The objective of this study was to evaluate and compare the load carrying performance of several frameless packs available to the ultralight backpacking community.
Frameless backpacks tested in this study include:
- Granite Gear Virga
- GoLite Jam
- Six Moon Designs Moonlight
- Wild Things AT
- Equinox Katahdin
- GoLite Dawn
- Osprey Aether
- McHale Supbop*
The McHale Subpop was used as a control, to illustrate the difference in load carrying performance for a pack with and without its aluminum frame stays, and comparing the differences between a flat foam back panel and a foam cylinder rolled inside the circumference of the main packbag.
Analysis of Pack Torso Lengths
As a foundation for understanding the methods described herein, the reader will be repeatedly referred to the detailed methodology described in the article, Quantitative Analysis of Backpack Suspension Performance, published previously.
The pack torso length was measured in an unweighted state as the vertical distance between the centerline of the hip belt and the apex of the shoulder straps (e.g., for packs without load lifters, the seam at which shoulder straps are sewn into the back panel, and for packs with load lifters, the seam at which the load lifters are sewn into the back panel).
Suggested torso lengths and the pack size (e.g., "Small", "Medium", etc.) were derived from the marketing materials provided by each manufacturer. In addition, we defined an arbitrary "optimum" user torso length, as the pack torso length minus three inches (to account for suspension collapse) plus one inch (to account for the optimum location location of the hip belt centerline, which usually occurs about one inch below the iliac crest). This definition is equivalent to the definition of user effective torso length defined in the article Quantitative Analysis of Backpack Suspension Performance.
This definition is based on a combination of observing industry averages of highly-rated packs, acceptable design conventions among reputable custom pack designers, and experiential observations of performance from our testing staff. It should be noted that this arbitrary definition does not impact the shape or extent of the torso collapse curves illustrated later in this article, nor does it impact the relative comparison of performance factors between different manufacturers. However, it does alter the derived value of a pack’s load carrying capacity (also presented later). More important, this arbitrary definition of optimum user torso length does not alter the comparison of load carrying capacities of packs among the different manufacturers.
Pack torso lengths (Tpack,unweighted), manufacturer-reported torso lengths (Tmanufacturer), and optimum user torso lengths (Toptimum) are outlined in Table 1.
|Pack Size||Pack Torso Length|
It should be noted that Granite Gear (Virga), Osprey (Aether 30), and McHale (Subpop) suggest torso lengths that are in sync with industry guidelines (as do Wild Things and Equinox, although specific recommendations were not available from these manufacturers), while GoLite (Jam and Dawn) underestimates user torso lengths for their packs by a half to full size (1.5 to 2.0 inches). Six Moon Designs (Moonlight) appears to be an unusual anomaly, with an optimum user torso length (15.0", size M) that is one and a half to two full sizes below the industry standard of 18-20 inches (size M).
1. Comparison of Techniques for Improving Frameless Pack Load Carrying Capacity
Manufacturers employ a variety of techniques for improving the carrying capacity of their frameless packs. Such techniques include hip belt load distribution fins (e.g., Jam) or hip belt "snugger" straps (e.g., Katahdin), wide hip belts (e.g., SubPop), fixed foam padded back panels (e.g,. Jam), etc. Some pack makers (e.g., GVP Gear and Six Moon Designs) have also added external pad pockets, into which a sleeping pad can be folded against the back and remain accessible without unpacking the pack (i.e., as a sit pad at rest stops).
In addition, manufacturers use other techniques that are less important at load transfer, including load lifter straps for the shoulders (e.g., Virga). Load lifter straps are often designed to transfer load between the hip belt and shoulders, but in a frameless pack, tests in our lab have shown that they do not contribute significantly to load transfer unless an internal frame is in place – i.e., a foam back panel or a foam pad rolled as a cylinder are insufficient to allow load lifter straps to work effectively. Many pack designers are quick to admit (and ask to remain anonymous) that load lifter straps are used on frameless packs primarily to extend the torso range of a pack and make a particular size fit a wider range of people, thus allowing a company to deliver a limited size range of packs to the market and keep production costs down.
Users also employ various techniques for improving the carrying capacity of their frameless packs. Packing gear tightly (or using smaller packs) and using a sleeping pad rolled as a cylinder in the main packbag are the two most fundamental and popular of these techniques. Some users opt to fold a sleeping pad and insert it into the pack against the back panel.
Herein, we evaluate the effect of rolling a sleeping pad and inserting it into the pack’s main packbag, and packing gear into the interior of the rolled cylinder. This is the most common method used by ultralight backpackers for improving a pack’s load carrying capacity.
