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Pack compression
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Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
Pack compression on 03/21/2014 21:45:57 MDT Print View

I'm working on yet another Zimmerbuilt pack and I'm stumped on what to do about the compression system. This would be my primary bear can/water hauler pack that (hopefully!) will be my PCT pack for 2015.

I'm drawn to the aesthetics of the ULA Ohm style zig zag cord side compression, as well as the simplicity of it. But I wonder if that is going to be sufficient to contain the full bear can or several liters of water for my desert hikes.

The other issue is that one of the side pockets will be a tall one for my shelter...if I do the zig zag cord, that can go under the pocket; a traditions webbing strap design would have to go on top of that pocket. Maybe that's a good thing...I haven't decided.

And those of you who work with such things...if I went with the webbing straps, which direction should they go? (I'm having a heated discussion with my engineering father about this....) do they angle upwards towards the back, or downward towards the back????

Any opinions here?

Marko Botsaris
(millonas) - F - MLife

Locale: Santa Cruz Mountains, CA
Re: Pack compression on 03/21/2014 22:50:14 MDT Print View

Aesthetically and economy-wise I do like the zig-zag pattern, but before I got my Kalais built I decided against it. Independent is probably much better and more versatile - you have to think of the zig-zag as like a shoe lace - there is only so much you can do to customize the compression that way - and all the best ways involve tying knots and in effect making the system work again like independent straps. Probably with a bear can a "custom" compression is more important than a for a pure "fluffy cloud" type SUL load without the huge boulder in the pack.

All I got at the moment. The angle thing is above my pay grade, though I'm thinking horizontal (no angle). Or down toward the back. Definitely not up towards the back, I'd stake my life on it....Maybe.

I can only imagine the discussions with you father. Strap angles - sounds like another of those "if there is a truck filled with birds, and the birds start flying inside the truck, does the truck get lighter?" questions. Answer, no. It weighs exactly the same.

Your father will understand. :-)

Edited by millonas on 03/21/2014 23:46:34 MDT.

Stephen Komae
(skomae) - MLife

Locale: northeastern US
horizontal straps on 03/21/2014 23:43:26 MDT Print View

After trying and owning an embarrassingly large number of packs I have come to the conclusion that:

- zig-zag straps are cute, but useless for real compression
- two straps on either side are a must
- straps angled up towards the back seem slightly better

Of all the packs I've owned and own, the Boreas Buttermilks does the best job of going from multi-day pack to day pack once it's shed of its base camp gear. It also compresses the easiest with partial loads. I attribute both of those features to the way the compression straps are designed and distributed on the pack.

Packs I've owned that use zig-zag compression tend to bend in weird ways with partial loads, or do not compress enough to prevent flopping around. It's also harder to compress due to compressing 2+ parts of the pack at once.

I should note that some packs like most of the Ospreys have their exterior pockets stitched with a hole on either side where the webbing strap is mounted so the user is given the choice to route it on the inside or out. This might be a solution for you.

Alex H
(abhitt) - MLife

Locale: southern appalachians or desert SW
Re: Pack compression on 03/22/2014 06:51:41 MDT Print View

I agree with the webbing better than zig zag, just can't get good compression with the cords. At least two straps, three better if possible. Horizontal or slighty up to the back. Tall side pockets are a problem but you could design it as Stephen says with a slot to access/accommodate straps. Please no straps over pockets, might as well not have the pockets.

My Kalais has three and they seem to work well

kalais side view

Doug I.
(idester) - MLife

Locale: PNW
Re: Re: Pack compression - Millonas on 03/22/2014 07:11:26 MDT Print View

"sounds like another of those "if there is a truck filled with birds, and the birds start flying inside the truck, does the truck get lighter?" questions. Answer, no. It weighs exactly the same."

