Franco- how correct. There is a continueum from comfort to pain. Each person has a different point where comfort progresses to discomfort which progresses to pain. And each person has a different tolerance for discomfort and expectations of what level of pain they are willing to tolerate when backpacking. A pack can feel comfortable when put on, uncomfortable by early afternoon and downright painful by evening. I swear gremlins put rocks in my pack as the day goes on! In my opinion, no pack is totally comfortable at the end of a long day.

In general, for me, frameless packs (simple rucksacks) are OK to about 20 pounds. Light internal frame packs with some support (stays, backframes, etc) Ok to 30 pounds. Beefy suspension system on internal frame up to 40 pounds. Above 40 pounds I prefer external frame packs. All the above assumes the pack fits well. And there are issues of how the pack handles a bear cannister.

I think someone with bigger and sronger shoulders than myself would be able to carry more weight in frameless packs.

And you have to consider how many hours you plan on having the pack on your back each day. If doing a large through-hike where you plan on a pack on your back for 12 hours a day, for weeks at a time, you need to look at greater comfort for the weight (because those little gremlins DO put rocks in your pack!)

And you also have to consider how the pack is packed. To get the maximum comfort out of any pack, you have to be very careful how to pack it. The lighter packs require much more attention to this detail.

I take all weight capacity statements with a grain of salt.