Many ultralight backpackers (and ultralight pack manufacturers) claim that the "rolled cylinder" method is "just as good as a frame" for transferring loads between the shoulder straps and hip belts. We hypothesize that these types of statements comprise unjustified hyperbole that needs to be challenged. And so, herein, we offer objective evidence that challenges these assumptions and illustrates the differences in load carrying comfort in the same pack (McHale SubPop) with different methods of load suspension:
- Two 7075-T6 aluminum frame stays (1/8" x 1/2") with closed cell foam (1/2") back panel
- Closed cell foam (1/2") back panel only
- Closed cell foam pad (7/16" thick x 20" wide x 45" long) rolled as a cylinder inside the pack.
Using the method of packing and measuring defined in Quantitative Analysis of Backpack Suspension Performance, we collected torso collapse data and derived the suspension performance factors (also defined in that article) for these three suspensions. This torso collapse data is presented in Figure 1.
Figure 1 shows that the use of a rolled cylinder vs. a standard foam backpad has little impact on the load carrying performance of this particular pack. This was not surprising, since other observations with this pack (not discussed herein) have shown that the hip belt construction is a major feature contributing to load carrying performance. The data does show, however, that the use of a rolled cylinder may slightly improve load carrying performance at lighter weights (less than 20 pounds), and that it may actually hinder load carrying performance at heavy weights (greater than 35 pounds). Our observations are consistent with our intuition – at lighter loads, a rolled cylinder packed tightly maintains excellent load stability, while at heavier loads, collapse of the cylinder and reduction of its surface area in contact with the user’s back results in poor load transfer performance. However, the reader is cautioned that the data for loads less than 20 pounds is not statistically significant for the pack using a rolled cylinder vs. standard foam back panel.
Clearly, Figure 1 shows that gear packed* inside a rolled cylinder results in very poor suspension performance relative to the same load density packed in the same packbag with two internal frame stays. The differences become statistically significant at loads in excess of 15 pounds, and correlates to substantial differences in perception of user comfort at loads greater than 20 pounds (data not shown, cf. Quantitative Analysis of Backpack Suspension Performance).
* Even at a relatively "tight" soft goods density of 2.1 oz/L, with the majority of additional weight distributed as flat steel plates along the inside back panel.
2. Comparison of Suspension Performance of Various Backpacks Using Closed-Cell Foam Rolled Cylinder-Based Suspensions
The second objective of this study was to compare the performance of various frameless packs available to ultralight backpackers using the ‘rolled cylinder’ technique of improving the suspension performance of the packs.
All packs were packed to a soft goods density of 2.2 oz / L (+/– 5%) in their main packbag, with additional weight added in the form of flat steel disks secured to the inside back panel and distributed evenly along the back panel (refer to Quantitative Analysis of Backpack Suspension Performance for more detail). The rolled cylinder foam pad used for this portion of the study was the same pad used above in Section 1. The only pack for which the rolled cylinder method was not employed was the Moonlight, which is designed with an external pad pocket on the back panel, into which a foam pad can be folded and inserted into the pocket. For this pack, an 8-section Z-Rest was folded accordion style and inserted into the pad pocket, as per manufacturer recommendations.
Figure 2 compares the collapse of pack torso lengths for each pack.
Figure 2 shows that no pack using a frameless, foam pad suspension was able to resist torso collapse to the extent of a pack with a frame (McHale Subpop w/frame) at weights meaningful to most lightweight backpackers (15 to 30 pounds). At 30 pounds (considered by many to be the very upper limit of frameless pack load carrying capacity), no frameless pack was capable of resisting a torso compression of less than 10%, which generally equates to a full frame size reduction (e.g., "medium" to "small"), and is a point at which comfort is severely compromised. Near the edge of this boundary are the Subpop (no frame), Jam, Moonlight, and AT packs, which appear from Figure 2 to be among the best at resisting % torso collapse at this load range.
A more telling comparative analysis of suspension performance can be made from the data shown in Figure 3, which compares the performance factors of each pack (see Quantitative Analysis of Backpack Suspension Performance for a discussion of how performance factors are derived).
Performance factors are proportional to the ratio of torso collapse in a loaded pack relative to the torso length of an unloaded pack. They are normalized such that a performance factor = 1 indicates zero collapse (i.e., at a load of zero) and a performance factor = minus infinity indicates total collapse of the torso length to a length of zero (i.e., at a load of infinity). The performance factor = 0 axis (indicated by the red horizontal axis in Figure 3) is the point at which the pack torso length collapses to the user effective torso length (defined as the optimum user torso length for the purpose of this comparative study, as defined earlier in this article) and load can no longer be distributed off the shoulders completely to the hips. See Quantitative Analysis of Backpack Suspension Performance for a detailed discussion of the mathematical and engineering basis for defining the performance factor.
In short, the weight at which a particular pack’s performance factor curve crosses the performance factor = zero axis is arbitrarily defined as the pack’s load carrying capacity. Although we have previously correlated the load carrying capacity as defined in this manner with perception of user comfort (in Quantitative Analysis of Backpack Suspension Performance), the reader must understand that other factors can influence load carrying comfort and capacity.
Thus, Figure 3 shows four distinct (statistically significant) groupings of backpack load carrying capacities.
