Best Center of Gravity?

[TooOld2Hike_EP]

In the world of flying, we had to calculate the Center of Gravity for the airplane (with passengers, fuel and baggage) because CG made a big effect on how the plane flew. (Could be deadly if out of CG.)

In the world of backpacking, everyone says (correctly) that heavier items should be in the "middle" of your pack, close to your back. I could make an argument that, like an airplane, if loaded wrong (heavy items at the top) a backpack could be dangerous/cause imbalance/falling.

But is there an optimal Center of Gravity for a loaded backpack?

(Actually, there are two CG's/moments here. One is the more obvious, the weight distribution horizontally, going up the pack. But there is also the moment vertically from the weight being close (or too far) from your back. For now I'm only asking about the first.)

My intuition says that if I were to hold my arm out to balance my pack on my arm, my back should balance about a third up, about where the lumbar support ends.

(I am talking about a frame backpack.)

But perhaps it should be at the 50% point, at the midpoint of my back?

Or perhaps even 2/3's up? Because when I carry a 40 pound bag of salt pellets, that's where I usually drape the sack. Although that seems high from a "keeping your balance" perspective.

I've experimented a little. But I don't know how to quantify "comfort" or what feels right. Although I can tell when it's not right, at the extremes.

As I write this, I'm thinking that I should try strapping a bag of salt to my back to see where it feels best.

But perhaps you can save me the trouble. (And mess.) Is there a study quantifying the optimal CG for a backpack?

[LindaAnn]

A simple online search will yield about 4,938,137 results all telling you that the heaviest items should be located roughly between your shoulderblades, as close to your back as possible. Given your post history, you’re going to argue about conventionally accepted wisdom and practices, so the answer to every question is “do whatever the heck you want and what feels best for you as long as it doesn’t affect anyone else” because, quite frankly, you can load your pack however you want, and suffer the consequences if you do a crap job of it.

[Mountain_Rat]

Here are a couple of resources:

https://www.sandboxx.us/blog/how-to-pac ... ree-hikes/

[ youtube video ]

Please direct any questions or complaints to the Commandant himself.

[big_load]

A simple online search will yield about 4,938,137 results all telling you that the heaviest items should be located roughly between your shoulderblades, as close to your back as possible.

Yep. It's all about fighting the torque (static and dynamic) resulting from a load for which the CG is cantilevered out from the supporting members. I don't think anybody ever said "Gee, I wish this 40 pounds stuck out another foot or two from my back". There's a little more wiggle room vertically, especially with a decent suspension and the hipbelt being used to best advantage, which not enough people do.

[LindaAnn]

@big_load
Yep, I tend to carry heavier items slightly lower, but it’s easy to adjust and rearrange based on whatever I happen to be carrying.

[TooOld2Hike_EP]

Yes, I know the general rule that the 5 million hits will say. I was simply looking for a simple, common sense way to ensure/know that one's pack is packed optimally.

Has no one ever thought to check where their pack balances? 'Cause I, for one, can't tell when I put mine on whether it is right or not. It takes me a little bit of walking until I begin to feel that it's uncomfortable. But by then, who wants to re-pack their pack during a hike?

[TooOld2Hike_EP]

@big_load
Agreed that no one should cantilever the heavy stuff far away from their back. So I don't need a balance test for that.

It's the wiggle room for the vertical stacking that I'm focusing on. I'm going to guess that this is an asymmetric thing. That is, too high becomes bad quickly. Whereas too low is less critical?

(I'm trying to think of what the problem is if the CG of the load is, say, below your hip belt. I suppose that a way low CG pulls on your hips (instead of pushes) and it also puts a torque aft on your shoulders.)

One of the sites cited said to put the stuff that you don't need quick access to down low. While that makes sense from a practical standpoint, it doesn't take weight into consideration. (So then you would put your recharge brick near the bottom of your back?) Also, backpacks that have the zipper access to the mid-inside of the pack obviate the quick access issue.

[TooOld2Hike_EP]

@LindaAnn
Okay, thanks. Carrying slightly low confirms what my intuition was guessing.

[Mountain_Rat]

Did we really just go through all that to establish that heavy stuff goes to the bottom?

[LindaAnn]

@TooOld2HikeQ
I’ve never even weighed my pack, much less spent time trying to figure out if it’s balanced. Why bother? Just make an adjustment if needed. It takes less than five minutes to repack if something is uncomfortable, and then you know not to pack it that way the next time. Following the standard (tried and true) packing guidelines has always been fine for me. I can think of only one time my pack was too top-heavy and I had to redistribute weight while on the trail, and that was my own fault because I was careless when repacking in the morning and had swapped some gear between my and my partner’s packs.

My time is much better spent actually outdoors (and I have remarkably few problems on hikes or backpacks given the amount of mileage I do) rather than sitting in my house or online worrying about every little detail. Time is one thing you can never get back, I try not to spend mine on things that ultimately aren’t a good use of that irreplaceable time. I’d rather spend five minutes repacking on a trail once every 2500 miles vs hours spent trying to balance my pack at home.

[Mountain_Rat]

Time is one thing you can never get back

So, does that mean there are no refunds on this thread for this afternoon?

[big_load]

One useful check is to hold the pack up by the haul loop and verify that it hangs straight up and down (not heavy on one side). A heavy side causes more trouble than a load too high or low.

A problem you can have with a high load is it will be tippier in response to acceleration with respect to the vertical axis, for example side-to-side motion from your gait, or from turning, or sudden tips back or forward due to uneven terrain. Your core muscles work harder to counteract those forces than if the weight is lower down.

[xsproutx]

It takes me a little bit of walking until I begin to feel that it's uncomfortable.

