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You are exactly right. This article suffers the common misconception that two wheels spinning in opposite directions and same speed cancel out the gyroscopic st

by hristov 16y ago

You are exactly right. This article suffers the common misconception that two wheels spinning in opposite directions and same speed cancel out the gyroscopic stability. And this is simply not true. In fact two wheels spinning in opposite directions and same speeds will actually double the gyroscopic stability (as opposed to one wheel of the same speed). However, two wheels spinning in opposite directions may cancel out other spinning wheel effects.

jules 16y ago
Which other spinning wheel effects?
- yread 16y ago
  Precession
  - jules 16y ago
    Precession is caused by the gyroscopic effect.
    - lincolnq 16y ago
      Wait, so if you have two identical wheels on one axis, spinning in opposite directions, and you rotate the axis, are you saying you'll experience precession or you won't? My intuition (and physics understanding) says you won't. But the top-level post is saying you will still experience "gyroscopic stability". Is that true?
      - RiderOfGiraffes 16y ago
        You're right, your understand is right, you won't experience "gyroscopic stability." I've done it. You don't. I believe that people who believe otherwise are working on misunderstandings of perceptions. It's nototiously easy to confuse several effects in this. Careful experiments are required with tight controls on error bars, etc. It's easy to be fooled. Eric Laithwaite is a classic and high profile example: http://en.wikipedia.org/wiki/Eric_Laithwaite http://en.wikipedia.org/wiki/Eric_Laithwaite
      - jules 16y ago
        You don't. That's because the gyroscopic effect cancels out in that case. But if you use a single spinning wheel you will. I'm just saying that precession isn't a different effect than the gyroscopic effect; it's a specific case of it. The same forces that cause a wheel to feel like it's hard to turn (i.e. the "gyroscopic effect") cause precession. I asked: > Which other spinning wheel effects? [other than the gyroscopic effect] And he replied: > Precession To which I replied that precession is not a different effect. I did not mean to imply that precession happens when you have two wheels spinning in opposite directions, in fact I believe it does not because the forces that cause a single spinning wheel to precess cancel out if you have two wheels spinning in opposite directions.
- hristov 16y ago
  Another effect is the tendency to tilt to the side if you accelerate or decelerate the rate of spin. This effect will be canceled by counter spinning wheels.
  - jules 16y ago
    Can you elaborate on this? I have never seen a wheel tilt when you accelerate or decelerate it.
    - deleted 16y ago
      [deleted]
RiderOfGiraffes 16y ago
It is a common belief that two identical wheels spinning in opposite directions and the same speed have a net zero gyroscopic stability. The basis for this belief is usually false, but the assertion is true. I've done both the math and the experiment - it's in one of the lectures I give on "Spinning Things". In short, I believe you to be wrong, and this is based on my personal experiments and calculations. I would be interested to know the basis of your claim. When computed in detail, gyroscopic stability is really the effect that torque applied in one plane manifests itself as movement in another. The direction of this movement depends on the direction of spin, and they cancel each other out when you have two wheels spinning in opposite directions. My independent experiments and calculations were done before I read anything formal about it, and it agrees with the work of people who design devices that run on satellites and the ISS. I'd need a lot of convincing that I'm wrong. This is independent of the other effects that help to keep a bike upright, such as the steering geometry. I have experience of that too, having raced bikes with very small rake, and toured on bikes with a larger rake. The difference is unmistakeable, and unrelated to the gyroscopic issues.
- hristov 16y ago
  Well here is an experiment that I have done. I used a toy double blade helicopter, like these ones: http://www.rc-airplane-world.com/coaxial-rc-helicopters.html http://www.rc-airplane-world.com/coaxial-rc-helicopters.html And it sure seemed to have a lot of rotational inertia. If you were right, then I could touch one side of the helicopter slightly and since it has no rotational inertia it would tilt to the side and completely change direction and fly off somewhere into the wall or the floor. This did not happen however, even when I tried to tilt it the helicopter remained horizontal. And please do not tell me that a $50 toy has an active stability system with internal gyros and all the required electronics and super fast servos. And now that I think of it, I remember that the russians have been making counter rotational helicopters for ages. See for example this one: http://en.wikipedia.org/wiki/Kamov_Ka-25 http://en.wikipedia.org/wiki/Kamov_Ka-25 How does this helicopter stay horizontally stable in the air and does not fly off on a tangent when tilted by the slightest cross wind? Note that this one was designed in the late 50's before any electronic active stability systems could have been invented.

