The Black Diamond Axiom 40 internal frame backpack on a summer backpacking trips in southern Colorado’s Weminuche wilderness. The Axiom 40 has 40 liters (2441 cubic inches) of volume, weighs 2.6 pounds (1.19 kg), and costs US$150. The women’s model is the Astral 40.
Black Diamond has introduced an extensive line of well-engineered backpacks during the past two years. Since Black Diamond is a climbing oriented company, many of their packs are designed to directly or indirectly support climbing. However, some of the packs in their line are designed for backpacking. We previously reviewed the lightweight Infinity 60 and Innova 50 backpacks, which feature Black Diamond’s innovative ergoACTIV freedom of movement suspension system.
The Axiom 40 is a lighter weight and smaller volume internal frame backpack designed to comfortably carry moderate loads. It intrigued me because of its light weight, clean design, useful feature set, freedom of movement suspension, and optimum volume for weekend lightweight backpacking. This review will assess its performance and compare it with other sub-3-pound internal frame backpacks.
|Year/Model||2011 Black Diamond men’s Axiom 40 (Women’s model is the Astral 40)|
|Style||Built-in internal frame, top loading with floating top pocket|
|Volume||40 L (2441 cu in) for size Medium, 42 L (2563 cu in) for size Large (tested); measured total volume size Large 50.8 L (3096 cu in)|
|Weight||Measured weight (size Large) 2 lb 12.8 oz (1.27 kg), manufacturer specification 2 lb 10 oz (1.19 kg)|
|Sizes Available||Men’s M, L; women’s S, M|
|Fabrics||210d ripstop nylon and twill, 70 x 210d Dolby|
|Frame Material||HDPE framesheet with attached peripheral 6061 aluminum rod frame|
|Features||SwingArm shoulder straps, OpenAir backpanel, V-Motion framesheet, floating top pocket with zippered access (key clip inside), flared top opening with drybag closure, two stretch nylon side pockets, large front stretch nylon and fabric kango pocket, two fabric hipbelt pockets, two front tool holders, two ice axe/trekking pole loops, four side compression straps, load lifters, hipbelt stabilizer straps, adjustable sternum strap with whistle, pulley-type hipbelt, internal hydration retainer loop, one center hose port|
|Volume to Weight Ratio||57.2 ci/oz (based on 2563 cubic inches and measured weight of 44.8 ounces for size Large)|
|Maximum Comfortable Load Carrying Capacity||25 pounds estimated comfortable load for an average person carrying the pack all day|
|Carry Load to Pack Weight Ratio||8.9 (based on 25 lb and measured pack weight of 2.8 lb)|
Frame and Suspension System
What’s unique about the Axiom 40 is its suspension system, so let’s cover that first.
The complete Black Diamond ergoACTIV suspension system, which we tested in the Infinity 60/Innova 50 (cited above), consists of three design elements: an ergoACTIV hipbelt that is connected to a pivot hub on the backpanel that allows it to swivel, SwingArm Shoulder Straps that are connected to each other by a cable and housing that allow the shoulder straps to move from side to side in tandem with the hipbelt, and a V-Motion Framesheet that transfers weight to the hipbelt. These three components working together allow the backpack to freely move from side to side and twist to the right and left with the user. It’s claimed by Black Diamond to be “the next advancement in backpack comfort technology”. In our review of the Infinity/Innova we had mixed feelings about the pivoting hub design because it concentrated all of the weight of the pack in one place, which placed a lot of leverage on the hipbelt with heavier loads. I understand that Black Diamond has since improved the ergoACTIV hub design, so our issues may be moot.
The Axiom 40 (women’s Astral 40) does not have the full-on ergoACTIV suspension; rather it only has the SwingArm shoulder straps and a lighter version of the V-Motion frame, which eliminates some weight and may be sufficient for a pack designed to carry moderate loads. The swiveling shoulder straps allow the pack to move with you as you twist and lean from side to side. The ends of the shoulder straps are connected to each other by a cable system very similar to a brake cable on a bicycle, providing about 1.5 inches (3.8 cm) of travel. See the video below for a demonstration.
The Axiom 40’s SwingArm shoulder straps are demonstrated in this video:
The pack’s frame consists of a HDPE framesheet and attached peripheral curved aluminum rod to create a very supportive unit in the vertical direction while providing torsional flexibility to conform to the user’s movements. The frame design is the same as the Infinity/Innova, but the peripheral aluminum rod used is smaller in diameter. The frame unit is bendable to create a customized anatomical contour to match the user (see photo below).
