Backpack Construction

Backpack Frame Types

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 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.

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 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.

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. 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.

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 of a poor Internal Frame would be loss of balance and wasted energy wrestling with the backpack at every movement.