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Black Diamond Axiom 40 Backpack Review

A lightweight sub-3-pound internal frame backpack that’s well designed, durable, comfortable, and features freedom of movement innovations.


Overall Rating: Recommended

The Axiom 40 deserves our Recommended rating based on its excellent design, materials, construction, innovations, and comfort – provided you use it for moderate loads or less. With heavier loads around 30 pounds (13.61 kg) the hipbelt slips and the comfort ends, and our rating drops to Below Average.

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by Will Rietveld |


Black Diamond Axiom 40 Backpack Review - 1
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/Model2011 Black Diamond men’s Axiom 40 (Women’s model is the Astral 40)
StyleBuilt-in internal frame, top loading with floating top pocket
Volume40 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)
WeightMeasured weight (size Large) 2 lb 12.8 oz (1.27 kg), manufacturer specification 2 lb 10 oz (1.19 kg)
Sizes AvailableMen’s M, L; women’s S, M
Fabrics210d ripstop nylon and twill, 70 x 210d Dolby
Frame MaterialHDPE framesheet with attached peripheral 6061 aluminum rod frame
FeaturesSwingArm 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 Ratio57.2 ci/oz (based on 2563 cubic inches and measured weight of 44.8 ounces for size Large)
Maximum Comfortable Load Carrying Capacity25 pounds estimated comfortable load for an average person carrying the pack all day
Carry Load to Pack Weight Ratio8.9 (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:

Black Diamond Axiom 40 Backpack Review - 2
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).

Black Diamond Axiom 40 Backpack Review - 4
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.

Pack Description

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.

Black Diamond Axiom 40 Backpack Review - 5
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.

Black Diamond Axiom 40 Backpack Review - 6
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.

Field Testing

Black Diamond Axiom 40 Backpack Review - 7
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:

ComponentMeasured Volume L (cubic inches)
Main compartment (with drybag closure rolled once and closed)38.4 (2340)
All Pockets (six total)12.4 (756)
Total50.8 (3096)

Black Diamond Axiom 40 Backpack Review - 8
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.

Black Diamond Axiom 40 Backpack Review - 9
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).

PackVolume L (cubic inches)Weight Pounds (kg)Cost US$
Black Diamond Axiom 4040 (2441 )2.63 (1.19)150
Elemental Horizons Kalais47.8 (2920)1.63 (0.74)190
Granite Gear VC 6060 (3661)2.13 (0.97)200
Osprey Exos 4646 (2808)2.31 (1.05)179
REI Flash 5050 (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.

What’s Good
  • 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.


"Black Diamond Axiom 40 Backpack Review," by Will Rietveld. (ISSN 1537-0364)., 2012-06-26 00:00:00-06.


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Black Diamond Axiom 40 Backpack Review
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Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Black Diamond Axiom 40 Backpack Review on 06/26/2012 19:48:05 MDT Print View

Companion forum thread to:

Black Diamond Axiom 40 Backpack Review

Stuart .
(lotuseater) - F

Locale: Mountains
Black Diamond Axiom 40 Backpack Review on 06/27/2012 12:29:27 MDT Print View

Welcome back, Will. Glad to see your review is self-contained, not "evolving". Your return raises the bar for equipment reviews back to where it was until a few months ago. Long may it remain high.

I. Chhina
(ichhina) - MLife

Locale: Puget Sound, WA
Yay! Will is back. We've missed you on 06/27/2012 12:45:52 MDT Print View

Glad you're back! You are the king of product reviews, and the faithful have missed your expertise.

Warren Greer
(WarrenGreer) - F

Locale: SoCal
Nice review on 06/27/2012 21:26:47 MDT Print View

Glad to see you here and contributing. I've always appreciated your writing style and you hit the points that so many of us are interested in. Really like the comparison to other gear. Very informative. And glad to see Ryan in here, even if he's only uploading the article. Nice to see you Ryan.

Will Rietveld
(WilliWabbit) - MLife

Locale: Southwest Colorado
Re: Black Diamond Axiom 40 Backpack Review on 06/28/2012 11:47:21 MDT Print View

Hi all, thanks so much for your kind words. I want to verify that I will still be around and continue to contribute to BPL, but to a lessor extent than before. The articles I write from now on will be limited to gear and techniques that really intrigue me. Have a great summer and get in lots of hiking. Best, Will

(mountainwalker) - MLife

Locale: SF Bay Area & New England
Will R. how do you like your Altra Lone Peaks? on 07/15/2012 18:22:58 MDT Print View

Will so glad that you're still involved in BPL and sharing your rich experience and knowledge. Have learned so much from your very informative reviews and forum comments.

Now I know this is outside the scope of your review but I've got to know - how do you like your Altra Lone Peaks? Can you offer any fit guidelines? What size do you wear for backpacking compared with your actual foot size?

My wife and I have been considering them as replacements for our totally beat up trail shoes (with Shoe Goo I may get just a month's more use out of mine).

We've been looking at shoes with roomy toe boxes, good cushioning (doesn't have to be thick but not minimalist), neutral insole, quick drying mesh, durable outsole and preferably lower heel to toe drop. Saucony Xodus 3.0 (4mm drop with good cushioning) is also in the running, as well as possibly New Balance 810 and 1110.

(The New Balance shoes depend on getting correct info from NB - one NB rep said the 810 has an 8mm drop and the 1110 a 4mm drop, while another NB rep claimed 12mm for each shoe. Wish they'd list that info).

Edited by mountainwalker on 07/15/2012 18:31:00 MDT.

Monty Montana
(TarasBulba) - MLife

Locale: Rocky Mountains
New: Saucony Outlaw on 07/16/2012 22:40:21 MDT Print View

Hi EJ! Check out the Saucony Pro Grid Outlaw: it has pretty aggressive lugs, a D ring at the base of the laces for attaching gaiters (it also has an internal, built-in gaiter, but I still like to use an external one), and also features a 4mm differential. Talk about comfort!