A couple of good points are made above.
"Twist isn't going to happen at big loads because the load itself is stiffening the pack." Obvious, but one I had not thought about.
" It is very difficult to design a pack that is comfortable with both a 25 lb load and with a huge 80 lb load."
While waiting for David's Paradox review, (excellent, by the way}, I carried out a very unscientific trial on 5 packs, (all well regarded of their type}, basically to jog my memory, and to consider the last point in particular.
Steadily increasing loads were put into each pack, mainly tent material and water bottles,with the pack compressed to give an equal thickness top to bottom. Weights quoted are load weights, not total pack weights. An attempt was made in packing not to add to pack stiffness.The packs were carried for a relatively short distance, over both flat roads and very rough and steep bush tracks.
The results were as you would expect, and in agreement with conventional wisdom. There are also personal, and other people will find differences.
The first of these packs was a thirty litre frameless I use as a day pack. It uses a stiff foam pad to prevent torso collapse, and has an excellent hip-belt.
It was the most comfortable up to about 8 lb, 4Kgs,with almost all weight on the hips. Above that,it became the most uncomfortable with torso collapse.
The three internal packs were nominal 50 litres,(Deuter 1.6 kg), 70 litres,(Macpac 3Kg)and 80 litres,(Fairydown 2.5Kg). At low loads the difference in pack weights was noticeable but by about 15 lb and above this largely disappeared. At this level all three were similar in comfort. However at about 30 lb the difference between the packs was evident,not that any was unusable.
By 40 lb load the Macpac was the most comfortable, despite its heavier total weight, largely because of better hip and lumbar padding. The Deuter is new, and I am not yet sure it is ideally set up for me, but I felt it was starting to reach its comfort limit even on flat ground.
Above this, all became less and less comfortable on rough terrain as the hip belts all restricted movement despite being correctly adjusted for flattish ground. They all had to be eased off considerably, and the packs then moved in a less controlled fashion on my back.
The external frame is from the late 50's and I used to carry one many years ago a lot, with very heavy loads. It is extremely rigid, uses webbing across the back, which can be adjusted to suit the back shape. The lowest cross-webbing can be positioned to provide both torso length adjustment and lumbar support. No hip belt is provided. Weight about 2Kg. This one belonged to my wife, so both back shape and torso length was wrong. I did not adjust these.
At low loads this was clearly the most uncomfortable. All weight was on the shoulders, and the pack bounced around on my back in an uncontrollable manner. However by 20 lb this had started to settle down, but there was still too much shoulder weight. By 40 lb the weight was spread evenly across my back, the pack stayed in place, and it was at least if not more comfortable than the internal frames, as far as weight distribution went. The lack of hip belt restriction was a decided plus. I felt had the pack been correctly set up for me, a lower comfortable weight limit could be obtained.
I feel we can draw a few conclusions, most of which we already know:-
Given the state of the existing market, internal frame packs (including UL versions such as the Arc Blast) are the best choice for most people. Frameless and externals have their place for certain types of trips, and for those willing to pack appropriately.
The trial weight used when considering purchasing a pack, should be much higher than I thought necessary, at least 35 to 40 lb, (as this seems a reasonable maximum weight for all seasons in all climates), and the carry should include several flights of stairs. A lower weight (such as the manufacturer's recommended maximum)could be chosen if absolutely sure this would never be exceeded under any circumstances.
The design of the hip belt is extremely important at modest loads, to carry most of the weight. But as the terrain becomes rougher, and the load increases, it becomes much less important (and in fact can become a problem), and the shape of the back support should provide the stability required. Internal packs seldom provide the appropriate shaping.
The Paradox pack tries to bridge the gap between internal and external. The problem I have is the cost that although competitive with its peers, is higher than the two packs, (a light weight internal plus an external) that would cover the same range, and probably at light loads provide more comfort. For a hunter of course a two pack combination is not appropriate.
