Strainin' In The Sweat Yard

Strainin' In The Sweat Yard

How backpacks work.

– This is work? –

You could say that what makes backpacks work is that they don't know any better. Because they are only great big empty spaces you can shove things into. Conveniently.

Conveniently, they don't have muscles, or bones, or teeth, so they pretty well have to go along with you, and can't fight back. Whatever you want to do, they have to do it. Hey, realistically speaking very few packs are ever semi-conscious, even occasionally.

But there is more to it than that.

Backpacking is more than equipment abuse. Because a pack is passive and helpless isn't an excuse to be thoughtless about it. If you are thoughtless you'll find out all too soon that things don't work just any old way. Maybe a pack can't fight back or bite but it can fail, and by failing leave your butt high and dry somewhere (or low and wet, more likely). Den you be up dat proverbial creek, Busta.

And there is even more to it than that. Even more. Packs aren't passive at all, not really. No! A pack may not have bones or muscles or teeth, or a nervous system, or motors, batteries, wires, or gears, but it is really an engineering miracle. Maybe a minor miracle, but a dependable one. A useful one. Dependable and useful miracles are the best.

While you are out walking around admiring the leafy whispering cool trees and looking at the high, snow kissed distant clean peaks your pack is still there, riding quietly behind you and doing its job, facing the other way, seeing the boring stuff, looking at where you've been. The pack's job is a little like the job an average bridge does. Both perform important functions if you intend to get anywhere. And both of them deal with a lot of stresses coming from every whichway, and do it quietly and professionally. If they are treated well. Mostly, you don't notice, as long as things just work.

Packs hang for a living. That's the main thing. A pack hangs, but it's more than a hanging bag. Stresses flow through a pack from the top down, and from the bottom up. And from side to side.

– Verticalize my stresses, please. –

The main thing you'll notice if you use an external frame pack is that the pack bag hangs from the frame. There are two pieces — bag and frame. This hanging puts vertical stress on the pack bag at every point where it attaches to the frame. There might be eight or 10 of these points. This vertical from-the-top-down stress is handled more subtly by internal frame packs. An internal frame pack has a more intimate connection with its frame, which is inside the pack. That internal frame contacts the pack bag continuously at many points, and the stresses are more evenly distributed. But the stresses are still there. They are stretching or tensional stresses.

Another kind of stress is from the bottom up. Most of this comes up from the hip belt. The bottom of the pack scrunches down on top of the hip belt, which sits on you and won't budge. If the hip belt wasn't there to help support the pack then all the pack's weight would be on your shoulders and you know how that feels. The pressure of everything stopping at your pelvic bones creates an upward, compressive force that pushes through the pack from the bottom up. This works to keep the pack in place. Whether the frame is internal or external most of this force goes right into the frame.

– Radial stress. (Sounds scary!) –

Aside from these vertical forces acting through tension and compression, there are others.

Stuffing a pack full stretches it from the inside out, creating tension on the fabric of the bag. The fabric is always a little elastic, and absorbs the tension. Weight in the pack exerts a force that runs down through the fabric and into the frame (and then through your skeleton) but the tension of being stuffed full stays with the pack bag. The pack has to cope on its own, without help from your body. There is nowhere for this stress to go unless the pack gives up and explodes, spewing your undies across the landscape. You probably don't want that to happen. And we don't want to see it. Not even in our imagination.

But stuffing isn't the whole story. Most packs also have some kind of compression device, like straps, to tighten things down and keep the whole load compact and solid. These straps add more stress all around the pack. As with the tensional stresses from stuffing, compressive stresses are mostly confined to the pack bag, and to the compression straps and the points where they are anchored.

In this case it's a little different though, since compression from outside the pack body, as from the compression straps, flows through the pack bag and into its contents, where it stops. You pull, the straps squeeze, the contents compress, and so on, until you can't do any more. At that point you've reached dynamic equilibrium, where squeezement is balanced by pushification, and you're done.

Then your pack explodes.

There is some tension involved here too, and not only in guessing at which moment your pack goes nuclear. To work, a compression strap has to be pulled tight, which means that it is pulled from its anchor point toward its free end. The other point the strap attaches to, which is usually a buckle linked to another section of strap, anchored somewhere else, pulls in the opposite direction.

Counterintuitively then, the more tension you put on a strap the more compression it generates. Life is like that. Luckily for all of us, we're backpackers and this is just a bit beyond our depth, so we can simply stand there, staring, scratching ourselves in random locations, and that's all anyone will ask of us.

– Special ops. –

Oops! Don't forget the shoulder straps. For some packs these are the main support, or the only support, so they can end up dealing with a lot of stress. Shoulder straps get to know every variety of grief that exists. Each and every one.