The pack’s OpenAir backpanel (left) provides ventilation and conforms to the user’s back; 2.5 inch (6.4 cm) wide shoulder straps (right) are contoured and well padded. The suspension system on the women’s Innova pack is anatomically contoured for women.
The Axiom 40 has a fixed torso length and comes in two sizes, Medium and Large (Small and Medium for the women’s Astral 40). The measured pack torso length of the size Large pack I tested is 20.25 inches (51 cm) by the conventional manufacturer method (underside of shoulder strap to bottom of the hipbelt), and 18 inches (46 cm) by the BPL method (underside of shoulder strap to center of the hipbelt). The load lifters allow some additional latitude.
Views of the Black Diamond Axiom 40: The frontpanel (top left) has a large capacity stretch nylon and fabric kango pocket; the backpanel view (top right) shows the pack’s OpenAir ventilated backpanel; each side (bottom left) of the pack has a stretch nylon pocket and two compression straps; and the top view (bottom right) shows the pack’s roomy floating top pocket. The top pocket attaches with side-release buckles so its easily removed to yield a lightened pack with a drybag top closure. The Axiom 40 has a total of six pockets – front kango, two side stretch nylon, top cap, and two on the hipbelt.
Key Features: The pack’s large stretch nylon front kango pocket (left) will expand to hold larger items like a jacket, or a wet shelter or rainwear. The floating top pocket is easily removed to save 3 ounces (85 g), and a drybag closure (right) seals the top of the main compartment.
I tested the Axiom 40 backpack on several summer backpacking trips and two ski trips to a mountain hut (shown) in the southern Rockies, where I carried loads ranging from 20 to 30 pounds (9.07 to 13.61 kg). My testing included on- and off-trail backpacking.
While hiking on-trail, the pack’s freedom of motion feature is obviously working but it’s not that noticeable. The benefits are more tangible and apparent when hiking off trail or traveling on skis. The pack leans sideways with you, and twists as you twist, which is appreciated. I am quite happy with this version of the ergoACTIV suspension system without the hub mechanism; it’s simple, lightweight, and effective.
Although Black Diamond rates the Axiom 40 to comfortably carry up to 35 pounds (15.88 kg), I personally found the pack comfortable up to about 26 to 27 pounds (11.79 to 12.25 kg) and problematic with heavier loads. On one backpacking trip, I carried 26.5 pounds (12 kg) comfortably, but on a ski trip to a mountain hut carrying a load of 30 pounds (13.61 kg) I had to tighten the hipbelt to an uncomfortable level to keep it from slipping below my waist. To verify this issue, I loaded the pack with 35 pounds (15.88 kg) and carried it on a day hike up a local mountain, and again found hipbelt slippage to be a significant problem.
The hipbelt tapers down to a 2-inch (5 cm) band (see views photo above) where it attaches to the lumbar pad. The design is apparently meant to coordinate with the SwingArm shoulder straps and V-Motion framesheet to enhance freedom of movement, but the downside of this design is hipbelt slippage under heavier loads. The outcome is the Axiom 40 is a delight to carry with loads under about 26-27 pounds (11.79 to 12.25 kg), but not so great with heavier loads. This result may vary upward for people with a stronger back and/or a more pronounced Iliac crest (hipbone).
With loads in the 20 to 25 pound (9.07 to 11.34 kg) range, which is the pack’s sweet spot for me, I found the pack conforms to my back very well (and holds that shape), the pulley-type hipbelt tightening system works very well, and the pack transfers all the weight to my hips. If your typical pack weight falls into that range, which is the case for many lightweight backpackers and weekend warriors, the Axiom 40 is a pack you will love. If you need to carry more weight I recommend getting something else.
The Axiom 40 seems larger than its 40 liter rated capacity (42 liters for size Large tested) – actually it seems more like a 50 liter pack – so I decided to measure the actual pack volume. I used the method I described in a previous article Lightweight Frameless Backpacks State of the Market Report 2011: Part 1 – Choosing and Using a Frameless Pack. Briefly, I fill the pack and pockets with durable packaging peanuts, then dump them into a tall cardboard box and measure their volume. Using this method I found the pack’s total volume (main body and all pockets) to be 50.8 liters (3096 cubic inches), which is 21% larger than the pack’s 42 liter specification for size Large. Note that stretch pockets are not normally included in manufacturer’s volume measurements, but I include them here to show the pack’s total volume capacity. The breakdown is as follows:
|Component||Measured Volume L (cubic inches)|
|Main compartment (with drybag closure rolled once and closed)||38.4 (2340)|
|All Pockets (six total)||12.4 (756)|
I measured the Axiom 40’s total volume using packaging peanuts (left) and found it to be nearly 51 liters (right), 21% larger than the specified 42 liter volume for size Large. Note that the pack’s top opening is funnel shaped for easy loading. The volume of the main compartment is very close to the specification, and the volume of the main compartment plus the top pocket (not shown) is almost dead on. Manufacturers do not normally include expandable exterior pockets in their volume specification, but I include them here to illustrate the point that many backpacks provide more volume than their specification indicates.
