# The Naked Scientists Forum

### Author Topic: Why is it easier to balance on a bicycle that is moving?  (Read 9805 times)

#### chris

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##### Why is it easier to balance on a bicycle that is moving?
« on: 25/01/2008 12:51:42 »
What is a moving bike so much easier to balance than one which is held stationary?

Chris

#### lightarrow

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #1 on: 25/01/2008 13:15:41 »
What is a moving bike so much easier to balance than one which is held stationary?

Chris

Physics of bycycle movement is very complicated. There are essentially two effects of stability: the first is gyroscopic, the second is due to how all the forces act on the front wheel considered that the front fork is made in a specific way and with an inclination. The firs effect is the same that prevent a whirligig from falling down when in motion, that is angular momentum conservation.

#### Soul Surfer

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #2 on: 26/01/2008 18:56:15 »
The critical feature is that the inital motion of the forks as the bicycle is leaning is to correct the lean by steering into it an effect of negative feedback.

#### lyner

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #3 on: 26/01/2008 19:01:38 »
We did this one recently, didn't we?
I don't believe gyroscopic action is significant with lightweight cycle wheels because the moment of inertia is so small. The stability is there for very small wheels with even less anguylar momentum. In any case, the precessional moment would be in difference senses, depending on whether you tipped to the left of to the right; it is due to a cross product of vectors.
What I think is (have just read s-s's contribution) that the front forks are curved so that wheel spindle is not in line with the top bracket (shaft from handlebars). When you tip to one side, the wheel steers itself 'into the curve' a bit. You are then traveling forward with a force from the road pushing against your direction of motion.  This provides a moment which will right the cycle. The faster you are going, the stronger is the force and the more stable you are.

#### lightarrow

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #4 on: 26/01/2008 19:23:58 »
We did this one recently, didn't we?
I don't believe gyroscopic action is significant with lightweight cycle wheels because the moment of inertia is so small.
Take even a small bycicle wheel alone, put it on vertical on the road and give it a movement; you will see it won't fall down until its speed is almost zero.

This man has made a deep study on bicycle stability. The results are quite complicated:
http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys7221/vol59no9p51_56.pdf
A video showing bicycle auto-stability:
« Last Edit: 26/01/2008 19:54:20 by lightarrow »

#### lyner

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #5 on: 28/01/2008 16:06:20 »
Quote
Take even a small bicycle wheel alone, put it on vertical on the road and give it a movement; you will see it won't fall down until its speed is almost zero.
Yes - I agree. BUT, in the case of an isolated wheel, the moment of inertia is the same about 'both' axes and the correcting moment would be large enough.  In any case, the gyroscopic action doesn't actually keep the wheel upright, directly. As the wheel tips to the left, say, (about a horizontal axis in the direction of motion)  precession makes it steer to the left (about a vertical axis) because of the cross product of the two rotation vectors. Then you get a righting moment due to the friction against the ground. A wheel on a smooth surface would fall over very quickly; the friction is necessary to provide a correcting moment. The faster the wheel is spinning, the more stable the situation because the resulting correcting moment will be greater for a given tilt.
btw, I was wrong to dismiss, completely the gyroscopic effect in my earlier post; it is just very small, by comparison with the effect from the fork design, because of the massive M.I. of the bike and rider.
The forwards offset of the forks must be the major factor. People have made bikes which are virtually impossible to ride by having a zero offset or, even a rearwards offset.

#### lightarrow

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #6 on: 28/01/2008 18:07:37 »
Quote
Take even a small bicycle wheel alone, put it on vertical on the road and give it a movement; you will see it won't fall down until its speed is almost zero.
Yes - I agree. BUT, in the case of an isolated wheel, the moment of inertia is the same about 'both' axes and the correcting moment would be large enough.  In any case, the gyroscopic action doesn't actually keep the wheel upright, directly. As the wheel tips to the left, say, (about a horizontal axis in the direction of motion)  precession makes it steer to the left (about a vertical axis) because of the cross product of the two rotation vectors. Then you get a righting moment due to the friction against the ground. A wheel on a smooth surface would fall over very quickly; the friction is necessary to provide a correcting moment. The faster the wheel is spinning, the more stable the situation because the resulting correcting moment will be greater for a given tilt.
btw, I was wrong to dismiss, completely the gyroscopic effect in my earlier post; it is just very small, by comparison with the effect from the fork design, because of the massive M.I. of the bike and rider.
The forwards offset of the forks must be the major factor. People have made bikes which are virtually impossible to ride by having a zero offset or, even a rearwards offset.
I don't believe it until I see it!
(No, seriously, I believe it if you say so, it's just that I would like to improve my intuition on it).
« Last Edit: 28/01/2008 18:09:43 by lightarrow »

#### lyner

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #7 on: 28/01/2008 21:54:37 »
Take a toy gyroscope. Run it at the sort of speed that a bicycle wheel  rotates and stand it so that its rotational axis is just-off vertical. Let it go and it will fall over - precessing on the way down. To make it stand up (or, to be accurate, to precess around in a small angle to the vertical), it would have to be rotating at significant speed.
For a bicycle wheel to stay up, after tilting through a similar angle, you need some extra moment to keep it up. Also, remember that the resulting couple on the wheel is not actually against the tilt but around a different axis (normal to both existing axes) i.e. tending to steer the wheel, not pull it up. That cross product thing.
Rotational motion is by far the hardest to understand; it completely confused me at uni, I remember. I only sussed some of this stuff out much later - and I have a long way to go still. It is just not intuitive.

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##### Why is it easier to balance on a bicycle that is moving?
« Reply #7 on: 28/01/2008 21:54:37 »