How is "Backpack" Weight Capacity Determined???

BPL Article . HOW PACKS WORK .

since this is in the MYOG forum, it’s the max weight you can comfortably carry, which is subjective

getting close to 20 pounds you need a waist strap which transfers load to hips

going much over 20 pounds you need a frame which keeps the pack from collapsing so weight is better transferred to hips

What about a pack with a weight rating that is not a “modern backpack,” say, frameless and without a hip belt?

Sorry, this is a hypothetical materials question.

I bought Dyneema from a site that rated the tensile strength. It was over 200lbs. I suppose one could use this material to make a carrying device for a strong machine and rate that carrying device for a high weight based on the tested strength of the carrying device’s materials and how they are connected (the build).

For my myog frame packs its a rough calculation, based on experience,  with a margin of error thrown in.

So I’ll make a pack and use it 50 times or so with 40 lbs.  If everything works and nothing breaks I’ll say its good for up to 30 lbs.

Daryl! That is a very logical a answer!

• I guess your process is pretty scientific too… lots of testing sounds reliable.

Thanks for the reply!

The tensile strength of the fabric is not the determining factor, it’s how much your shoulders and back hurt

Fabric can abrade, or get ripped by a branch, or seams rip out.  Not directly related to tensile strength.

Generally, there are a lot of things involved. Perhaps the least of these is the actual weight capacity (as measured by the actual weight a pack can carry without breaking) as opposed to the carry capacity (which is often conforming to different rules under the heading of “comfort.” Personally, I am NEVER truly comfortable carrying ANY load.)

Comfort levels on a frameless vs foam frame vs stays vs internal vs external framing all change and is pretty meaningless. What does comfort mean? If comfort levels are basically meaningless in this context, maximum weight carrying is meaningless, too. The point at which seams fail, shoulder straps rip loose, hip belts tear off, etc can very wildly between the same pack model. That said, I have never seen any pack fail with double the maximum weight carry capacity. Terrain can effect this too, with hopping up & down rocks placing large impact strains on pieces of gear that smooth, walking trails do not…or bushwhacking placing extreme tearing strains on packs not found on open trails.

I use small packs, with a virtual foam frame. Typically these are rated at maximum loads of 20-25 pounds. However, I use these packs for longer two week trips, with 23-25 pounds. I use the same pack for exercise carrying about double the “maximum load” rating. For example I use a 2012 Murmur rated at a 20pound load capacity with ~45pounds as an exercise pack, easily doubling the maximum load.

Would I ever send it back because of some failure? No. The rating says 20pounds. In essence, the manufacturer will not guarantee the pack because I used it beyond their spec. The manufacturers are simply protecting themselves. In most cases, I believe this is the biggest reason manufacturers list a maximum load capacity. Indeed, this extends to pack dimensions, also. Example: if you continually carry a larger 9″ bear ball in a small pack with a 4.5″ maximum width, you are putting stress on the pack where it was not designed to hold it allowing thinner fabrics to take abrasion where it was not designed to. For example a Bear ball in the bottom of a pack and stiff side pouches do not mix well. You cannot fit a water bottle in the pouch without really stressing the fabric/seams.

While not much of a concern with traditional packs (>2 pounds,) we see smaller packs running into different limits, strength of fabrics, quality/type of sewing, design/dimensional limits, limited use of heavy duty laminated fabrics, etc. For someone who does 90% of their backpacking UL, it becomes important, though. Every ounce of base weight matters. Packs are one area that a UL person can easily save a pound of weight or more over a traditional pack at over 2 pounds. (Cloths, tents, sleeping gear, cook gear being the others. Lowering the weight of food, fuel and water doesn’t effect base weight.) Packs are built to less durable, but lighter standards: lighter fabrics, lighter construction techniques, lighter hip belts, etc. They will NOT hold as much maximum load.

So, in answer to your questions, I don’t think it is either of the two reasons you mention. It is simply the manufacturer protecting themselves from a lot of repair work.

Sometimes, it is mostly salesmanship. A HMG Windrider 2400 has a spec or 40pounds maximum load. Yet a HMG Windrider 4400 has a spec of 60pounds maximum load. Same materials/same design in either case. It doesn’t make sense when you think about it. But paying more for something means you think you are paying for more.

Subjective rating with the maker’s experience thrown in. This usually only comes up with cottage industry ultralight packs. I don’t think durabilty/warranty adds into it much, but customer comfort and satisfaction does. The weight ratings given are more of a reality check for the inexperienced buyer.

