Help with the maths

[mfolch]

I’ve been comparing packs and pockets, and I’ve noticed some interesting mathematics, figures that don’t add up.

Take, for instance, the dimensions of Stone Glacier’s camp pocket. The website says they’re ‘6" x 9" x 3.5" (Height x Width x Depth)’

But then they say the volume is: ‘322 Cubic Inches’

Now, it’s been 20 years since I studied math, but last I checked, volume is Height x Width x Depth, which in this case is 189c”. So, what am I missing? Where did the extra 133” come from?

Probably the answer is obvious...


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[beachbunny]

i think you and i went to the same school cause that is the way i remember it also.
maybe it's the "new" math.
i'm curious also

 

[bpctcb]

I hear you on the math stuff. But our figuring is for rectangles made out of hard sided material, not Cordura.
I suspect they stuff their packs/pockets full of something (beans?) then measure the volume of the beans. Not sure here; just guessing because I’ve heard of the Kifaru guys using beans to measure pack volume.

BP

 

[beachbunny]

now that sounds reasonable, thanks

 

[Gr8bawana]

All that math makes my head hurt!

 

[pods8 (Rugged Stitching)]

I hear you on the math stuff. But our figuring is for rectangles made out of hard sided material, not Cordura.

This. You might make a pack with rectangular dimensions but when stuffed it will round out and a circular shape of the same perimeter as a rectangular one is going to have more area in it.

 

[dotman]

Yeah I know Kifaru uses beans, assume the others do as well.

 

[Gunnersdad49]

Yeah I know Kifaru uses beans, assume the others do as well.

With their loyal following, they should use coffee beans. Measure a pack, call them adventure beans or something, and people would buy them up! LOL.

 

[COlineman78]

I hear you on the math stuff. But our figuring is for rectangles made out of hard sided material, not Cordura.
I suspect they stuff their packs/pockets full of something (beans?) then measure the volume of the beans. Not sure here; just guessing because I’ve heard of the Kifaru guys using beans to measure pack volume.

BP

Makes sense as a way to measure volume, however a unless the pack is made out of latex a 40% increase in volume is not possible, so one of their measurements is wrong. If you make a vessel out of rectangular cross sections there is no way no matter how you sew it that you can increase the size by deforming it other than the material stretching.

As far as volume measurements though, some companies like Exo don't measure external pockets and only use the volume of the main bag as their rating. For example their 3500 bag is probably closer to 4500-5000 once you include the external pockets and lid.

 

[pods8 (Rugged Stitching)]

Not sure if I misread your statement but yes something designed as a rectangle from flexible materials which will round out will increase in volume when stuffed. For example a circle of the same perimeter as a true square will encompass 127% of the area the square does.

 

[Rokwiia]

The material doesn't need to even stretch in order to achieve increased volume. All you need to do is have one side of the rectangle made of excess material so that the material fully expands when the rectangle is filled.

 

[pods8 (Rugged Stitching)]

Its not an excess material issue its that while you might make something out of 4 sides the fabric isn't ridged so it doesn't form a rectangle when stuffed, the sides round out unless somehow constrained, this is because rounding out forms more volume as you cram stuff into it.

 

[Kevin_t]

I'll chime in a bit. There is an official spec , some companies follow the spec to a T and some not at all. Some companies have a good reason for not following the spec , especially in the UL world. We (Seek Outside) try to give an accurate representation against other packs that is very very close to spec. So for instance, a Brooks is pretty close to spec , a Divide is in our world a little more closely aligned to how the UL companies do it.
The best comparison is bag sizing but that still is not perfect and there are variables

In essence, it may never make perfect sense.

 

[boom]

With their loyal following, they should use coffee beans. Measure a pack, call them adventure beans or something, and people would buy them up! LOL.

then we would get threads about the best damn beans on the planets and how they make your 16 oz coffee feel like 12oz.

 

[boom]

i think their math is simply wrong. if one size squeezes out, another side squeezes in..

 

[pods8 (Rugged Stitching)]

i think their math is simply wrong. if one size squeezes out, another side squeezes in..

Not in a rectangular fashion which is what was originally referenced dimensionally, the rounded shape encompasses more area.

Example. A 12"x12" square pack profile has an area of 144sqin which if 20" tall would calculate as 2880cuin if all 90deg angle flat sizes. Now that 12"x12" square has a perimeter of 48". A circle with a perimeter of 48" has a diameter of ~15.3" and an area of ~184sqin so in that same 20" height would yield 3680cuin. This of course is ignoring the bottom of the pack is somewhat constrained by the bottom piece and also the frame will limit how much distortion occurs on one face of the bag, etc. but shows how sides bulging out and rounding can increase the area of the profile and thus the volume of the container.

 

[boom]

Not in a rectangular fashion which is what was originally referenced dimensionally, the rounded shape encompasses more area.

Example. A 12"x12" square pack profile has an area of 144sqin which if 20" tall would calculate as 2880cuin if all 90deg angle flat sizes. Now that 12"x12" square has a perimeter of 48". A circle with a perimeter of 48" has a diameter of ~15.3" and an area of ~184sqin so in that same 20" height would yield 3680cuin. This of course is ignoring the bottom of the pack is somewhat constrained by the bottom piece and also the frame will limit how much distortion occurs on one face of the bag, etc. but shows how sides bulging out and rounding can increase the area of the profile and thus the volume of the container.

makes perfect sense..i was thinking more rigid.