Katahdin (11 pounds). This pack had a significantly lower load carrying capacity than other packs tested. Two reasons for this are its very large circumference and capacity, and the use of hip belt snugger straps that pivoted through a sliding loop at the hip belt attachment point, causing the pack to sag downward in response to added weight. The snugger mechanism on this pack did an inadequate job of transferring load from the main body of the pack to the hip belt and preventing packbag sagging in the vertical plane.
Dawn (15 pounds), AT (16 pounds), Aether (16 pounds). The Dawn has a large circumference:torso height ratio (increasing the length of the load moment arm around the spinal plane) and a thin (1") hip belt that prevents adequate load transfer around the hip bones. The AT pack is a very tall pack that is subject to a greater extent of torso collapse than packs with shorter vertical heights. The Aether hip belt-to-pack attachment point failed to prevent excessive sagging of the packbag in response to weight.
Virga (23 pounds), Subpop/No Frame (27 pounds), Jam (28 pounds). These three packs performed above the acceptable industry standard for frameless pack load carrying capacity (typically, 20 pounds). The Virga is hindered by load sagging resulting from load lifter straps and an unnecessary "detached flap" to which shoulder straps are sewn in. Without these "features", it could carry a heavier load. Otherwise, the Virga offers unremarkable load carrying features, and is an acceptable performer. The Subpop carries loads well in its frameless mode as a result of its very wide hip belt (5 inches) and padded lumbar region (made of Cordura-based surface contact materials that are "sticky") that allow the entire hip contact surface area to be used in load transfer to prevent packbag sagging. Finally, the Jam is a narrow, trim-profile pack with well-designed hip belt fins that provided the best load transfer (most resistant to sagging) of all packs that employed webbing belts.
Moonlight (34 pounds). The Moonlight performed significantly better than all other frameless packs in this study. The external pad pocket is placed between the pack’s back panel and the user’s back. Further, and most notable, shoulder strap attachment points and hip belt attachment points to the pack are behind the pad pocket. Consequently, as the shoulder straps and (especially) the hip belt are tightened, the entire load can be compressed against the user’s back. Clearly, this load transfer system is very effective. Subjectively, we observed this system to be very effective even when only the hip belt was tightened, and the shoulder straps were left loose. Unfortunately, the Moonlight suffers from a very short torso length that is a full size or more shorter than conventional industry standards. We hope that Six Moon Designs elects to produce taller packs that are more in line with conventional torso sizing conventions. It should be noted that Six Moon Designs claims that the Moonlight can carry a load of up to 35 pounds. Initially, we viewed this claim with incredulity. Based on the results presented in this study, we cannot dispute this claim. However, the reader must be cautioned that the load carrying capacity that we derived (34 pounds) is based on a user sizing up at least one, and probably two, sizes over Six Moon Designs recommended torso sizes.
Using a rolled cylinder closed cell foam pad relative to a closed cell foam backpad does not appear to improve load carrying performance of a McHale Subpop without its frame stays. The effects of the rolled cylinder method of packing are unknown for other packs. However, there appears to be no justification that the rolled cylinder method is comparable for resisting torso length collapse relative to twin aluminum stays, thus providing evidence for disputing common user claims that a "rolled cylinder pad is just as good as a frame".
Four of the eight frameless packs tested herein (excluding the McHale Subpop, which is not designed to be a frameless pack) exceeded conventional expectations of a 20-pound load carrying capacity using a rolled cylinder foam pad technique for improving backpack suspension performance (except the Moonlight, which uses a folded pad back panel).
The suspension system of the Six Moon Designs Moonlight appears to offer substantial increases in frameless backpack load carrying performance relative to packs that employ the rolled cylinder foam pad technique, with a load carrying capacity that was 66% higher than the average load carrying capacity (20.4 lbs, n = 7) of all packs (excluding the Subpop) in this study.
A Final Note About Pad Type. It should also be noted that differences in the type of pad used in the rolled cylinder technique may influence backpack suspension performance. We have been objectively testing various pad and pack combinations to examine these effects. The most effective combinations have been pads that have the highest friction coefficient between the pad and the pack fabric (e.g., "sticky" pads and "sticky" inner pack fabrics). The reason for this is that the overall load is less likely to "slip" at the pad-pack fabric interface.
In this study, the best performing pads have been a very sticky version of EVA foam (and also very heavy – with a 2/3 length pad weighing 14 ounces) with a sticky surface combined with polyurethane-coated Cordura pack fabrics. The worst performing combinations have been self-inflating sleeping pads with "slick" outer surfaces combined with packs made with silicone-impregnated nylon fabrics. The most remarkable conclusion from these studies is that for pack weights greater than 15-20 pounds, the pad and pack fabric combination (and thus, the friction coefficient between the two) made little difference in the pack’s load carrying ability.
Field evaluations and reviews of these packs will appear soon at BackpackingLight.com. Please check our Editorial Calendar to check the publication schedule.