Here's the rundown. Frameless packs are generally comfortable to about 20 lbs. Ultralight thru-hikers on the PCT can get their baseweight down to between 7 and 10 lbs. You're going to be carrying varying amounts of food and water. How do you decide whether to opt for a frame or frameless backpack on the PCT?
Basically, we need to estimate the amount of time we'll be carrying over 20 lbs. and decide whether it's worth it or not to get a frame pack that weighs somewhat more, but is more comfortable. Let's put the threshold at 25%. If your packweight is going to be above 20 lbs. more than 25% of the time, then get a frame pack.
Let's do a bit of math.
We'll assume that:
- one day of food weighs 2.5 lbs and provides near 5000 calories
- you're among the faster hikers and finish in roughly 130 days (like I did)
- you're carrying an average of 2 lbs of water at any given moment
- you're carrying an average of 0.5 lbs of packaging and garbage at any given moment
- you choose the same town stops I did, in particular dropping down to Independence to resupply in the High Sierra
We'll then look at three scenarios:
1. The SUL thru-hiker with a 7.5 lbs. baseweight.
2. The UL thru-hiker with 10 lbs.
3. The lightweight thru-hiker with 12.5 lbs.
In the first case, to be over 20 lbs. the hiker will need to have over 4 days of food (7.5 + 0.5 + 2 + 4 x 2.5 = 20). In the second case, over 3 days of food (10 + 0.5 + 2 + 3 x 2.5 = 20). In the third, over 2 days.
1. SUL thru-hiker: using my calendar, he'll be carrying more than 4 days of food roughly 15% of the time. Conclusion: use a frameless pack.
2. UL thru-hiker: he'll be carrying more than 3 days of food roughly 27% of the time. Conclusion: use a frame pack.
3. Lightweight thru-hiker: he'll be carrying more than 2 days of food roughly 45% of the time. Conclusion: definitely use a frame pack.
As you can see, the cutoff for comfortable use of a frameless pack for the PCT is a baseweight of roughly 9.5 to 10 lbs, assuming a fast pace (finishing in 130 days or less). You can get down to this baseweight only if you practice classical UL principles and shave down all excess weight.
Remember that you'll also be carrying maps and spare batteries and things like that. Oh — and a heavy bear canister for much of the Sierra. If you're using a frameless pack and not dropping down to Independence, your first week or more in the High Sierra could be pretty uncomfortable.
If doing the math shows that you can comfortably use a frameless pack, then I'd go for one of the really light ones (under 12 oz.), such as the Zpacks Blast series. I'd probably get the highest-volume one and store my sleeping bag/quilt semi-compressed to preserve it's insulating capacity.
If you see that you're going to need a frame for added comfort but are going to be under 25 lbs. at least 75% of the time, then consider frame packs from Gossamer Gear (Gorilla and Mariposa Plus). These are completely comfortable to about 25 lbs. The ULA Ohm is totally comfortable to about 28 lbs. These packs are among the lightest frame packs out there and weigh in at under 24 oz. (680 grams).
If your baseweight is 15 lbs. or higher, you're most definitely going to want a pack with a heftier frame capable of carrying 30+ lbs. comfortably. There are many more relatively lightweight commercial options in this category that weigh 2 lbs. or more.