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Hmmm, site seems a bit slow - must be the HN hug. I love the bizarre contraptions/models made to demonstrate tensegrity, but they always seem very impractical.

Hmmm, site seems a bit slow - must be the HN hug. I love the bizarre contraptions/models made to demonstrate tensegrity, but they always seem very impractical. However, they've got pages on bicycle wheels and suspension bridges though the term "tensegrity" is rarely mentioned with them. I do have issue with their description of bicycle wheels though: > It makes no difference where you compress the rim of the cycle wheel; the load is equally distributed through the spokes to the hub. The load is not distributed equally, but the spokes directly below the hub will be under greater compressive stress (though still in tension).

dcanelhas 4y ago
Since the spokes would buckle at the merest whiff of compressive stress, we can assume the hub to be "hanging" by the spokes that attach to the top of the wheel. However, if that was the only force acting on the wheel, it would allow the rim to deform into an ellipse, so the spokes that radiate sideways are also under tension to hold the rim into a circular shape. I think the description of even load might not be too far off, but I haven't looked at it in depth.
- ndsipa_pomu 4y ago
  > we can assume the hub to be "hanging" by the spokes that attach to the top of the wheel That's a common misconception of how bike wheels work, but it's more accurate to consider the hub to be propped up by the reduction in tension in the spokes below it. There's a more detailed analysis here: https://www.astounding.org.uk/ian/wheel/ https://www.astounding.org.uk/ian/wheel/ > From these figures, I conclude that it is perfectly reasonable to say that the hub stands on the lower spokes, and that it does not hang from the upper spokes. It is also wrong to say that the force distributes all around the rim and all the spokes contribute to holding up the hub - over a third of the spokes have an effect that pulls the hub down!
  - mitthrowaway2 4y ago
    It's fine to subtract out the preload if you're talking about stiffness rather than simply strength, and that's a valid way of using words and a valid way of analyzing a bike wheel, but not so uniquely-valid that hanging from the top spokes can be called a misconception.
    - johnwalkr 4y ago
      Yeah, the link ignoring preload (but using preload as an argument as to why the spokes don’t buckle) to say the lower spokes hold the bike up isn’t convincing to me. The lower spokes are still in tension during normal loads, therefore they are pulling the hub down, not pushing it up. And the rubber band thought experiment is no longer required when these rope-like spokes are now available.[1] [1] https://berdspokes.com/collections/spokes https://berdspokes.com/collections/spokes
      - ndsipa_pomu 4y ago
        From: http://disco80.at.ua/book/bicycle_wheel_jobst_brandt.pdf http://disco80.at.ua/book/bicycle_wheel_jobst_brandt.pdf > A wheel with wire spokes works the same as one with wooden spokes except that the built-in force in its spokes is different. In a wooden-spoked wheel, force is transmitted from the ground to the hub by compressing the bottom spoke. This spoke becomes shorter as it furnishes the upward force to the hub. As in a wooden-spoked wheel, the bottom spokes of a wire wheel become shorter under load, but instead of gaining in compression, they lose tension. With the same load, the net change in force is the same for both wheels. The algebraic sum of negative and positive forces (compression and tension) is the same. > That the bottom spokes support the wheel need not be taken on faith. An experiment will show that only a few spokes at the bottom of the wheel are affected by a vertical load. The relative tension of a spoke can be found by plucking it like a guitar string. The pitch of a spoke, just as the pitch of a guitar string, increases with more tension and decreases with less tension.
        johnwalkr 4y ago
        I’m still not convinced that a significantly large decrease in tension of a few lower spokes and more distributed increase in tension of other spokes means that the bottom ones are logically pushing up on the hub. That difference in forces is just the nature of a point load being reacted to at a distant location. If you push on a string or spoke that is in tension, it doesn’t push back, it just pulls less. Maybe the issue in trying to describe this system is that saying the wheel hangs from the upper spokes is also not a totally correct simplification. It’s more correct to say all the spokes are in tension and in equilibrium with each other.

dcanelhas 4y ago

Since the spokes would buckle at the merest whiff of compressive stress, we can assume the hub to be "hanging" by the spokes that attach to the top of the wheel. However, if that was the only force acting on the wheel, it would allow the rim to deform into an ellipse, so the spokes that radiate sideways are also under tension to hold the rim into a circular shape. I think the description of even load might not be too far off, but I haven't looked at it in depth.

ndsipa_pomu 4y ago
> we can assume the hub to be "hanging" by the spokes that attach to the top of the wheel That's a common misconception of how bike wheels work, but it's more accurate to consider the hub to be propped up by the reduction in tension in the spokes below it. There's a more detailed analysis here: https://www.astounding.org.uk/ian/wheel/ https://www.astounding.org.uk/ian/wheel/ > From these figures, I conclude that it is perfectly reasonable to say that the hub stands on the lower spokes, and that it does not hang from the upper spokes. It is also wrong to say that the force distributes all around the rim and all the spokes contribute to holding up the hub - over a third of the spokes have an effect that pulls the hub down!
- mitthrowaway2 4y ago
  It's fine to subtract out the preload if you're talking about stiffness rather than simply strength, and that's a valid way of using words and a valid way of analyzing a bike wheel, but not so uniquely-valid that hanging from the top spokes can be called a misconception.
  - johnwalkr 4y ago
    Yeah, the link ignoring preload (but using preload as an argument as to why the spokes don’t buckle) to say the lower spokes hold the bike up isn’t convincing to me. The lower spokes are still in tension during normal loads, therefore they are pulling the hub down, not pushing it up. And the rubber band thought experiment is no longer required when these rope-like spokes are now available.[1] [1] https://berdspokes.com/collections/spokes https://berdspokes.com/collections/spokes
    - ndsipa_pomu 4y ago
      From: http://disco80.at.ua/book/bicycle_wheel_jobst_brandt.pdf http://disco80.at.ua/book/bicycle_wheel_jobst_brandt.pdf > A wheel with wire spokes works the same as one with wooden spokes except that the built-in force in its spokes is different. In a wooden-spoked wheel, force is transmitted from the ground to the hub by compressing the bottom spoke. This spoke becomes shorter as it furnishes the upward force to the hub. As in a wooden-spoked wheel, the bottom spokes of a wire wheel become shorter under load, but instead of gaining in compression, they lose tension. With the same load, the net change in force is the same for both wheels. The algebraic sum of negative and positive forces (compression and tension) is the same. > That the bottom spokes support the wheel need not be taken on faith. An experiment will show that only a few spokes at the bottom of the wheel are affected by a vertical load. The relative tension of a spoke can be found by plucking it like a guitar string. The pitch of a spoke, just as the pitch of a guitar string, increases with more tension and decreases with less tension.
      - johnwalkr 4y ago
        I’m still not convinced that a significantly large decrease in tension of a few lower spokes and more distributed increase in tension of other spokes means that the bottom ones are logically pushing up on the hub. That difference in forces is just the nature of a point load being reacted to at a distant location. If you push on a string or spoke that is in tension, it doesn’t push back, it just pulls less. Maybe the issue in trying to describe this system is that saying the wheel hangs from the upper spokes is also not a totally correct simplification. It’s more correct to say all the spokes are in tension and in equilibrium with each other.