Two of the major forces acting on our body under a loaded pack are the actual weight of the load, and the leverage that load exerts on our body. While we cannot change the actual weight of the load we need to carry, we can manage how much leverage that weight creates. Generally speaking, this is felt as leverage pulling us back, causing us to lean forward and engage our core to maintain balance. This added force can be as high as 40% of the total load weight on a poorly loaded pack, even more on a pack frame with short or no load lifters. This extra leverage also makes the load more unstable, requires more effort to remain balanced and in control. In technical country where every step counts, or while on crampons, it can also add an unneeded degree of risk.
It is hard to put an exact number to the additional force created, so we built a jig to better test and understand the mechanical advantage. The jig simulates a 25” tall pack frame with a base 12” wide by 11” deep, the approximate size of many 6000 cubic bags. To simulate the fulcrum created at our lower back with a loaded pack, the frame pivots at the hip belt area and has a scale attached to the load lifter to record the force. There are 6 sand bags with a combined weight of 76 pounds.
The first photo simulates loading the meat the full dimension of the bag bottom, the second photo simulates loading the meat in a shelf and compressing it to the frame. Simulation #1 generated 31.87 pounds of force, Simulation #2 with a shelf configuration generates 14.21 pounds. So with 76 pounds of meat, an extra 17.6 pounds of force is created on your body simply by how the pack is loaded.
Loading the pack vertically and minimizing the horizontal distance between the load CG (center of gravity) and our body CG is the key. This is further demonstrated by applying the basic rule of a 1st class mechanical lever that states every time you double the distance from the fulcrum, you also double the force that weight creates. Example: a load that exerts 15 pounds at 5 inches from the fulcrum will increase to 30 pounds at 10 inches from the fulcrum. While these are simplified examples of the actual forces at work, these examples show the major impact of load placement.
An ultralight backpack's weight advantage is not realized if not loaded correctly. This is the primary concept behind the load shelf, it is a load positioning system, keeping the heaviest portion of the load as close to our body CG as possible. The closer we come to aligning the body CG and the pack CG, the more we reduce the effects of felt leverage. While the Stone Glacier load shelf does add expandable cubic inches to each pack design, the primary function is to maintain the position of the heavy portion of your load to minimize excess force. Less force equates to more comfort, less fatigue, and more stability in the back country.