between a moving and a not-moving bicycle is the fact wheels spin, in the first case. So if you want to find a cause of the different equilibrium, you have to look for here.

[lightarrow]

I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.

Don't know what to say, because I really have difficulties to believe it.

[Geezer (OP)]

To LeeE and Geezer: if you remove unessential things from the physical model of the problem, that is air friction ecc, the only difference between a moving and a not-moving bicycle is the fact wheels spin, in the first case. So if you want to find a cause of the different equilibrium, you have to look for here.

Lightarrow - As we might say, baloney! (You might prefer Balogna of course [])

Please refer to the second post in this thread. How much stabilization would wheels that size produce? Yet I'm sure that bicycle actually worked.

The human operator (cyclist) and the bicycle become a control system. While the bicycle is moving, the operator makes exquisite movements of the steering and his/her body mass to maintain equilibrium. We are so good at it, we hardly even know we are doing it, although we do have to learn how to do it. If the bicycle was self stabilizing, why would we need to learn how to balance it?

When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering) so it becomes almost impossible to maintain equilibrium.

[Bored chemist]

I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.

Don't know what to say, because I really have difficulties to believe it.

Seeing is believing.
http://www.rainbowtrainers.com/default.aspx?Lev=2&ID=34
"Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires. "

[lightarrow]

When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)

Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.

[lightarrow]

I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.

Don't know what to say, because I really have difficulties to believe it.

Seeing is believing.
http://www.rainbowtrainers.com/default.aspx?Lev=2&ID=34
"Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires. "

With wheels put in that way, you certainly don't have exactly zero gyroscopic effect.

[Geezer (OP)]

When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)

Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.

I'll try (sorry about the baloney comment - I could not resist it.)

Imagine you are cycling in a straight line at constant speed. Your body mass will tend to continue in a straight line. Now, the path of the tires deviates slightly so that it is no longer directly beneath your center of mass. This causes you, and the bicycle to tilt very slightly. I'm not sure exactly how the brain detects this tilt, but somehow it does. Because our brain is "trained" to maintain the bike in balance, we make a slight adjustment of the steering so that the path of the tires moves back under our center of mass.

I suspect, when we cycle in a straight line the tires actually trace a very small amplitude sine wave while our bodies in fact do travel in almost a straight line. This would be the same behaviour that a servo control system would exhibit. There is always a small error that it tries to cancel out.

The speed we travel at has some effect on the "gain" of the system - in other words, how quickly a certain steering input repositions the tire path under our center of mass. I suspect the "gain" increases with speed, but that might be baloney! If it does, when the speed is zero, the system has no gain.

Either way, you can see that if the bicycle is stationary, moving the steering is not going to be able to reposition the path of the tires relative to the center of mass of the rider (and bicycle).

[JimBob]

Just an observation - a bicycle will not function without a rider. The rides needs to push the pedals to alternative sides of the center of gravity to make the thing move (only "center" if it were still.) Thus to BE a functional bicycle, it must be moving. I watched part of the Tour de France this summer. The front wheel had to move on every one of the bicycles.

[RD]

Just an observation - a bicycle will not function without a rider.

As someone who has dismounted a bicycle at high speed by grabbing onto a rope swing I can say the above is incorrect:
 my bicycle continued riderless for at least 50 meters.

[Geezer (OP)]

JimBob - Good point. The bicycle and the cyclist become a single "unit", and it does get a lot more complicated when the rider is working hard. Going round bends adds further complications.

The case I was describing is possibly the simplest where the cyclist is cruising down a slight gradient in a straight line at constant speed with no, or almost no, force applied to the pedals. That case is hard enough to understand. When you start adding all the other variables it gets really complicated. The interesting thing is that we can figure all this out when we are about four years old, and we don't even know we are doing it!

Wonder if anyone ever made a control system, or robot, that can "go" a two wheeler? Sounds like an interesting AI challenge project. Anyone got a million quid to spare for the prize?

EDIT Neil's pretty "pally" with that Branson geezer. He should be good for a few quid, don't you think?

[LeeE]

I think we need to go back and remember the difference between static and dynamic stability.

A box sitting on the ground will be statically stable; as long as its CoG remains within the plane of contact it won't topple over.  When we walk or cycle anywhere though, we rely upon dynamic stability: in either case, while we are moving we are not statically stable, and if we were to suddenly 'freeze' we'd fall over, whether walking or riding a bicycle, but because we're moving and anticipating where we will be, even as we start to move to that point, we're already subconsciously planning how we will react to ensure that we don't fall over when we get there.  Cycling, just as with walking, is really more a case of being in a state of controlled falling rather than being in a state of stability.

If someone were to make a bicycle with fixed handlebars and forks, so that the bicycle could only go in a straight line, and someone was catapulted away on it, they'd fall over sideways just as quickly as if they were standing still.  Unless the wheels had significant mass, or were spinning at unfeasibly high speeds, the gyroscope effect would be insignificant.

[RD]

Wonder if anyone ever made a ... robot, that can "go" a two wheeler?

 [ Invalid Attachment ]

[Geezer (OP)]

Wonder if that's RD doing his famous robot impersonation?

Edit: If the gyroscope effect on a bicycle was quite large, what might happen when you actually tried to change direction?

Edit: RD, are you up for a small experiment? LeeE has something in mind involving a bike with no steering and a steam catapult. I think the idea is to launch a volunteer off the deck of an aircraft carrier or something, and I thought you were well qualified based on your past experience.

EDIT (again!!): I think I may be full of it (chorus of "We already new that!"). When you get moving at any reasonable speed, I don't think you really turn the handle bars at all. (What - Geezer's lost it.) I think you actually transfer your weight which causes the handle bars to turn. It's a subtle distiction I know, but here's the proof. You can easily cycle great distances without actually touching the handle bars. Look mum, no hands!

[lightarrow]

When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)

Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.

I'll try (sorry about the baloney comment - I could not resist it.)

Imagine you are cycling in a straight line at constant speed. Your body mass will tend to continue in a straight line. Now, the path of the tires deviates slightly so that it is no longer directly beneath your center of mass. This causes you, and the bicycle to tilt very slightly. I'm not sure exactly how the brain detects this tilt, but somehow it does. Because our brain is "trained" to maintain the bike in balance, we make a slight adjustment of the steering so that the path of the tires moves back under our center of mass.

I suspect, when we cycle in a straight line the tires actually trace a very small amplitude sine wave while our bodies in fact do travel in almost a straight line. This would be the same behaviour that a servo control system would exhibit. There is always a small error that it tries to cancel out.

The speed we travel at has some effect on the "gain" of the system - in other words, how quickly a certain steering input repositions the tire path under our center of mass. I suspect the "gain" increases with speed, but that might be baloney! If it does, when the speed is zero, the system has no gain.

Either way, you can see that if the bicycle is stationary, moving the steering is not going to be able to reposition the path of the tires relative to the center of mass of the rider (and bicycle).

Geezer, I still can't understand why all that you wrote is impossible to do when the bicycle is stationary. First answer me this question: which is the *only* difference between stationary and moving bicycle? Don't tell me that it is the bicycle speed or linear momentum because you can always take the frame of reference where the bicycle is not moving and nothing must change, for what we are considering here.