External pack frame flexibility.

All my diy carbon fiber frames have flexible connections (to one degree or another) where the verticals meet the top cross bar.

Works for me.

The old school external packs were welded: any flex came from the mesh back panels and waist belt mounts. Some were as noisy as squeaky doors too. Jansport went through all kinds of gyrations trying to increase movement, culminating in the "D" series packs with wings to support the hip belt.

Black Diamond has made some interesting swivel suspensions, with a single bolt to hold the hip belt and adjustable limiter straps. The shoulder straps have a sliding cable arrangement to aid motion.

Give the Osprey Exos series a good look for more ideas. They use the trampoline back and very flexible strap/hip belt construction and attachments to pull off their rigid frame.

>"culminating in the "D" series packs with wings to support the hip belt."

Popularly called, "torture tabs".

Perhaps I should clarify.

We cannot always go UL.

This is for a pack to carry 25lb and above.

I've wondered aboy making a carbon fiber "pan" to attach the suspension and pack bag onto. Vacuum laminated carbon fiber sheet with foam stiffening channels could be very light and have all sorts of configurations. It would be easy to make it adjustable for torso sizes.

Alister,
Will respond as if posted on MYOG.

I've been working on an hourglass frame using Easton tubes for the perimeter, but obviously had to have some cross tubes to keep the pack contents from sticking into my back and rendering the mesh backpanel useless.

So I looked in many hardware stores and websites for barbed nylon hose Tees used to connect plastic tube or rubber hose. They come in many sizes, but what was important was the Inner Diameter (ID) of the Tee, that would fit snugly over the tubes. (The diameter used to size the fittings is the ID not of the Tee, but of the hose or tube, so is not helpful.)

Since I'm using Easton .340 7075-T9 tent pole tube, the best fit came from a 1/2" Tee sold in Home Depot stores under the brand "WATTS," and labeled Nylon Hose Barb A-381. The tube is loose in the barb portion of the Tees, but if that is cut off, the remainder can be bored for an extremely tight fit over the tube. The nylon is flexible, but strong, which sounds like what you are looking for.

For other sizes of alloy or carbon tube, you will have to do your own hunt, but I've been able to find Tees that can be bored for just about whatever diameter tube I want to use. The key is to have a good set of quality drill bits in numerous sizes. Yet to be decided is whether to bond the tubes in the Tees, and if so, what adhesive to use. Since the nylon and the alloy are so different in composition, some bonding tests would be needed to try to find something that adheres to both the nylon and the alloy. I fear if you install pins or fine screws in the alloy, it will lead to cracks and failure, so ruled that out. The 5/8" tubing used by Jansport and others for frames can handle some drilling a tapping, but the ultra-lite materials are not as rugged.

Note that the hose fittings also come in polyethylene and other plastics, but I chose the nylon for its strength. Daryl has posted a number of times about using hose tees that fit carbon tubes, so you might want to PM him for info also. And you could always make your own Tees as Roger Caffin did – you can email him at BPL.

One question: To keep the pack away from your mesh backpanel, won't you need curved or bent crosstubes, which I haven't been able to find in carbon fiber. (There are some pages on eBay though for makers of tiny drones that use a variety of shaped carbon fiber parts.) So you might need to use alloy for the cross tubes because it can be prebent to the shape needed. Once having taken that step, it may lead to using alloy for the vertical tubes also, to achieve a shape that conforms to the back, as on the pack frames of yore. If you end up bonding alloy to alloy, JB Weld sold in WalMart is great stuff, as suggested by BPL member Dale W.

Finally, you might be tempted to try the many flexible fittings sold by the Kite sites, Goodwinds being one of the most popular. I found these fittings don't hold up very well to repeated flexing under weight, and eventually split. Also, any boring makes them even more likely to fail.

There are a number of threads in the MYOG archives about this stuff.
Good luck.

Flexible packs are old news. Flextrek and Steve Climber have beat you to it:

Yes I am aware of the use of nylon Tees. I have read as many of the MYOG pack threads as I can find, but none actually answer the question I have raised.

Does flexibility add comfort?

I am also aware of the Flextrek pack, but again this did not answer the question. In fact that was what caused me to raise it.
It is always difficult to tell fact from Marketing on a manufacturers web site.

I have also previously corresponded with Roger, but that was mainly regarding the making of a pack which could be dis-assembled for air travel, based around a variation of the one he has published.

I have the capability of making curved carbon fibre tubes, rod, or sheet in whatever form I want, so this is not really a discussion for the MYOG section. Although it may become so eventually.

I have a number of packs of all styles,frameless, internal framed and external framed. I have always assumed that for heavy loads, rigidity was a good thing. In fact the first pack I ever made was an welded aluminium external, which turned out to be less satisfactory than those available commercially. It was certainly very rigid!

So having decided to attempt a light weight external for heavy loads, I thought this forum may have good advice to offer.