Some smaller issues: 1) The hipbelt pockets (left) are tight; they are large enough to hold a mini digital camera but are simply too tight for my compact digital camera. They are only useful for smaller items and energy bars. 2) The lower side compression straps interfere with inserting a water bottle into a side pocket (center), releasing the compression strap solves that problem but then a full water bottle easily falls out when you bend over, which is an annoyance. And 3) the shoulder straps could use another elastic loop to hold a drink tube where you need it (right).
Overall, the Black Diamond men’s Axiom 40 is a very nice backpack, provided you carry less than about 26-27 pounds (11.79 to 12.25 kg). It is exceptionally well designed and constructed to fill the needs of lightweight backpackers. I especially like the pack’s fit, contoured backpanel, hipbelt tightening system, durability, comfort, large front kango pocket, large floating top pocket, and drybag closure.
I am neutral on the benefits of the freedom of movement feature while hiking on a good trail. It’s nice, but it doesn’t make the load any lighter or easier to carry. However, the freedom of movement feature is appreciated much more while hiking over rougher terrain and traveling on skis or snowshoes.
Weight-wise, the Axiom 40 compares favorably with similar backpacks. However, there are lighter similar-sized internal frame backpacks to be found, as covered in Roger Caffin’s state-of-the-market series on Lightweight Internal Frame Backpacks. Some of the closer comparisons are summarized in the following table (manufacturer data for size men’s Medium).
|Pack||Volume L (cubic inches)||Weight Pounds (kg)||Cost US$|
|Black Diamond Axiom 40||40 (2441 )||2.63 (1.19)||150|
|Elemental Horizons Kalais||47.8 (2920)||1.63 (0.74)||190|
|Granite Gear VC 60||60 (3661)||2.13 (0.97)||200|
|Osprey Exos 46||46 (2808)||2.31 (1.05)||179|
|REI Flash 50||50 (3051)||2.50 (1.13)||150|
Highlights from table:
- The Axiom 40 is the heaviest pack in the group and matches the REI Flash 50 for lowest cost.
- The Elemental Horizons Kalais weighs a pounds less than the Axiom 40, but it costs US$40 more.
Overall, this group of sub-3-pound (1.36 kg) internal frame packs is quite diverse in terms of design, volume, and features and the final choice gets down to individual preferences. The main point to be made here is that the Black Diamond Axiom 40 compares favorably among its peers in this group of lightweight packs, and is a good choice for off-trail backpacking and ski travel.
- Innovative freedom of motion suspension
- OpenAir backpanel is contoured to fit the back and provides good ventilation
- Lightweight durable fabrics and frame materials
- Large front kango pocket is very handy for stuffing a jacket or carrying a wet shelter
- Numerous pockets for organizing and convenient access
- Fits well
- Comfortably carries moderate loads
What’s Not So Good
- Hipbelt slips with heavier loads, unless very tightly fastened
- Lower side compression straps interfere with inserting a water bottle
- Hipbelt pockets are too small and tight
- Needs an additional drink tube loop on shoulder straps
Recommendations For Improvement
- Larger hipbelt pockets
- Revise the hipbelt to support heavier pack weights
- Add a second drink tube loop on each shoulder strap, or lower the existing one
- Route lower side compression straps behind the pockets
Disclosure: The manufacturer provided this product to the author and/or Backpacking Light at no charge, and it is owned by the author/BPL. The author/Backpacking Light has no obligation to review this product to the manufacturer under the terms of this agreement.
Will Rietveld holds a Ph.D. in plant science and worked as a research scientist, national program administrator, and university professor for 33 years. Now retired, he is an outdoor writer and senior editor with Backpacking Light since 2004. Will has 54 years of backpacking experience, 13 years going ultralight. While field testing gear Will is a volunteer US Forest Service wilderness ranger and a Gossamer Gear Trail Ambassador where he promotes Leave No Trace and lightweight backpacking principles to hikers he meets on the trail. His trail name is Willi Wabbit.