by hristov 16y ago

jules 16y ago

Which other spinning wheel effects?

yread 16y ago
Precession
- jules 16y ago
  Precession is caused by the gyroscopic effect.
  - lincolnq 16y ago
    Wait, so if you have two identical wheels on one axis, spinning in opposite directions, and you rotate the axis, are you saying you'll experience precession or you won't? My intuition (and physics understanding) says you won't. But the top-level post is saying you will still experience "gyroscopic stability". Is that true?
    - RiderOfGiraffes 16y ago
      You're right, your understand is right, you won't experience "gyroscopic stability." I've done it. You don't. I believe that people who believe otherwise are working on misunderstandings of perceptions. It's nototiously easy to confuse several effects in this. Careful experiments are required with tight controls on error bars, etc. It's easy to be fooled. Eric Laithwaite is a classic and high profile example: http://en.wikipedia.org/wiki/Eric_Laithwaite http://en.wikipedia.org/wiki/Eric_Laithwaite
    - jules 16y ago
      You don't. That's because the gyroscopic effect cancels out in that case. But if you use a single spinning wheel you will. I'm just saying that precession isn't a different effect than the gyroscopic effect; it's a specific case of it. The same forces that cause a wheel to feel like it's hard to turn (i.e. the "gyroscopic effect") cause precession. I asked: > Which other spinning wheel effects? [other than the gyroscopic effect] And he replied: > Precession To which I replied that precession is not a different effect. I did not mean to imply that precession happens when you have two wheels spinning in opposite directions, in fact I believe it does not because the forces that cause a single spinning wheel to precess cancel out if you have two wheels spinning in opposite directions.
hristov 16y ago
Another effect is the tendency to tilt to the side if you accelerate or decelerate the rate of spin. This effect will be canceled by counter spinning wheels.
- jules 16y ago
  Can you elaborate on this? I have never seen a wheel tilt when you accelerate or decelerate it.
  - deleted 16y ago
    [deleted]

RiderOfGiraffes 16y ago

It is a common belief that two identical wheels spinning in opposite directions and the same speed have a net zero gyroscopic stability. The basis for this belief is usually false, but the assertion is true. I've done both the math and the experiment - it's in one of the lectures I give on "Spinning Things". In short, I believe you to be wrong, and this is based on my personal experiments and calculations. I would be interested to know the basis of your claim. When computed in detail, gyroscopic stability is really the effect that torque applied in one plane manifests itself as movement in another. The direction of this movement depends on the direction of spin, and they cancel each other out when you have two wheels spinning in opposite directions. My independent experiments and calculations were done before I read anything formal about it, and it agrees with the work of people who design devices that run on satellites and the ISS. I'd need a lot of convincing that I'm wrong. This is independent of the other effects that help to keep a bike upright, such as the steering geometry. I have experience of that too, having raced bikes with very small rake, and toured on bikes with a larger rake. The difference is unmistakeable, and unrelated to the gyroscopic issues.

hristov 16y ago
Well here is an experiment that I have done. I used a toy double blade helicopter, like these ones: http://www.rc-airplane-world.com/coaxial-rc-helicopters.html http://www.rc-airplane-world.com/coaxial-rc-helicopters.html And it sure seemed to have a lot of rotational inertia. If you were right, then I could touch one side of the helicopter slightly and since it has no rotational inertia it would tilt to the side and completely change direction and fly off somewhere into the wall or the floor. This did not happen however, even when I tried to tilt it the helicopter remained horizontal. And please do not tell me that a $50 toy has an active stability system with internal gyros and all the required electronics and super fast servos. And now that I think of it, I remember that the russians have been making counter rotational helicopters for ages. See for example this one: http://en.wikipedia.org/wiki/Kamov_Ka-25 http://en.wikipedia.org/wiki/Kamov_Ka-25 How does this helicopter stay horizontally stable in the air and does not fly off on a tangent when tilted by the slightest cross wind? Note that this one was designed in the late 50's before any electronic active stability systems could have been invented.