So although your replies are greatly appreciated, I'm not really further on. It seems to me that the flexibility built into lightweight MYOG packs has occurred by happistance rather that deliberately. Am I wrong?

Just a further thought, it seems from the comments above that flexibility does not HARM comfort at lower weights, and is possibly a good thing if not overdone.

But does this hold true at greater weights?

Alister, look into the Paradox Packs Evolution frame. It does what you suggest, and works very well.

If you make all the T-junctions flexible the whole frame will collapse when the mesh is tensioned. So I make all my joints 'rigid'. They aren't that strong, but they are strong enough. The mesh (Leno Lock) sprung across the frame works very well.

Cheers

Alister,

"Would comfort be improved?"

I would rate the comfort of my old somewhat flexible Jan Sport external frame pack and my diy very flexible carbon fiber frame about the same with 40 lbs in the pack.

I don't have experience with a frame less flexible than the Jan Sport and I don't have experience with more than 40 lbs in my diy carbon fiber frame.

The flexibility of my diy frame is more happenstance, as you say, than by design.

Flexibility in my diy frame also allows my to use lighter materials than if I had rigid joints. When stressed the frame "goes with the flow" rather than breaking. (think palm tree versus pine tree).

I do get some pleasure from feeling my backpack frame move with me. This is particularly noticeable when I'm using my arms to climb a steep slope and grabbing on to bushes or a rope for assistance.

I don't use mesh between pack and back. My old MSR frame pack, which I used for many years, had a rope mesh and great separation between pack and my back. I didn't like it. The cool breeze running up my always wet back was usually uncomfortable. I sweat more than most, however, so I assume this won't apply to most people. I just added this info because I like to type and couldn't stop.

Roger, thanks for your comment. That is exactly what I feared would happen if care was not taken to build in flexibility in the right way. Simply using flexible "rods" would not work.

Daryl, again thanks. As you can see from my comment below, use of mesh has disadvantages when it comes to flexibility. It limits either the load that can be carried or the rigidity of the frame. The manufacturers that use mesh only do so on their lighter weight packs, switching to padding as the size goes up.

The Zpack Arc-blast is interesting, effectively using pre-tensioning and carbon fibre straps rather than round rods to achieve a good result.

David, your reference to looking harder at Paradox caused me to spend several hours chasing down ever reference I could find on the Internet. If I understand correctly their frame has three main points.
1) An S shape to fit the back better like any internal frame
2) A rotating joint on the lower cross member to allow the verticals to change angle with regard to each other, (I call this "twist".
3) No horizontal cross member that I can see to provide added stiffness to prevent "lozenging". There may be a top member but with rotatable joints which would allow "twisting", so I could be wrong.

The interesting thing is that the rotating joint is loaded, so as the weight goes up it is more difficult to move. This is just what is needed, and is the reverse of what would happen if fully flexible rods were used.
However again in this pack, substantial padding is used, and just the frame assembly is quite heavy.

I was following my wife up a steep and rough bush track a couple of days ago, and thinking about this problem. I noted that her back twisted considerably as she pulled herself up by convenient trees, ie hips and shoulder did not stay in the same vertical plane.
Her spine also bent so hips and shoulders did not stay in parallel , but the difference was not be so great.
So either the pack had to change shape ie lozenge and twist, or the pack had to move on her back and hips.
This explains why, in ancient times, when we used only rigid external frames, modifying them with hip belts was counter-productive, and they were quickly removed again. With heavy loads, your back angle sloped more forward, and the load was more evenly distributed vertically across your back, so there was no need for a hip belt to carry load. The packs could also "roll" on your back.
In fact I don't remember my pack causing more shoulder and back discomfort at 60lb than 30. Pain elsewhere, definitely! (Yes, we did carry that weight at the start of a 12-15 day alpine trip).
Reading Roger's pack article again, I think with his narrow hips and straight back, that is why his rigid frame pack works for him, with the mesh taking up minor variations. But I know some ladies simply could not get on with the old packs, possibly because the frames could not be bent to their back shape, and the webbing used then did not have adequate adjustment. Maybe that is why Sue found the macpac more comfortable despite its weight.
I don't have any conclusion at present. Certainly I know Roger's style pack works for me with adequate comfort. I would still like to know if a frame that could lozenge and twist but using mesh would be better. In fact could it be done?

I would appreciate any comments.

There is a carbon rod in the upper encasement of the Paradox which adds stiffness. Not connected directly to the alu frame. My forthcoming article will have photos to fully illuminate this.

At over two pounds the Evolution frame isn't super light, but considering what that weight includes (a rigid frame, good hipbelt, and a whole bunch of webbing for the compression straps) I think it is quite light. There certainly aren't any obvious places to save weight without sacrificing long term durability (mainly in the form of seam strength, I'd imagine).