Okay, forget Jennifer's questions, explain this answer! I'm not an engineer and never understood this answer, elaborate please!

peter vacco

Locale: no. california
thread drift. on 03/22/2014 08:43:32 MDT Print View

" Okay, forget Jennifer's questions, explain this answer! I'm not an engineer and never understood this answer, elaborate please! "

i suspect what we have here originally is an inaccurately posed question.

because : if the birds are sitting on the truck, and their little feets are pushing down on the truck, then the truck weighs what it weighs Plus the birds. so that there is the weight of the truck .. truck with it's birds. thusly : truck+birds=weight.
weight normally considered to be the amount of how hard something is pushing down. you need gravity (or close to it) to have weight in common terms. no gravity = no weight. and hence the moon-shot like term "weightless".

now, for us earthbound people, living as we do at the bottom of an ocean of air, if the birds take off and fly around inside the truck, or if the truck has open windows, or even if the birds are/were sitting in the back of a pickup, and are now flying all Around the truck (but not touching it), once the birds take off and are no longer pushing down (remember their little feets) on our truck. with flying birds, the truck will indeed weigh less if one is using the common and correct use of the term "weight".

but ... if what the original question meant to use was the was the word "mass", then that's a related, but different thing. because the mass of our truck+birds is the same regardless of if the birds are sitting, or flying, or even near the truck, their combined Mass will not change, nor will it change in the vacuum of orbital space, nor even if the entire lot is partially buoyant in our atmosphere.

much like managerial stupidity, mass is fairly durable stuff in the blue collar world. i assure you, that if peter attempted altering the mass of random objects within the sacramento city limits, if would almost immediately attract the attention of the Authorities. at any rate, being as it's california, no doubt i'd need a permit.


Stephen Komae
(skomae) - MLife

Locale: northeastern US
One more complication on 03/22/2014 09:30:39 MDT Print View

Ah, but we are forgetting that to stay aloft the birds are actually displacing their weight in air by flapping their wings the air must move somewhere, and naturally the truck bed is there to absorb some of that energy. Must take this into account when we consider the force with which the truck is exerting in the ground, which is our typical definition of "weight."

Dave -
(FamilyGuy) - F

Locale: Up there
Re: One more complication on 03/22/2014 10:00:40 MDT Print View

In order to get lift, the birds must exert force. This force must be significant enough to offset the effects of gravity thereby replacing the effects of gravity with the same pull / push. If you stand on a scale and push off, the scale needle will show your weight temporarily with you in the air. If you were able to hoover over the scale with the force necessary to lift you, that force would be pushing down on the scale, reading your bodyweight as a constant even though you would not be standing on the scale.

Newton and all that.

Dave -
(FamilyGuy) - F

Locale: Up there
Re: Re: One more complication on 03/22/2014 10:01:33 MDT Print View

Oh - this all falls apart if the truck 'box' is open, however.

Daryl and Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Call in the drones on 03/22/2014 10:36:12 MDT Print View

Perhaps a drone owner would like to experiment with this?

How does the scale read with:

(1) drone on scale
(2) drone flying above scale in open environment
(3) drone flying above scale in sealed box (with clear lid for observation)

Edited by lyrad1 on 03/22/2014 11:37:00 MDT.

Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
Best thread hijack ever on 03/22/2014 11:49:53 MDT Print View

And it is awesome.

I saw the force of the air displacement will NOT be the same "weight," because the air is a gas and can disperse without increasing overall air pressure inside the truck (unless the truck was a vacuum, in which case the birds would decidedly not be flying, but rather dead).

The feet of the birds are going to be greater PSI than the air displacement, so no, the truck will weigh less if the birds are flying.

Now, if you are in an elevator and it falls uncontrolled to the ground, and you jump very high right at the moment of impact, will you be able to survive the fall?

Daniel Pittman
(pitsy) - M

Locale: Central Texas
Re: Best thread hijack ever on 03/22/2014 12:19:23 MDT Print View

Reminds me of a customer I had at the hardware store. Brought back a window unit air-conditioner that he claimed made his room hotter. Upon questioning, it turned out his room was windowless and he had simply placed the unit on top of a cabinet. it took a while to explain that one to him; not sure if he believed me.

Marko Botsaris
(millonas) - F - MLife

Locale: Santa Cruz Mountains, CA
Holy hijack batman! on 03/22/2014 12:28:03 MDT Print View


I go away for five minute and you kids have gone off on a tangent. Don't look at me! I blame Doug, aka the usual suspect.

Let me just say that the physics is moderately simple (application of Newton's 3rd law), but the "paradox" comes (as it often does in such things) from neglecting things that can not be neglected. Hint: the original apocryphal story is about a bird shipper who wants to want to save weight fees when his truck is weighed. So in this simplified case the bird are flying inside the enclosed truck. Things are a but more complicated if the van is open in various ways. As a homework assignment explain why.

