Zpacks Arc Blast sizing question

Long story short, there's an acceptable range of torso length. What you prefer will depend on how you like to wear your pack and what you do with it.

Torso lengths towards the shorter end of the spectrum (what I assume your HMG would look like loaded with 25 pounds) tend to put a bit more on the shoulders at heavier loads, and provide what I think is a more direct connection with your COG. Generally favored by folks who ski, hike off trail, and have stronger upper bodies, or those who only carry very light loads.

Torso lengths toward the longer end (Zpacks explicitly states they're in this camp) keep weight off the shoulders more completely at big loads, at the expense of being able to put weight on the shoulders should you want to (see above).

I've tired the Zpacks route in a few packs, and disliked it quite a bit. It may just not be for you.

In any case, the load lifter attachment on that Arc Blast looks a bit off. Needs to be a few inches further forward.

David,

That's an interesting point your raise here. Do you think the vise-grip feeling I'm getting with the Arc Blast is a result of my trying to shorten down the torso length in a manner that is inconsistent with the pack's original design?

With regard to preferences within the torso length spectrum, I'd say that I prefer it to be snug rather than loose. As a matter of fact, I personally prefer the weight to rest on my shoulders equally (or more so) than on my hips. I have lower back issues, but a relatively strong upper body, so maybe that's way I prefer it that way. When it comes down to it, I'd prefer a slightly sore shoulder than an injured low back that puts me out of commission. Additionally, I rarely travel with more than 20 lbs total weight in my pack.

I do want the Arc Blast to work for me–setting aside the cost I paid for it, it saves me around 12 oz over my HMG Southwest pack. It also seems to have a suspension designed for heavier loads, in case I decide to up my pack weight to around 25 lbs for a much longer hike.

One thing I forgot to ask is whether you think I adequately tensioned the "arc" in the pack. When loaded there is about a 2.25" gap between the mesh backpanel and the curved frame on either side at the middle.

Joe V has probably seen this before, so I would put a lot of value in his suggestion.

Strictly from the peanut gallery, it seems that you like your waist belt relatively high on the iliac crest of the hips, and therefore the top of the pack and the shoulder straps' attachment points are also kind of high.

In the meantime, perhaps experiment with reducing the amount of arc and pulling the load lifters a bit tighter to bring the pack closer to your back. But your side stays are definitely a lot higher than mine, which are about even with the tops of my shoulders. Lucky for me that I am medium/regular/average in fit, but I have relatives that have frequent battles getting backpacks and bicycles to fit properly.

"As a matter of fact, I personally prefer the weight to rest on my shoulders equally (or more so) than on my hips. I have lower back issues, but a relatively strong upper body, so maybe that's way I prefer it that way. When it comes down to it, I'd prefer a slightly sore shoulder than an injured low back that puts me out of commission."

I can commiserate on the lower back issue. However your analysis here is backward. If you are in fact getting lower back pain rather than merely anticipating it, then it may be caused by something else. When you use a hip belt the load is placed on your hips, NOT your back. Your spine rests on your hips, not the other way around. Weight put on the shoulders is transferred to the spine. The muscles that support your lower back are ones that support your spine while the ones that support you hips are primarily the upper leg and gluts. So you should expect MORE back strain, not less, with the weigh on your shoulders. This is actually the whole point once you get past the trivial (IMO) issue of sore shoulders.

With the weight more on your hips rather than your shoulder you also have a lower center of gravity, so the gravitational torque on your body is less, and consequently the need is lessened for your stabilizing muscles, including those in your back, to work harder when you hike, especially over uneven ground, unless the pack is so loose that it wobbles on your back then all bets are off. Don't do that.

Long story short, the less weight on your shoulder relative to the hips, the LESS strain on your spine and back muscles. There are lots of people over the years who have chosen to put the majority of the weight on their hips precisely to avoid stressing the areas you mention. Does it matter with UL loads – not that much. Use what feels best to you, only you might as well get the rationale for using it straight so that if you do start feeling pain when you use a pack for extended period of time you can make an adjustment in the right direction.

However, if you do a lot of scrambling (or especially climbing) then having some of the weight on your shoulders can be better, especially when control trumps other issues, as is often the case with UL loads.

BTW Joes says 2.5 inches on the arc, but I like it more like where you have it. But that will shorten the torso length and potentially get more weight on your shoulders. Also based on experience asking the same question Joe will likely say that how it is in your pictures is how he personally prefers it – unloaded, straps a little above shoulders, when fully loaded, settling down to where they are level or just barely above the shoulders. If it were me I would prefer the torso length to be maybe 1" shorter, but I don't think you can get all of that by increasing the arc. But as David said, you like what you like. Just try it a bit before you prejudge. I don't think you will get a fair idea unless you actually use a pack for a while. However, I do get it that if it feels "weird" to you at first it might be hard to get over that threshold.

It looks like it fits just about right. Once you load it up and hit the trail with even the slightest slippage down on the upper buttocks it is going to load the shoulders a bit. I wouldn't want it any shorter.

David P, I think part of the problem is the length of the stays. They come so high above your shoulders that the mesh is holding the shoulder attachment points away from your back. You could have Joe shorten them.
The slipping hipbelt can be helped by having the hipbelt attached nearer to the centre of the pack – a bit late for you but something I'd recommend to people with slim hips. ZPacks' lumber pad might help.

"Do you think the vise-grip feeling I'm getting with the Arc Blast is a result of my trying to shorten down the torso length in a manner that is inconsistent with the pack's original design?"

Makes sense to me. The pack just might not suit you, and no harm in that. I imagine the resale value will be favorable.