Backpack Frame Types
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
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 faced by manufacturers selling External Frame backpacks.
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.
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 improvements in harness designs of Internal Frame Backpacks
do provide greater 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
harness systems only contacts the body with Air Mesh, through the
Harness Shoulder Straps, lumbar padding, and hip belt of the backpack
allowing adequate space for air circulation.
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.
The Internal Frame should provide weight transfer to the backpack
so that approximately 80% of the weight is transferred to the lower
back. 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. Some backpack harness systems have an Interlocking
Cross Frame to prevent the possibility of rounding out or pack twist.
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
of a poor Internal Frame would be loss of balance and wasted energy
wrestling with the backpack at every movement.