However, this gives me a chance to direct you to possibly the most hilarious episode of myth-busters ever, where they investigate this "paradox" experimentally. In particular their attempts at gadgets to get the birds to fly inside the "van" are a hoot - think dangerous ceiling fan.

Daryl, they in fact tried your drone idea first! But where is the fun in that. Since they are the most-coolest arch-geeks on the planet they just would not stop until they tested with actual live birds. :-)

Daniel, I thought for a second you were gong to say he had it backward in the window, in which case it really would have been a heater. LOL

Jennifer, your elevator scenario reminds me of my favorite "existentialist" joke, a guy is parashooting and is deciding to wait for the right moment to pull the cord. He waits and waits until he is 10 feet from the ground. At which point he say something like "nevermind, I can jump from here".

I hope to be able to say that on my death bed!

Edited by millonas on 03/22/2014 13:04:52 MDT.

Dave -
(FamilyGuy) - F

Locale: Up there
Re: Holy hijack batman! on 03/22/2014 12:49:21 MDT Print View

That was a great episode, which showed that the weight stayed the same.

Sorry Jen. Your premise only works if the truck door is open. Newton was right.

As far as the elevator question, are we in the Matrix when this occurs?

Oh, and I like standard horizontal compression.

Todd Hein
(todd1960) - MLife

Locale: Coastal Southern California
Straps for sure... on 03/22/2014 12:51:09 MDT Print View

I have many packs, including the Kalais. The straps on the Kalais work great. Threaded cords do not compress well. Go for the straps like the Kalais....

Ken T.
(kthompson) - MLife

Locale: All up in there
Re: Pack compression on 03/22/2014 17:22:11 MDT Print View

Never use them. The sleeping bag just gets more room.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: Re: Pack compression on 03/22/2014 19:23:29 MDT Print View

Jennifer wrote, "The other issue is that one of the side pockets will be a tall one for my shelter...if I do the zig zag cord, that can go under the pocket; a traditions webbing strap design would have to go on top of that pocket. Maybe that's a good thing...I haven't decided."

You could still do straps under the pocket, adding some for a tail and an exit hole a la Osprey. The narrow webbing straps on the Osprey Exos worked fine for me and you get to use smaller hardware too. I think that is a good compromise between wider webbing and small cord.

I agree with Ken that you can allow your insulation to fill in for excess space. Straps can help stabilize stuff that tends to bounce like water and bear cans. Note Golite's trick of using hooks and loops at the bottom to reduce volume.

I dont. on 03/22/2014 19:34:02 MDT Print View

I too have yet to ever really compress a pack, other than by rolling the top down tighter.
I removed the compression cords from my Ohm even. Just dont compress the sleeping bag, or maybe put CCF pad inside to when theres room. When the foods gone, well, the pack doesnt have anything heavy in it, its just a soft pillow on the back.

Admittedly, have never tried to use my pack as daypack full of water bottles or such.

I guess I dont see it as anything worth really worrying about. I do prefer the horizontal side straps though if carrying tent poles in side pocket, for obvious reason.

Edited by livingontheroad on 03/22/2014 19:35:47 MDT.

Jennifer Mitol
(Jenmitol) - M

Locale: In my dreams....
Re: Re: Holy hijack batman! on 03/22/2014 20:19:02 MDT Print View

Dave...the truck door wouldn't have to be open...there is plenty of ventilation and it is not a vacuum ;)

As for the compression guys have brought up a big reason why I ask the question. Since this pack will be a tad bigger to accommodate a heavy bear can and large water loads, it IS the heavy, bulky stuff I'm trying to contain. I'm in no way worried about my quilt, shelter or clothes....they'll be fine.

And of course, the closer the weight is to your back the less force there is for you to manage, so a deeper pack will actually require more work than a shallower one. So when dealing with that bulky, fully loaded can, or maybe 6 liters of water, I wondered if I I could get away with the obviously inferior compression system of the zig zag cord...or if I did need to go ahead and do the actual straps.

Franco Darioli
(Franco) - M

Locale: @Tarptent
Pack compression on 03/22/2014 20:30:43 MDT Print View

So why exactly do you want to compress a pack full of live birds ?