Originally Posted by
cyccommute
Your guitar example has described, almost exactly, what happens in a wheel. When you load a wheel with weight, you press on the rim but not the spokes just as you press on the guitar neck and sound box but not the strings. This reduces the tension on the spokes as it does the strings but doesn't result in compression of the strings or spokes.
Then I think we've reached an agreement of sorts. You seem to be aware at least what happens in a physical sense, just not of how this is commonly described (again I am reminded of why I am not a logical positivist, but that's neither here nor there). Opposing forces can, actually, operate
'in the same plane'. Just look at a bouncing basketball. Elasticity of the ball, gravity, the push of my hand, the elasticity of the floor, all in the same up/down line. And since tension and compression are forces, they can operate in the same plane. Remember that they do have components, like many other forces. But you don't always need to know these to solve a particular problem, just the resulting force or relevant component will do.
Every example that you have used is an example of tension reduction and not compression.
Reduction of tension is a negative change in tension. Negative tension is effectively compression, because tension cannot actually be negative.
This is what I meant when I said that we seem to actually agree on this: the net stress within the spoke is tension, which keeps it in place. If you can only wrap your head around it if I call it 'tension reduction' that is fine by me, because the physical reality is the same. There is no change in tension in the upper spokes and a reduction in tension in the lower spokes. If a bike 'hangs' in the upper spokes, those spokes would be subject to more tensional stress. As this is not the case, the bike does not hang in the upper spokes.
There is nothing "pushing" on the end of the spoke.
And that is what makes a bicycle wheel a remarkable thing. If I cut all spokes except the lowest ones, then those floating nipples of yours will cause the spokes to be rammed into the tube/tyre - and some other unpleasantness as well. But, as you aptly described at the beginning of this discussion, spokes are locked firmly in place by pretensioning them. This makes the wheel behave like a wheel. To stick to the lingo that you prefer, the weight of the bike is absorbed as a reduction in tension by the lower spokes. And surely the rim also, which I have mostly disregarded because it is not important for the question regarding if the bike hangs in the upper spokes.
You are absolutely wrong that you can push on both ends of a tensioned rod and still keep the rod under tension if the forces are in the same plane. They are forces in the opposite directions.
So what? Lots of forces are in opposite directions. If that was impossible, everything would be accellerating all the time in every direction. What a strange world that would be.
Aristotle thought that a constant force on an object made it move at a constant speed. Newton buried that. If something moves at a constant speed, forces
have to be operating along the same axis in opposite direction. And if some more force is exerted the object accelerates. Just like other forces, tension and compression can operate along the same axis.
(I think a 'plane' is actually a surface? Forces are typically vectors, so they'd be more like a line rather than a plane...)