6 ms·

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

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 3y 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 3y 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 3y 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 3y 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 3y 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.
        ndsipa_pomu 3y ago
        The forces aren't distributed equally around the spokes. There's barely any change in force to the spokes above the hub, and the majority of the change in force when putting a load on a wheel is in the three or so spokes below the hub. For the hub to be considered hanging from the rim, then there should be a significant change in the tension of the upper spokes when the wheel is loaded, but that does not happen. It may sound funny, but the reality is that there is only a significant change in tension in the bottom spokes.
  - ndsipa_pomu 3y ago
    However, if you model/examine the forces involved, the load is carried by the spokes under the hub and the spokes do not experience greater load when they are at the top. From https://www.cyclist.co.uk/in-depth/the-science-behind-spokes https://www.cyclist.co.uk/in-depth/the-science-behind-spokes > There’s vigorous disagreement over whether a bike in effect hangs from the upper spokes (those above the hub as you view the bike from the side) or rather is being supported by the lower ones, acting like tiny pillars. ‘The latter view, odd as it seems, is definitively the case,’ says Jim Papadopoulos from Northeastern University’s College of Engineering in Boston, USA, and the co-author of Bicycling Science. > While it’s easy to believe a bicycle spoke would simply collapse under the weight of bike and rider, he goes on to explain that the tension created in a spoke during the wheel building process (called ‘pre-tension’) is what allows the lower spokes to bear the load without buckling, as they would if there was no pre-tension. ‘Every spoke on the unloaded wheel has a tension of the order of 100lb [445N]. When the axle is pressed towards the ground with a force of 100lb, the only significant effect on spoke tensions is to reduce those directly below the hub – typically, one reduces to about 50lb and spokes to each side of that one reduce to about 75lb. This is exactly what one would see with solid wooden spokes like an old wagon wheel – the bottom one would carry 50lb and those to either side of it would carry 25lb. The difference with wire spoked wheels is that a wire spoke cannot carry a compression load – it will collapse. So all spokes are ingeniously pre-tensioned. A wire cannot carry a compression load of 50lb, except when it already carries a tension load exceeding that.
    - mitthrowaway2 3y ago
      I'm not sure if you realize that you're agreeing with me. Go ahead and apply enough force to the hub to exactly cancel out the preload. Then when the bottom spoke has zero tension, cut it, and observe that the bicycle is unaffected. Is your position that the load is then being carried by a spoke that doesn't even exist? If so that's fine, but it is a stretch to call any other view a misconception.
      - ndsipa_pomu 3y ago
        I don't believe we are agreeing. In your scenario, I'd expect the wheel to either collapse or to spread the load to the remaining spokes below the hub. It's not a separate system where you can remove a spoke without affecting the whole structure (though you can remove some spokes and the wheel will still function, but won't be as strong). The easiest way to test it is to pluck the spokes when no-one is sat astride a bike to determine the rough tension/pitch of the spokes (choose just one side if it's the rear wheel as spokes will be at different tensions for the different sides if the wheel is dished). Then get a friend (or enemy) to sit on the bike and again pluck the spokes to see if the extra load has resulted in an increase of tension in the upper spokes (i.e. hanging from the top of the rim) or whether the lower spokes have a decrease in tension (i.e. supported by the bottom spokes with a compressive load).

ndsipa_pomu 3y 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 3y 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 3y 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 3y 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 3y 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.
      - ndsipa_pomu 3y ago
        The forces aren't distributed equally around the spokes. There's barely any change in force to the spokes above the hub, and the majority of the change in force when putting a load on a wheel is in the three or so spokes below the hub. For the hub to be considered hanging from the rim, then there should be a significant change in the tension of the upper spokes when the wheel is loaded, but that does not happen. It may sound funny, but the reality is that there is only a significant change in tension in the bottom spokes.
- ndsipa_pomu 3y ago
  However, if you model/examine the forces involved, the load is carried by the spokes under the hub and the spokes do not experience greater load when they are at the top. From https://www.cyclist.co.uk/in-depth/the-science-behind-spokes https://www.cyclist.co.uk/in-depth/the-science-behind-spokes > There’s vigorous disagreement over whether a bike in effect hangs from the upper spokes (those above the hub as you view the bike from the side) or rather is being supported by the lower ones, acting like tiny pillars. ‘The latter view, odd as it seems, is definitively the case,’ says Jim Papadopoulos from Northeastern University’s College of Engineering in Boston, USA, and the co-author of Bicycling Science. > While it’s easy to believe a bicycle spoke would simply collapse under the weight of bike and rider, he goes on to explain that the tension created in a spoke during the wheel building process (called ‘pre-tension’) is what allows the lower spokes to bear the load without buckling, as they would if there was no pre-tension. ‘Every spoke on the unloaded wheel has a tension of the order of 100lb [445N]. When the axle is pressed towards the ground with a force of 100lb, the only significant effect on spoke tensions is to reduce those directly below the hub – typically, one reduces to about 50lb and spokes to each side of that one reduce to about 75lb. This is exactly what one would see with solid wooden spokes like an old wagon wheel – the bottom one would carry 50lb and those to either side of it would carry 25lb. The difference with wire spoked wheels is that a wire spoke cannot carry a compression load – it will collapse. So all spokes are ingeniously pre-tensioned. A wire cannot carry a compression load of 50lb, except when it already carries a tension load exceeding that.
  - mitthrowaway2 3y ago
    I'm not sure if you realize that you're agreeing with me. Go ahead and apply enough force to the hub to exactly cancel out the preload. Then when the bottom spoke has zero tension, cut it, and observe that the bicycle is unaffected. Is your position that the load is then being carried by a spoke that doesn't even exist? If so that's fine, but it is a stretch to call any other view a misconception.
    - ndsipa_pomu 3y ago
      I don't believe we are agreeing. In your scenario, I'd expect the wheel to either collapse or to spread the load to the remaining spokes below the hub. It's not a separate system where you can remove a spoke without affecting the whole structure (though you can remove some spokes and the wheel will still function, but won't be as strong). The easiest way to test it is to pluck the spokes when no-one is sat astride a bike to determine the rough tension/pitch of the spokes (choose just one side if it's the rear wheel as spokes will be at different tensions for the different sides if the wheel is dished). Then get a friend (or enemy) to sit on the bike and again pluck the spokes to see if the extra load has resulted in an increase of tension in the upper spokes (i.e. hanging from the top of the rim) or whether the lower spokes have a decrease in tension (i.e. supported by the bottom spokes with a compressive load).