Volume measurements are my pet peeve.

Dale, good points. Thanks for helping me hone in on reality.

And yes… -gotta love those volume metrics ;-)

@James Marco, I agree with you regarding salesmanship in general.

Re: HMG packs. I do not have specific knowledge. However, I remember reading that some HMG packs have beefier frame stays, particularly Porters and larger capacity packs 4400 and up as compared to the smaller packs. May explain the higher capacity of the 4400 Windrider. Whether the change has any real life impact is also another story and not something I have any experience with either.

I don’t believe the frames/stays have anything to do with maximum weight handling. They contribute to comfort by GREATLY influencing the distribution of the weight in a pack.

For weight handling you need to look at the weakest points(not in any order):

1. Strength of the shoulder strap attachments
2. Strength of the hip belt attachments
3. Seams
4. Strength of material used
5. Strength of the bottom of the pack

The maximum weight is a function of these things (and perhaps some others I forgot.) Not really a function of how the weight is distributed.

The weight recommendations are for maximum comfortable carrying weight, not for a weight that cannot be exceeded due to fabric/component failure. I doubt that any cottgagr makers do failure testing and rely on field tests with prototypes and samples.

It’s rather antithetical to build an ultralight pack and then load it up up with 200-300% of what a typical ultralight load would be. It just doesn’t make sense. Anyone who has backpacked would understand that it would be very uncomfortable and beyond the scope of the design.

And yes stays are for weight transfer and part of the whole suspension and effect the  weight comfort level.

I don’t think that normally, failure of seams or strap attachments or whatever is what limits the maximum weight

It’s how much of the load is transferred to the hips.  A wide enough hip belt helps.  A frame will prevent pack collapse.  If the pack “collapses” then your shoulders will carry more weight.

And wide enough shoulder straps where they rest against shoulders helps.  Narrow straps cut into shoulders and are uncomfortable.

Many companies list a maximum weight for carrying. Gossamer Gear does so (they actually list comfort and max load separately.) Zpacks says “The material and design can handle loads up to…”  HMG says “Load capacity: Up to …”  ULA says “<b>MAX LOAD:</b> …”  Even Aarn packs lists a “maximum load…”

Maximum weight is often the maximum weight that can be carried for any length of time, ie more than a couple days. I have had packs blow out exceeding those values. (For example, the old GG G5, I ripped a shoulder strap lower buckle loose carrying gear for my wife when she twisted one of her knees.) The maximum weight was 15 pounds, I exceeded it by a 233% overload with about 35pounds. The old spinnaker pulled and ripped loose, but not quite off, it was repairable. (Material failure)

Last year, a 12 year old pack was being used to carry tools into a lean2rescue site (we picked these up on a trail intersection.) The upper seam on a shoulder strap partially failed over about 7 miles to the site carrying about 60 pounds. The pack was only rated for 20pounds or 300% overload. Again, repairable. (Seam failure)

About 35 years ago, I had a pack on while carrying a canoe along a portage for about 3/4mi. The pack was loaded with about 30pounds of gear and I loaded the boat, then slipped the yoke onto the pack. The shoulder strap started pulling loose right through the fabric after carrying the ~95/100 pounds to the end of the portage. Roughly a 238% overload. Repairable. (Material failure)

In every case there was well over 220% overload of the maximum weight listed for the pack. The first two were from a single on-line manufacturer and the third from a local company.

I do not know that they are testing for maximum possible weights, but I suspect they are…even if it was similar to what I did for Gossamer Gear testing their prototypes sorting out problems. Every time I have exceeded 220%, I have had some sort of real failure: Blown-out side pouch, ripped out bottom, loose shoulder strap, blown out side panel, ripped out hip belt or something. A 200% overload has never caused a failure on the trail, but I wouldn’t care to try it.

Yes, these were mostly Light and UL packs, except the traditional 45year old Tough Traveler at just over 2 pounds (2#2 as I remember…) Again, lower weights mean lighter materials. Some failures at 200% are expected and at 300% almost guaranteed over about a week on a trail.

Stays, virtual frames, internal frames, etc don’t really prevent these types of failures. They are caused by raw weight stressing material, seams and design. For example the Aarn packs. While a balanced weight carry (strap and belt and front packs) can help, putting on a heavy pack can often stress one component or the other leading to a failure. Lifting a pack by one strap to sling it over your shoulder means the ENTIRE weight is on the one seam near the top, for example. Comfort and load carrying are two different animals.