Originally Posted by
elcruxio
Why is the elbow localized but the butting points aren't? Is is because one is straight but the other isn't.
In order for the elbow to actually move during use (assuming it's conforming to the hub flange via elastic deformation) the spoke elastic stretch would have to release almost completely, ie. the spoke would have to go almost slack. That would be a problem sure, but no just for the elbow.
Here's a fun analogy. If it takes 5000N to fully compress a spring and you add 6000N of force after which you cycle the force between 5500N and 6000N, how much does the spring wear out? It doesn't, because it doesn't move. It's the same with the elbow. If it's impossible to say whether it's plastically or elastically deformed before detensioning the wheel, the elbow is "fully compressed" when tensioned.
That's a dumb analogy, not a fun one. A spoke is in tension, not compression. It is not analogous to a spring compressed until the coils are touching each other, because that is a mechanical limit. A spoke or spring in tension doesn't have a mechanical limit - it can keep stretching until it breaks. A fully compressed spring can't break from compression.
A sprung elbow can "move" for exactly the same reason that the rest of the spoke can elongate - because tension changes the shape of everything based on where the forces are applied. A straight section - like the end of the butt - is going to elongate in proportion to the amount of material. The butt isn't a stress riser because there is no bending force against it. The spoke is equally weak right next to the butt as it is mid spoke.
The sprung elbow is touching the flange, but is not necessarily at its mechanical limit in how short a distance it can make that curve. So as tension increases, that partially collapsed curve does get smaller and flatter, trying to compress into the flange, and it is doing that under a strain that is greater on the outside of the curve than the inside. So as tension changes, the bend experiences changes in that relative strain.
Which is why we are having a conversation about elbows breaking, instead of having a conversation about why elbows never break. If you actually believed your BS, you would say that there is no force that could break an elbow.