An external frame backpack looks like a regular satchel attached to a metal frame. It usually has extra space above and below for attaching sleeping bags and other gear. There are straps and netting between your body and the metal frame that prevent the metal frame from coming in contact with your back. There are many advantages and some disadvantages to using this style of backpack for carrying large loads across long distances.
Advantages
Since the satchel component can be removed from the frame, external frame backpacks can accomodate loads of varying sizes and weights. External frame backpacks also provide different places where you can hang additional gear or attach bulkier items such as sleeping bags and tents. In general, this style of backpack is less expensive than more sophisticated internal frame models.
Disadvantages
External frame backpacks usually take up more space when empty than traditional backpacks, due to their structured frame. In addition, the satchel portion of the external frame backpack tends to be smaller than alternative backpack styles.
Benefits While Wearing
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Because of its unique frame and strap configuration, which lifts the load away from your body, external frame backpacks have better ventilation and are cooler to wear. Since an external frame backpack extends from below your waist through the spine, it enables you to walk more comfortably and erect.
Negative User Experiences
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Users of external frame backpacks report that they can be heavier than normal backpacks and hard to maneuver on uneven terrain. The structured frame can become caught on trees while hiking. In addition, the metal frame can squeak and needs to be lubricated periodically.
References
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