There are special times, like when you are crossing streams, when the shoulder straps take all the weight, even if you have a hip belt, if you have any brains at all.

Why? Because you have unlatched both the sternum strap and the hip belt. You do this in case you fall, so you can get away from the pack. You want to get away from the pack so it can't pull you under or jam you on the wrong side of a log (underneath it, for example). If you get pulled under water and jammed there, you die. Don't do that unless you really mean to, and don't have dinner reservations for later, because you will miss dinner and annoy a whole bunch of people who were expecting big tips from you.

Another special time when shoulder straps have it especially tough is when you are putting the pack on or taking it off. This doesn't sound so bad, but that's because you've never been a backpack. Try it sometime. Then you'll see.

When taking off your pack, swinging it to one side while disentangling the opposite arm from its shoulder strap puts huge stress on only one strap, and can rip it. This is a tough area to get right if you make your own packs, and you learn to respect the makers of day packs. Day packs have to go through this routine hundreds or thousands of times while being handled by average people, who are even more clueless than backpackers.

Shoulder straps have to be really tough.

If the pack is on your back, unmoving, and only hanging there, well, not so bad. But when you're putting it on or taking it off, you have all the weight concentrated on only one part of one strap, if you go through this process the way most people do.

Worse. (Yes, it gets worse.) As you swing the pack by this one strap you are adding sideways motion to the stress. This is called shear stress. It's how you peel a banana, or what you do when tearing a sheet of paper. Shear is tough on gear.

Take a look at a shoulder strap, especially one that attaches directly to the pack body at the top. This is a simpler attachment than most packs use, but is common in very light packs, and easiest to understand. If you hang the pack from just this one strap you concentrate all the force not on the width of the strap (wide as the palm of your hand) but on only one side of it, an area narrower than a fingertip, where only a few stitches hold things together. So watch it, then, and take some care there, because it is really easy to rip a few threads, then a few more, and so on until the shoulder strap is useless.

– Extras! Extras! –

There are a few mostly incidental issues, stresswise.

External pockets hang on the outside of the pack bag. Each one is stitched on, and stitching weakens fabric. The pack has to handle all the weight — everything inside it, whatever you carry in its pockets, and anything else that you lash onto the outside of the pack.

Whether a pocket is made of see-through mesh with elastic at the top or solid fabric closed by a heavy zipper doesn't really matter. The pack still has to cope with the weight. The more you put into a pocket, and the heavier that stuff is, the more stress there is on the pocket and on the pack, not to mention on you.

All those extra loops and tie downs and daisy chains? Yes, the pack has to cope with them too. All those conveniences, all those gizmos for lashing on more things, are nothing but stress points for your pack.

– I'm just loopy for hauling. –

A lot of packs these days have a separate loop up top, between the shoulder straps. This is the haul loop. Clever name, no?

The haul loop doesn't get much use, but it should. The haul loop is for lifting the pack off the ground, or for carrying it around camp. The haul loop is the pack's official carrying handle, centered at the top of the pack, and put there as an easy thing to grab. Use your haul loop, and your pack hangs straight down from that loop, with all stresses spread evenly. Evenly spread stresses are less dangerous stresses.

Using this loop is a lot less stressful than carrying a pack by one shoulder strap. It's that particular kind of strain produced by the one-strap carry (a twisting pull to one side), that is really hard on the shoulder strap stitching. So while the haul loop needs to take a lot of weight it does that job well, because it has a haul of a good design, and (bonus points here) it is very light as well.

These haul loops are handy, but ignored a lot — the shoulder straps get abused no matter what the pack designer intended.

– Itsy bitsy teeny weenies. –

Some more bits and pieces handle minor stresses.

Load lifters (also called load levelers) are common additions to many packs. They connect the top of the shoulder strap to the top of the pack bag.

These lifting or leveling straps are there so you can do some fine tuning and dial in your own personal degree of comfort. Tightening the load lifters pulls the top of the pack bag forward, closer to your shoulders and more in line with your center of gravity while you are either ascending or walking on level ground. Loosening these straps lowers the pack a bit, and lets it fall away from your back a scoch, which improves balance on downhill stretches, and is handy when you're in the mood for a tad bit of ventilation in that general area.

A sternum strap runs across your chest, connects the two shoulder straps, and pulls them closer together, for comfort. Yours. This is really nice if you have an odd pack or an odd body and the shoulder straps want to slip away from each other and slough off your body. There's nothing wrong with that, but when it happens your pack falls to the ground and then you have to stop and put it on again. Bummer.

You don't have to do this more than three or four hundred times before you catch on, but catching on isn't enough, because after you catch on you have to hang onto one or the other of those shoulder straps to keep it (or them) from slipping off again. So tedious. So very, very tedious. A simple sternum strap takes care of this while your brain comfortably goes back to sleep, which is good for brain health. So get one. (A sternum strap. Brains are actually optional equipment for backpacking, and you can go somewhat lighter without one.)

