3 ms·

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 t

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.
    - johnwalkr 3y ago
      That you see more change in tension in the bottom few spokes is irrelevant to the explanation of how the wheel supports a bike. My professional experience is this area (preload of structures, beard turning grey) and I had to think a bit to gain confidence in my thinking and explanations. I'm not replying to prove you are wrong, I am an expert in this field and I am enjoying trying to elaborate on my reply. I understand that there is significant change (reduction in tension) in only a few spokes, but there _must_ also be a corresponding increase in tension to other spokes to keep equilibrium. Not distributed equally but absolutely distributed in a way that keeps equilibrium of forces within the wheel. It's distributed among many more spokes because that's how structures work. The rim's stiffness determines how this happens. A mechanical engineer designing a wheel knows that the wheel fails as soon as the bottom spokes have zero tension and fail by buckling. And the analysis works if you assume no buckling, ignore most of the spokes and just check the "red areas" in FEM or hand calculations and make sure the compression forces are less than the design preload. But this doesn't mean that in the physical world, the bottom spokes are "pushing up the bike". It's just a convenience of the problem. So let's simplify things. Imagine if there was only one spoke on the top and one spoke on the bottom, and the wheel is locked from rotating. Consider each spoke is a tension spring in tension, because they are. They can provide a tension force but are almost as useless as a string in compression (indeed, one of my comments linked to a new type of spoke that is literally a string). Each spoke is installed by tightening it to stretch it slightly and make a preload according to Hooke's law. Each spoke is tightened to get about 1000N of tension preload[1]. Note here that it's impossible to do this exercise with only one spoke, at least two spokes are required to tension them. State 1: The bike and wheel are held off the ground, and the wheel has negligible mass so each spoke has 1000N of tension. State 2: A person gets on the bike, resulting in a 500N force at the bottom of the wheel. This is roughly correct for a 100kg person, if front and rear wheel are loaded equally. The bottom spoke will compress and have a resulting 500N of tension preload. The top spoke will stretch and have a resulting 1500N of tension preload. Both spokes are still in tension and _pulling_ the hub to the wheel. But clearly the top spoke is stretched more than State 1 and the bottom spoke is stretch less than state 1. More stretch of a spring = more force, but less stretch of a spring = less force. How can it be said that the bottom spoke is holding the wheel up if basic physics says it is providing less force than before? State 3: leave the top spoke and cut the bottom spoke. The top spoke's preload goes to zero and the top spoke will actually have its tension reduced from 1500N to 500N. State 4: leave the bottom spoke and cut the top spoke. The bottom spoke's preload goes to zero and the bottom spoke will experience 500N in compression, immediately buckle and the rider will fall. [1]https://www.parktool.com/en-us/blog/repair-help/wheel-tension-measurement https://www.parktool.com/en-us/blog/repair-help/wheel-tensio...

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.
  - johnwalkr 3y ago
    That you see more change in tension in the bottom few spokes is irrelevant to the explanation of how the wheel supports a bike. My professional experience is this area (preload of structures, beard turning grey) and I had to think a bit to gain confidence in my thinking and explanations. I'm not replying to prove you are wrong, I am an expert in this field and I am enjoying trying to elaborate on my reply. I understand that there is significant change (reduction in tension) in only a few spokes, but there _must_ also be a corresponding increase in tension to other spokes to keep equilibrium. Not distributed equally but absolutely distributed in a way that keeps equilibrium of forces within the wheel. It's distributed among many more spokes because that's how structures work. The rim's stiffness determines how this happens. A mechanical engineer designing a wheel knows that the wheel fails as soon as the bottom spokes have zero tension and fail by buckling. And the analysis works if you assume no buckling, ignore most of the spokes and just check the "red areas" in FEM or hand calculations and make sure the compression forces are less than the design preload. But this doesn't mean that in the physical world, the bottom spokes are "pushing up the bike". It's just a convenience of the problem. So let's simplify things. Imagine if there was only one spoke on the top and one spoke on the bottom, and the wheel is locked from rotating. Consider each spoke is a tension spring in tension, because they are. They can provide a tension force but are almost as useless as a string in compression (indeed, one of my comments linked to a new type of spoke that is literally a string). Each spoke is installed by tightening it to stretch it slightly and make a preload according to Hooke's law. Each spoke is tightened to get about 1000N of tension preload[1]. Note here that it's impossible to do this exercise with only one spoke, at least two spokes are required to tension them. State 1: The bike and wheel are held off the ground, and the wheel has negligible mass so each spoke has 1000N of tension. State 2: A person gets on the bike, resulting in a 500N force at the bottom of the wheel. This is roughly correct for a 100kg person, if front and rear wheel are loaded equally. The bottom spoke will compress and have a resulting 500N of tension preload. The top spoke will stretch and have a resulting 1500N of tension preload. Both spokes are still in tension and _pulling_ the hub to the wheel. But clearly the top spoke is stretched more than State 1 and the bottom spoke is stretch less than state 1. More stretch of a spring = more force, but less stretch of a spring = less force. How can it be said that the bottom spoke is holding the wheel up if basic physics says it is providing less force than before? State 3: leave the top spoke and cut the bottom spoke. The top spoke's preload goes to zero and the top spoke will actually have its tension reduced from 1500N to 500N. State 4: leave the bottom spoke and cut the top spoke. The bottom spoke's preload goes to zero and the bottom spoke will experience 500N in compression, immediately buckle and the rider will fall. [1]https://www.parktool.com/en-us/blog/repair-help/wheel-tension-measurement https://www.parktool.com/en-us/blog/repair-help/wheel-tensio...