At the 'bottom' of the sweet spot
At the 'top' of the sweet spot
People don't understand why staying within the sweet spot is important.
I am currently weaving on a Baby Wolf which is a small floor loom with short distance between breast and back beam. It is a jack loom so the shed is formed by the shafts rising.
At the beginning of the sweet spot, the angle the threads are forming is about 20 degrees and the bottom of the shed floor is pretty much on the shuttle race. The shed is nice and clear and large. Plenty of room for the shuttle to pass through.
At the top end of the sweet spot, the angle the threads are opening is greater - about 28 degrees. (I didn't do a really scientific measurement, just an approximate.)
While the shed is still big enough for the shuttle to pass through, it is clear that the bottom of the shed has lifted significantly from the shuttle race and the threads are under more stress because of the more acute angle at which they are opening.
If I pushed beyond this point, the shed would become harder to open (require more foot pounds) and it would also become smaller. The bottom of the shed would continue to lift away from the shuttle race making it more likely that the shuttle would 'submarine' and pick up threads from the shed floor.
By pushing beyond the sweet spot, there is more likelihood that threads would break, especially the selvedge ends, which are under more stress than the rest of the warp to begin with.
Another thing that happens is that the warp tightens ever so slightly as the warp ends take up more and more. The force needed to beat the weft in will change with the tightening of the warp.
As the beat changes, there can be streaks in the cloth as the warp is pushed to it's limit, then released and rolled forward.
Much, much better to stay within the sweet spot and advance more frequently.
Currently reading His Majesty's Spy by Susan Elia MacNeal