Backpack Construction
Frame Types
External Frame Backpacks
FT1 What is an External Frame Backpack?
External Frame Backpacks are a design where the frame is a fully exposed
external frame and the pack bag itself is attached to the sides of the frame
using straps or aluminium pins. The frame material of choice is aluminium,
due to its light weight, although there are external frames constructed
entirely of plastic that provide flexibility while walking. The shoulder
straps and hip belt are attached to the frame using webbing slid through
the frame to provide an amount of adjustability. Some of the better designs
allow the shoulder straps to be adjusted up or down to compensate for various
torso lengths.
In External Frame Backpacks the vertical bars are slightly curved in
an S shape, with cross members attaching the two sides together. Some designs
have or shelf on the bottom, which provides a ledge for a sleeping bag.
The frame generally separates the backpack from the back providing ample
room for air to circulate between the back and backpack. Because of this
distance from the back, External Frame Backpacks are generally considered
to be cooler to carry in warm weather, but are then more difficult to balance
on your back.
Some External Frame Backpacks may also include a cross member that extends
beyond the top of the backpack and body height. This "Cross Member Extension"
can be used to lash extra equipment on top of the backpack, or make the
backpack easier to remove in providing another grab point to lift the backpack
off your back. The downside of this "Cross Member Extension" is that it
can get caught on trees and branches when travelling off trail.
A rigid aluminium frame locks together the opposing movements of the
hips and the shoulders and minimizes the twisting action of the torso. In
some cases, manufacturers have used a variety of techniques to address this
incompatibility of motions, and in some cases they haven't. Clearly, this
is the greatest challenge to backpacking comfort is faced by manufacturers
selling External Frame designs.
The basic External Frame Backpack has remained unchanged for many years
and this design is perfectly suitable for carrying equipment on the trail.
However, a perfectly rigid frame with permanent, non-flexible attachment
points works against the natural body movement. This design has a tendency
to tire the user quicker and increases the potential for sore muscles in
the shoulders, back and hips.
The more refined External Frame Backpack are constructed entirely of
plastic. This provides an amount of flexibility in the twisting motion and
a greater degree of comfort. Flexibility is the key to backpack comfort,
the more a backpack moves with your natural body motion, the greater the
comfort afforded the backpacker.
Internal Frame Backpacks
FT2 What is an Internal Frame Backpack?
Internal Frame Backpacks are a design where the Internal Frame is inside
the backpack. The Internal Frame is either two aluminium, plastic, fibreglass,
or composite staves that run vertically from the top to the bottom of the
pack. In some internal backpacks a ridged HDPE frame sheet is incorporated
in the design for extra stability and back protection. The Internal Frame
provides the primary means of support for the shoulder straps and hip belt,
and a structure to support the pack bag.
An Internal Frame Backpack usually includes a large integral compartment
on the bottom for your sleeping bag. This provides greater protection from
the weather (since the bag is actually inside the backpack) and eliminates
the chance of the sleeping bag being lost or damaged while hiking through
brush. In wetter conditions, this is a major benefit. Since the sleeping
bag compartment is integral with the backpack itself, Internal Frame Backpacks
are larger in appearance and heavier.
There is generally fewer pockets on the outside of an Internal Frame
Backpack, but plenty of lashing points to attach equipment. This is likely
to compensate for the loss of lashing points that might have been provided
by an External Frame Backpack. Extra pockets can also be purchased separately
and are often fully detachable. Many Internal Frame Backpacks use compression
straps to compress the backpack if it is not fully loaded. The compression
straps eliminate the extra space by compressing the entire load into a smaller,
tighter package and prevent load shift.
Regular users of Internal Frame Backpacks may actually find these backpacks
to be over padded and hotter to carry in the warm weather. But the latest
improvements in harness designs of Internal Frame Backpacks do provide greater
back ventilation, as in the White Mountain™
SSS and SHS Harness System. Some designs allow the user to remove lumbar
or yoke padding. Yet others may find the padding provides extra comfort.
The White Mountain™ SYS and SHS Harness Systems allows for the Yoke Cushion
to be removed, therefore providing better back ventilation.
Internal Frame Backpacks carry lower on the back, which increases balance
further and minimizes your chances of catching the top of the pack on trees
and branches when travelling off trail. Manufacturers have incorporated
a variety of design enhancements such as quick wicking fabrics, ribbed padding,
or enhanced harness designs to assist in making the backpack cooler to carry.
The White Mountain SHS Harness System after removing the Yoke Cushion, contacts
the body only at the shoulders and lumbar area of the body.
On Internal Frame backpacks the entire backpack is carried against the
back, there is a greater surface area to transfer weight between the backpack
and body. On External Frame backpacks, the weight is typically carried at
only four points on the pack frame, the hips and shoulders. With Internal
Frame Backpacks, you have a feeling that the backpack is an extended part
of your body.
Body motion considerations in design that offer a great deal of freedom
of movement, are the reason Internal Frame Backpacks have dominated most
of the backpack market, particularly in off trail, ski touring and climbing
situations.
FT3 What is the function of the Internal Frame?
Weight Transfer
The Internal Frame should provide weight transfer to the backpack so
that approximately 80% of the weight is transferred to the lower back and
hips. The Internal Frame will control the backpack shape and in profile
should maintain a close spacing between your body and the backpack. There
are backpacks that have Internal Frames that are too light, or soft to maintain
this crucial shape and these should be avoided when making your choice of
backpack.
Pack Rounding or Pack Twist
The Internal Frame should prevent the backpack from rounding out, a common
occurrence with daypacks, or frameless rucksacks that are over stuffed.
The White Mountain™ SSS and SHS harness systems have a
Interlocking Cross Frame to prevent rounding out or pack twist. The
EYS harness system has a large solid sliding yoke that performs a similar
function and the SYS harness system relies on the a smaller solid yoke with
a very stiff vertical webbing in its ladder design. All of the White Mountain™
harness systems are designed to prevent pack rounding and pack twist.
Flex
The Internal Frame should work in harmony with the body, with enough
flex to contract and spring back as required and allow natural movement
without excessive distortion. The Internal Frame if too soft or too stiff
would not provide the necessary dynamic synergy required in a well designed
Internal Frame Backpack. The result a poor Internal Frame would be a loss
of balance and wasted energy wrestling with the backpack at every movement.
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