> Reading Roger's pack article again, I think with his narrow hips and straight back,
> that is why his rigid frame pack works for him, with the mesh taking up minor
> variations.
Agree

> But I know some ladies simply could not get on with the old packs, possibly because
> the frames could not be bent to their back shape, and the webbing used then did not
> have adequate adjustment. Maybe that is why Sue found the macpac more comfortable
> despite its weight.
That was her opinion.

Cheers

David C.

When do you anticipate your article on the Paradox pack will be published?

We have prototyped lighter weight options on the Paradox, but the sacrifice was in long term durability, or long term comfort of a component like the hipbelt. At lighter weights, we could save a few ounces, but I'm not sure it's compelling.

Kevin

Flexibility in external pack frames is a game that has been played a lot of ways over the years, from Jansport's mechanical connections and relatively thin-wall tubing to the North Face "Back Magic" packs with a nylon flexible arm that attached the hipbelt to the pack, to the plastic frames that Coleman used to sell – which were pioneered by someone else – and the real trick is to have the right kind of flexibility. For the most part, if you are walking, your spine doesn't bend sideways or front to back a lot; what it mostly does is twist, and that's what a pack frame needs to do in order to be more comfortable. And to save energy, because if you are working against a rigid frame you will waste some energy on that resistance. And at the same time, the frame needs to have enough vertical load carrying capacity to transfer weight to the hipbelt. So balancing those aspects is the tricky bit, and as the load goes up it gets trickier. At really high loads even a welded frame is flexible – if you've ever had an external loaded with 70 or 80 lbs (it was mostly old tin cans I was packing out as trail cleanup) you can feel it, just stand the pack up on the ground and wiggle the tops of the frame. The same frame with a 40 lb load seems pretty rigid.

Sop if you're designing an external frame and want flexibility, think in that direction – how do you get a frame that twists easily but doesn't compress or bow too much under a comnpressive load. My first thoughts would be maybe a central column with arched crossbars clamped to it and tensioned bands on the arched bars, with the bands contacting your back. Or you could do like a the old Camp & external/internal frames, whihc were a "X" of flat aluminum bars with a mesh panel tensioned from the corners of the "X" and hipbel attached at the bottom corners of the "X".

> the frame needs to have enough vertical load carrying capacity to transfer weight to the hipbelt.

I will disagree. My external frame packs do not transfer much load to the hip belt at all. The hip belt really just functions as an anti-sway belt for scrambling. It's quite soft.

That said, the vertical part of the frame does have be strong enough to carry the load, which usually gets connected to the shoulder straps at the bottom corners of the frame – usually the bottom ends of the vertical rods.

Cheers

Ross, I'd imagine in the next few weeks. It is in the editorial mill as we speak.

Roger, I don't understand the point of a frame if you are carrying most of the weight on your shoulders. I am honestly curious why you want a frame. I hate carrying any weight on my shoulders (even 5 lbs) so the concept is foreign to me.

Hi Larry

> I don't understand the point of a frame if you are carrying most of the weight on
> your shoulders. I am honestly curious why you want a frame. I hate carrying any
> weight on my shoulders (even 5 lbs) so the concept is foreign to me.
I agree it sounds strange, especially if you have never carried an external frame pack before. But there is a reason why things like the Kelty external-frame packs (sometimes known as 'trapper frames') are used for serious gear-hauling. They work, way beyond what you can do with an internal frame pack.

Actually, I don't think the load is all on my shoulders. A lot of it, if not most of it, seems to go through the mesh or webbing across the frame and onto my back, partly by friction. You lean forward a bit from the ankles, and it works. Can't beat actual field testing.

Mind you, when I was hauling about 24 kg (~53 lb) of winter gear and skis in one time, a lot of the load did go on my shoulders – with my UL external frame pack. Things got a whole lot nicer when I took the skis, boots, stocks and base camp gear and reserve food off the pack!

Cheers

I have used several external framed packs (mostly Jansports) and I always carried most of the weight on the hips. I would adjust the shoulder straps so the top attachment point was above my shoulders. I also often loosened the shoulder straps and allowed the pack to barely rest on my back insuring the weight was on my shoulders (I hate weight on my shoulders, it hurts). The last one I had had the "wings". It kept the weight off my shoulders, which I like, but put all the weight on the hips at the attachment points. Combined with an under padded hip belt and I ended up with bruises. I was carrying a heavy load (over 50 lbs) for short distances (less than 2 miles, and down hill with the consumables).

It seems to me that with lighter loads the stiffness of the frame can be less, even for me who prefers all the weight on the hip belt. I still prefer a stiff frame, but it takes a lot less of a frame when the load is lighter. :^)

In earlier lifetime I used Kelty external frame

You can put most of the weight on your shoulders for a while, then shift most of the weight to your hips for a while, let the other rest some

But, I can do the same with frameless pack – sometimes more weight on shoulders, sometimes more weight on hips, but because there's so much less weight it doesn't matter as much

Yeah – friction causes weight to be carried directly from back. But, then I sweat more. Sometimes good for there to be air space between pack and back.