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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