Very interesting, Joerg. One thing on theory, though. There is a quiet revolution in physics going on with ideas about order-disorder and entropy. Very briefly, you haven't increased the order in the bands by stretching them. It seems that way unless you think about it more closely. You have long molecules in rubber that are crosslinked at random positions. In a resting state these long molecules are coiled and bent on themselves in a random way, true, and when you stretch them out it sounds like that would be more orderly- but in fact what you have done is make them start vibrating and jerking and affecting each other like demented violin strings- there is a lot of chaos going on. That is why the rubber heats up. More chaos than before, order has not increased. Where a particular element in a particular molecular chain might be in relationship to the others, which is how you look at order-disorder, has a greatly increased number of probabilities with all this motion. And if you imagine holding on to a rope tied to something- like a jump rope- and then imagine someone swinging that rope, that swinging will increase the tension, right? Same thing for all the molecular chains in the rubber. The cooling comes from these vibrations reducing, energy dissipating, and the overall tension reduces. Heat it back up, the vibrations increase, and the tension returns. It is actually a simpler way of understanding what is going on... Unfortunately it flies in the face of what has previously been taught, the wiki article being an example, but concepts about entropy-order disorder are already being taught differently in some places. Changes in thought like this are seldom neat, as people previously taught one way can have a very hard time letting go of it. But it is happening. There is a professor of chemistry named Lambert who has been quite influential about spearheading this change of thought. He says at least twenty new textbooks have been changed... If you want you can do a search on him and possibly get more information than you really wanted. ;-) I am an engineer, but wasn't taught this, I was taught the old way about order-disorder and entropy. I started seriously questioning relationships between order-disorder and entropy on my own back in 1990, but with regard to other topics, not rubber, not chemistry so much, either. I was interested in how order-disorder related to questions about life... I don't know if Lambert talks specifically about rubber, either, though he might. Last I knew, he wasn't at all interested in relating it to life, either. But for me it is all similar, just a matter of understanding basic concepts of how things move.
I know this is really going off topic and I'll understand if the moderator doesn't want it to stay. I am interested in crossbows, have made a bunch of them over the years with some unique designs, but never really satisfied with them. Constantly changing things. My last design seems to have solved a number of previous issues and if that proves out, I hope to post something about it eventually. I've been busy with the implications of this understanding about order-disorder and entropy in most of my free time since 1990 and don't spend as much time on crossbows or other projects as I might otherwise do. The word "entropy", obviously grabs my attention because of that. I'm also not much into rubber slingshots, but you certainly take them down some interesting paths, Joerg.