Backpacks are usually available in three categories: External Frame; Internal Frame; and Daypacks.
External Frame – typically constructed using a ladder-like frame of aluminum or plastic with a separate pack bag attached usually utilizing clevis pins and split rings. The hip belt and shoulder straps are attached to the frames. Volume capacity of this backpack ranges from 3,000-4,500 cubic inches (49-73 L). The external frame backpack allows space for attaching large items (eg sleeping bags) to the outside of the frame making the carrying capacity more than the volume of the backpack. Features to look for in a backpack include: good lumbar padding, a conical hip belt, recalled shoulders with good padding, and a chest compression strap.
Pro:
* Weight carrying capacity
* More air space so back does not sweat as heavily
* Weight driven higher in backpack allowing more upright post
* More outside pack capacity allowing for versatility in packing
* Generally less expensive when compared to internal frame backpack
Con:
* Raises center of gravity making hiker top heavy and less stable
* Does not hug body and risks wobbling from side to side
* Can cause difficulties with balance when skiing or snowshoeing
* Must be boxed up to prevent damage when traveling by plane
Internal Frame – uses materials such as aluminum stays, carbon fiber, plastic sheets, and foam to create a rigid spine to which the hip belt and shoulder straps are attached. The backpack extends the full height of the pack with compartmental partitions. Pack volume ranges from 3,000-7,500 cubic inches (49-122 L). Internal frame backpacks should have the same comfort features as the external frame described above. Also look for a fabric layer sewn around the top opening of the pack bag which allows the top pocket to be lifted up providing space for additional storage of gear and side compress straps that squeeze the pack down when carrying a smaller load.
Pro:
* Carries lots of weight
* Conforms to body for better balance
* More comfortable to wear for longer periods
Con:
* Back perspiration more common
* More fixed carrying capacity
* Bending over more from lower center of gravity
* May be more expensive
Daypacks – no frame, uses a foam or plastic sheet for back panel. To carry heavier weights check for the following features:
* Well-padded shoulder straps
* Foam hip belt as opposed to just a webbing strap
* Chest compression strap
* Volume capacity up to 3,000 cubic inches (49 L)