[pct-l] packs: comfort vs weight: Why packs work

[reynolds@iLAN.com (Reynolds, WT)]

Now that everybody has voiced their opinion of which pack is best, I'll
present some facts.

1. The HIP Belt
The big improvement in packs was Dick Kelty's invention of the hip belt.
Although there are other ways to relieve weight on the shoulders including
seriously reducing pack weight, the goal of the hip belt -- to transfer the
load from the shoulders to the pelvis -- is what makes carrying heavy loads
possible.

An ideal hip belt will transfer 100% of the weight to the centerline of the
pelvic bone. Take your thumbs and press them into your waist at the hip line
right in the middle of your body. That is where you want 100% of the weight
to rest. If the weight is behind the center point of the pelvis then the
pressure will rotate the pelvis [your a$$ rotates towards your private
parts] causing some degree of fatigue and discomfort. Probably the most
perfect hip belt is the Jansport swingarm system. In this old external
design a metal [later plastic] horizonal arm connected the pack frame to the
exact centerline of the pelvis. Other old designs included the A-16 and
Stephenson wraparound designs where the pack frame actually wraps around the
pelvic bone to deliver the weight to the pelvic centerline. A very simple
design was use by Kelty in his Tioga. A strap extended up from the end of
the pack frame to the pelvic centerline on the hipbelt.

Given the above geometery, lets examine some modern hipbelt designs.
1-Dual half belts.
This is a weight saving approach. Straps are sewn to the pack bag to create
a "hip belt". As a result the weight is transfered from the shoulders to the
A$$. The pelvic bone is under constant rotational pressure. However, a very
lightweight pack can be built in this manner, witness Glen Van Peski's
half-pound packs and many homemade packs. In my opinion the effectiveness of
this design is based on how much pelvic rotational pressure one can stand.
Based on talking to many thruhikers, the average maximum is less than 20
pounds. If the load goes over this, the solutiuon is to tighten the shoulder
straps, relieving pressure on the A$$. Additionally, the average maximum
weight that is tolerable decreases with time. A hiker may start out with
loose shoulder straps but end the day with very tight straps and even the
waist belt unbuckled.

The good news is that the total load is typically reduced by 4-6 pounds. Not
trivial.

2-Separate Hip belt
The goal of a separate hip belt is to wrap snugly around the waist, then
have the weight spread across the back as much as possible. Almost all high
quality packs [Dana for example] use this approach. Manufacturers have
become quite creative at desinging milti-layer belts with space-age foam
materials to see that the hip belt wraps snugly around the waist and spreads
the weight as evenly as possible. A variety of lumbar pads try to keep the
A$$ comfortable while very extensive shoulder strap systems make transfering
the weight to the shoulders less tiring. Additional, stabilizing straps run
from the pack to the pevvic centerline on the hip belt like the old tioga. 

A new design by Kelty seems to work well on people with straight hips. The
Kelty Cloud hip belt is stiff. It won't flex very much at all [making it
unsuitable for a woman's curved hips] this approximating the Jansport
swingarm system while space-age elastomer foam hugs the hips. 

It should be clear that all this cleaver design is to overcome the geometery
that says the weight is on the a$$. In fact, the designers have been
reasonably successful. Women, who typically can't find a proper fitting
external frame pack, tend to favor this type of pack. Still, for straight
trail walking, this design is inherently inferior to the old externals and
internal frame packs don't save weight over external frames. The general
rule is that comparing external and internal frame packs, for a given weight
and comfortable, 30% of the weight should be carried on the shoulders of an
internal frame pack wearer. To some this is no problem at all. To other it
is very uncomfortable.

My personal experiences:
One solution I tried was to use was to combine an external pack frame with a
lightweight pack bag. I simply stripped the old codura pack off a Jansport
Yosemite and created a lightweight replacement. This saved 2.5 pounds. The
resultant external frame pack was 3.5 pounds and carried better than any
commercial internal frame pack [Ladies: this may not be true for you, but
then it may]. Since old Jansport D3, D-5, Yosemite and other packs that used
the swingarm suspension are available at garage sales for cheap, I believe
that this is an approach for someone going not ultra-light. You may also be
able to find the old A-16 frame and belt.