UKH

The physics is clever but this backpack suspension system just looks weird!

youtube.com/watch?v=Xsth-rkFBxk&

Interesting idea, thanks for posting. I wonder if people will go for it.

I guess that putting a suspension system on a backpack could work for the same reasons it works on a car so the basic idea is sound.  It also seems obvious enough that other people will have tried it and since it isn't being done by the major players, even for military rucksacks, there must be drawbacks.  Presumably it is about tuning the suspension, the additional weight and size of the suspension and the increased cost and loss of reliability from moving parts. 

I have tried a prototype of something similar, and it did indeed work. Especially noticeable when running, so maybe a small pack for fell runners could make sense. The dampening would still have to be improved, as the bag bounced when you stopped moving. I also would not trust the mechanics as yet, and I would want a super stable lock down option before considering buying such a bag. 

For a heavy trekking bag, a pivoting hip belt as e.g. found on Macpac or Fjällräven packs makes more sense. 

CB

I wonder what level of activity it becomes counter-productive? 

That would be a pretty hard sell given that fell runners are mostly absolutely obsessed with weight.

But it does reduce the mass you have to accelerate at each step, which is particularly relevant when running. So even if the pack is a few grams heavier overall energy expenditure will drop. Maybe desert races where runners carry a substantial amount of water may be even more pertinent.

CB

Potentially it could also allow runners to maintain more of a pack-free gait as well, which could also increase efficiency of movement, rather than adopt the distinctive shuffle style that's necessary when carrying anything beyond a very light load.

Maybe there but regular fell runners carry very little so it's hard to see it catching on there. I suspect it will also be more than a few grams heavier and that some of the extra mass will be in the bit that you still need to accelerate each step as you put it. And regardless of whether you accelerate it each step you are still carrying it uphill.

Isn't it basic physics that there's no way you can reduce the energy in a closed system (runner/walker + backpack)? When you push off one foot, the backpack has to start travelling down, ie reach inertia and change direction. This requires energy which the runner/walker has to supply, so the height of the arc of the next pace will be slightly less. If this wasn't the case, then surely the heavier the backpack, the greater the advantage, which obviously can't be true!

Very neat.

What happens when you push it to far and it runs out of travel?  I’m imagining jumping off a 3 meter ledge and so storing a load of energy up in the spring load part of the rucksack, to then be dumped into my spine after the landing.   

Suspension on cars is great until you mis-judge a bridge jump...  

Imagine trying to run with a cup of tea. Before spilling most of it you'd have run some distance with the cup barely moving up and down at all, while you would still have been doing plenty of up and down motion. No work is being done to raise, lower or vertically accelerate the cup. The backpack surely is imitating the job done by your arm while running with the cup.

This was my second thought on watching the video. I'm pretty convinced my intuition is right too - it might be able to smooth out the downward force of the backpacks weight but it isn't going to reduce the overall energy of carrying something.

My first thought was of course "hahaha, here comes another gimmick".

My third was that if you are moving quickly and go down more sharply at some point (e.g. a big step down) then you are just going to adjust with your legs to catch your own weight and then fall over when a fraction of a second later the pack runs out of travel and you suddenly get an extra X kg of load slamming down on your shoulders at speed.

And unless it's extra specially cunning, it seems like to work properly you would have to adjust for the weight you are carrying every single time you put it on.

AND there is the extra weight to consider.

In fact, all in all, it looks like a horrendous idea!! 

True if the runners legs are a perfect spring.. so obviously not true

Yeah, my guess is that just like with cars the hard part will be tuning the suspension system between springs and shock absorbers so as to get a smooth ride in the 'normal' case without getting cases where it limits out or builds up an oscillation or returns energy in an unexpected manner and throws the wearer off balance.   

The kind of regular movement they demonstrate it with in the video is probably a near ideal 'soft suspension' case where they can set up the suspension to totally smooth out the vertical motion. 

Don't worry - some people will buy it.

Surely if you are holding a cup of tea and keeping it steady you are using energy, as in biceps curls. Try holding a cup of tea with a bent arm. Stay like it for a while and you will soon find if you are using energy when your arm gets tired. It may take a while but energy is still being used in a small amount.

I was referring to the energy used to do work, in a physics sense. If I were to stand with a cup of tea in an outstretched arm I would get tired from holding it there but the physics required to do so would require no work done and no energy expended, as it could equally be achieved by an inanimate table.

You get tired because of the energy expended holding it the against it natural desire to fall to the ground due to the force of gravity. You are using energy to resist gravity.

This pack is still on your back, and is still subject to the force of gravity so overall you will expend as much energy with this pack as any other, albeit in a smoother more even way. It will not cut down the energy needed to move it from A to B without it falling to the ground. 