– Frameless packs. Unique. Just like all the others. –

That's pretty much it for packs, except for the frameless ones. They're the same but different.

• The same: they have to handle stress too.
• Different: they don't have frames.

So?

What this boils down to is that frameless packs are maybe more like one piece of something. Think of a big foam rubber ducky.

No, a big one, really big. Garbage can size. That's more like it. Like one that wandered far, far off its diet plan. A big and foamy Ducky.

Now a frameless pack is like this here Ducky. With a frameless pack you don't have a separate frame and bag clipped together at a few points, or even a lot of points. No separate frame, no separate bag. At all. The whole pack is like big foam-filled Ducky Object, absorbing all the stresses evenly, as a single unit, the way a 20-pound, fully-loaded rubber ducky would.

A frameless pack is easier to carry too, because a frameless pack is a light pack, made for light loads.

Condition	Result
Lighter loads.	Less stress on the pack.
Less stress on the pack.	A pack made from lighter materials.
A pack made from lighter materials.	A lighter pack.
A lighter pack.	A pack that's easier to carry.
A pack that's easier to carry.	More fun while backpacking.
More fun while backpacking.	That's the whole point, innit?

A frameless pack can fail, but if it's made well and used with a little intelligence it can do as well as any other pack, and maybe better in some ways. Better because it will be soaking up stresses over its whole surface, and not getting them concentrated at the few points where it contacts the frame. And better because it's lighter overall.

Every pack works by transferring weight to your body. They only vary in how they do it. Hip belts are fine, but hip belts transfer weight through your pelvic region directly onto your leg bones, which have no elasticity. Slam one of these bones hard, and you have a Big Owie, which will be brought to your attention by the characteristic snapping sound. That's your clue there. Besides the pain.

Routing weight through the shoulders is OK as far as it goes, which isn't far. More owie time. A little weight over a little time, fair enough. A little weight too long? No.

A lot of weight anytime? Even More No.

Not even ultralight packs work well hanging only from the shoulders. Shoulders are really not good for hanging things from. They have too many bones coming in from too many different directions. They have too many joints, and too many nerves and blood vessels.

Luckily we, as a species, are bent.

A whole bunch of the weight of a frameless pack, whether medium weight, lightweight, or ultralight, or sooper dooper ultra flyspectacular weight, goes into your back somewhere between your shoulders and your waist. You lean forward a little, and the weight rests there on your back, your spine absorbs the weight, and springy thing that it is, it transmits the weight efficiently and safely down through the rest of your very own internal frame. Your body knows how to do this. As long as there isn't too much weight, and it's reasonably balanced, then things work.

This is an advantage of a light load carried in a frameless pack. The whole pack absorbs stress, and then comfortably and gently passes it on to you. And neither one of you has to do much.

But sometimes things just don't work out.

– Fail! –

If a pack is going to fail, it's going to fail for one of three reasons.

First: A pack will fail if it's not made right. Bad design, flimsy materials, or poor construction will kill a pack dead. The pack needs to be designed for its intended use and used within its design limits. The pack has to be put together well, using the right materials in the right ways.

Second: A pack will fail if too much stress builds up in too small an area. This applies to external frame packs more than any other design. The frame on these packs may be strong but that frame is joined together at only a few spots. If a weld or pin fails the whole pack fails. The pack bag, for its part, is attached to the frame at only a few spots. Same deal. If one grommet rips out the stress goes right to the remaining ones, right away, and no pack comes with spares.

Internal frame packs spread the stress better than external frame packs, but they still have hard parts and soft parts working against each other. Whenever you have a hard part and a soft part rubbing together you will have trouble. Tomorrow if not today, but you will have trouble.

Frameless packs don't concentrate stress so much. (Remember the Rubber Ducky Principle? Go Ducky!)

Third: A pack will fail if it degrades from use.

Degrading will really kill an ultralight pack. Its fabric might be one ounce per square yard or even less (roughly 30 g per square meter). Say 30 denier, in fabric terms.

On traditional packs you'll see 500 or 600 denier fabric, stuff an alligator would not ordinarily mess with. Drag an ultralight frameless pack over a few gnarly rocks and right away things get desperate. Drag a Kelty Tioga or a similar pack (either side up — doesn't matter) and laugh. Do it all day for a week and all that happens is you get tired of laughing.

That is the difference between an eight ounce pack (227 g) and a five pounder (2270 g). Which is the same as the difference between a blueberry muffin and a cannonball. You get it. Not interchangeable. If you want to go light and deal with the tradeoffs, things work. Otherwise they don't. Simple.

And: Any piece of gear will eventually fail if you throw enough alligators at it.