Yes. This is exactly right. You kind of look to see what people say works and then... test it out? See if it works for you? We're all different. Seems like you're recently retired with plenty of time on your hands so shouldn't be an issue? I'm pretty short for a dude at 5'6 but I have a 19.5" torso which is long for a guy my size. I also have stupid flat feet. These all affect my personal choices for gear and how I pack and I've figured it out by packing and walking.

[rcorfman]

I always just pack my bag the same way. My main concern is whether one side is heavy compared to the other like @big_load brought up.

My bag's small, frameless, and beltless, though so there's that. Just to be clear the back of the pack goes against your back and the front of the pack faces behind you.

When I pack it, first goes in the quilt, then my folded sleeping pad between the quilt and the back of the bag, then my clothes on top of that, then my tarp and netting, and first-aid/repair kit, then my food bag. Then I cram my fleece hoody and rain jacket between the food bag and the front and sides of the bag. That's usually everything in the bag. I have two 1-liter bottles in one side pocket and a 20oz gatorade bottle and another liter bottle in the other side pocket. The front pocket has a bunch of other crap, such as my wind jacket, head lamp, some food, poop kit, umbrella, bandana, knife, Spot, etc. Oh, I also have a zippered front pocket where I put electronic stuff such as cords and a battery.

[TooOld2Hike_EP]

I realize that most no one here cares about this. But I'm going to share anyway what research I've found so far on the CG of backpacks.

This is like Ben Franklin's "A stitch in time saves nine." Having an "out of balance" backpack can be a pain. If not dangerous. Since it's so easy to balance a backpack (once one has taken a little time to research it) that there's no downside to knowing about it and a lot to gain by doing it.

So, per a suggestion by a member here, I did what I should have done first. I did some searches for "center of gravity" and "backpacks."

The first link that I found was this one.

Forget the text - what drew my attention was the graphic at the top from Researchgate.net.

(As for the graphic proper - it shows the human-backpack interface as a mass-spring system.

Hmmm... I haven't thought that far ahead.

But at the speed that I travel at, I don't think that there's much springing going on in my backpack.

Although I did find this so-called "zero gravity" backpack.

(My intuition tells me that this is a bad idea. In our airplane, we had temperfoam under our seats for crash protection. Temperfoam is a dense foam that does not rebound. One might think that soft, squishy, reboundy upholstery foam would be best. But it's not. The rebound puts energy back into your body in a crash. I can make a similar argument for the bouncy backpack in the video. And what happens if you stop suddenly (rattlesnake!)? Do you topple over?)

So, I searched more, but this time limiting my searches to researchgate.net.

Wow. There are a lot of scientific papers on how to optimize backpacks!

The first image I saw was from here.

Someone has made a backpack jig, where one can adjust the CG. (They say the CG should be located between T-11 & -12. )

I might try to make something like this. (So far I've been experimenting with gallon size rectangular jugs, putting an empty at the bottom of my pack (horizontally), and then a full one above it. Go hike. See what it feels like. Then two empties below, one on top, etc.)

Or maybe they've given us the answer. Just try to have your pack balance between T-11 and T-12? Although the paper was focusing on school backpacks and young girls with adolescent
idiopathic scoliosis. (But their introduction is worth reading/generic enough and it introduces Center of Pressure, which I hadn't thought about before either.)

The full article is here.

Then it turns out that some researches sought to design the perfect mountaineer's backpack! Now we're talking.

Of note, and something that I should have thought of earlier, "The military has conducted many physiological, biomechanical, and medical studies to investigate the effects of carrying backpacks." And they give references, which I still need to read. (Although if you've seen the cockpit of a B-29 Bomber, you know that the military doesn't care about the comfort of its grunts. Nor if some break during use. And designs for "one size fits all.")

In the mountaineer paper, they modified an off-the-shelf backpack to be gourd shaped and they used accelerometers.

Last (for now) I found this graphic.

I hadn't thought about Center of Pressure before. But I think that this one is more obvious/intuitive. Cinch up your backpack in the horizontal to keep your load as close to your back as you can.

I don't have an accelerometer (yet). And even if I did, I don't know what the optimum acceleration should be. (I suppose the lower the better?) For now all I can do is to move water jugs around in my backpack to get a feel for where the best CG is - for me.

The perfect CG is a difficult metric to pinpoint.

In tuned circuits, there is something called "Q." It is a measure of how narrow or broad a tuned circuit is. A High Q circuit is one that graphs out like a pointy mountain, where you have to tune it right on frequency to hear a signal. Whereas a Low Q is like a low knoll. It seems to me that the perfect CG/CB solution is more Low Q, in that you can be on either side of perfect and not notice much difference. (Although as others here have implied already, I don't think symmetry applies here. It would be better to err on the low side than to have too high a CG.)

Armed with what little I've learned so far, I packed my backpack more carefully on my last trip. Maybe it's placebo effect. But it seemed lighter/more comfortable.

[Nighthiker]

Actually, I think it depends how far out your waistline is.

[rcorfman]

It seems your priorities are off. Instead of worrying about what might be the most optimum way to distribute the load in your back, think about how to lighten the load in it. Then, once your pack weight is low enough, it just isn't going to matter if its center of gravity is shifted up or down an inch or two.

[azbackpackr]

@TooOld2HikeQ
This video should be very helpful: [ youtube video ]

[chumley]

by azbackpackr » May 12 2023 07:08 am
This video should be very helpful:

@azbackpackr Don't be silly. Unless you're an ultra featherlight backpacker that would totally mess with your center of gravity.

Another HAZ Forum Flashback Friday Favorite — the hoverglide — is the real winner for maintaining balance and reducing weight.

[azbackpackr]

@chumley
So many choices, so little time!