Energy can neither be created nor destroyed.

Physics says you are resisting gravity, therefore you are doing work to hold the cup there. Hence getting tired. A weak table would collapse because of the work (have stood on a weak table, gravity worked harder than the table could resist, an expensive mistake!) it has to do to resist gravity. Most of our problems as climbers and walker are caused by gravity and it's constant efforts to get everything down to the centre of the Earth, or solar system etc etc.

Ok, so your understanding of 'work' in physics is clearly different to mine (using a force to move an object). I cannot accept that a table, regardless of how strong it might be, is doing any work. Perhaps you're using a layman's definition of 'work'?

But getting back to the backpack, the point is that because of the inherent inefficiency of the human body (compared to a table), even maintaining a load at a fixed height requires energy, though no work is actually being done. But in addition to that, to raise a load above a fixed height requires work, which expends more energy. And when the load is allowed to fall again to its previous height that energy is not recouped. It makes sense therefore to try to minimise the up and down motion of the load wherever possible, so all you're left with is the energy required by the human body to maintain its height. The question then becomes one of finding a way to achieve that without negating any benefit by having to carry a heavier load or having to run in a less efficient manner.

Check physics of latent energy and kinetic energy!

I bet the suspension weighs a fair amount, it will have a resonant frequency at which the pack would just become counter productive hopefully they've designed it well enough that you would never encounter it in normal use 

Pretty sure these guys have the physics worked out.  They've got experimental data and their stuff has been peer reviewed.  They got a paper in Nature and funding from US military.

When people walk they necessarily move vertical dimension with each step as well as the desired motion in the horizontal dimension.   The suspension system isolates the load in the backpack from this vertical motion so you don't waste energy accelerating it.

There's much more information on their website, they point out the principle of suspending the load is the exact same as is used by porters who suspend loads on bamboo poles. 

http://www.lightningpacks.com/lightningpacks.com/Ergonomic_Backpack_%7C_Lig...

So did these folks with their preposterous theory pulled out of their ass - https://www.nature.com/articles/s41586-018-0151-x - A suggestion of microscopically fractional charge is so exceptional it needs more evidence than “it’s a fudge factor that makes some models that are known to be woefully incomplete match observed reality”.   Then again if we stared insisting that theories of dark matter have any sort of correspondence to theories of particle physics, where would the madness stop?

So did these folks who, wait for it.... made an ASIC!  But they didn’t just make a chip, they stuck a claim on that doing more polynomials with their chip advances the rise of artificial general intelligence - https://www.nature.com/articles/s41586-019-1424-8 

Aspirational stuff...

I don't mean that the runner isn't going to expend energy; I mean that he's propelling his own weight + the weight of the backpack, and that no system on Newton's Earth will cause a backpack to weigh less than it actually does! 

The time averaged weight of the backpack doesn’t change.  The instantaneous peak weight of the backpack is however increased as the walker’s body rises with a step (a). This is balanced by the instantaneous minimum weight being lower as the walkers body drops a moment later (b).  The human body bobs up and down against the backpack.  

If the human body had no frictional losses and a linear food>work response, it would be able to average out the bobbing effect with no net cost in energy or work. However the human body is full of losses and so more is taken out of it by (a) than is offset by (b).  So it’s better to mechanically average out the cyclical weight with suspension that to do it through the human body.  This is what “timparkin” means when he says the human leg isn’t a perfect spring.   You talk about a closed system but the system isn’t closed - friction losses and metabolic losses in the human flow outwards to the environment, and suspension on the rucksack reduces those losses.

Its a bit like a human repetitively stop-start cycling.  With the right equipment, this would be more efficient with regenerative energy storage on the bike.

Thanks, that all sounds intriguingly logical!

Aarn Packs have a good system with balance pockets on the front - the result of which is that the centre of mass of the 'backpack system' moves inside your body (unlike with a normal backpack, where it sits outside your back). They also have a kind of 'half-floating' shoulder strap attachment point which gives a similar effect to the backpack in the video, but only for side-to-side motion.

Putting aside the physics of that weight bouncing up and down, what will happen once it's been soaked and dumped a few times in a corner to fester and go rusty?  It will start by squeaking maddeningly then by siezing up and becoming just an unnecessarily heavy rucsac.

Their 55L backack weights around 9lbs (4kg!!!) as stated on their ingiegogo site. No mention of the weight of the 30L daypacks...Even if all their statements come true, no way I am lugging a 4kg backpack when there is a plenty of ones weighing 1kg with same volume... Unless it'd really magical, but give me the benefit of doubt, and their backpack design is pretty bad, like something from the 80s...If they did a good pack with the system and brought it down in weight, might be different...

This goes into my "useless" bin along with L shaped bicycle cranks, whch were supposed to be more efficient.