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The Zerk 40 (Photo: Mountainsmith).

The Mountainsmith Zerk 40 is a rugged and flexible pack explicitly aimed at thru-hikers and those doing long days or big miles. Designed to be comfortable and stable with easily accessible pockets, the Zerk 40 straddles the line between minimalist, frameless packs and feature-rich packs with more load-bearing capacity. Thick, ergonomic shoulder straps take the brunt of the load while providing accessible external storage options. The upshot is a pack that helps minimize unnecessary stops.

Features and Specifications

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The author backpacking in Montana with the Zerk 40 (Photo: Beth Voss).


  • Wide, comfortable shoulder straps
  • Dual mesh shoulder pockets with drawstrings
  • Outer mesh side pockets
  • Inner, diagonally cut side pockets (nest inside outer mesh side pockets)
  • Wide roll-top closure with over-the-top webbing
  • Large rear stretch pocket
  • Elastic gear attachment points located high on shoulder straps
  • Integrated shoulder strap safety whistle
  • Removable back foam panel (no frame sheet or stays)
  • Removable non-load bearing waist belt (webbing strap only, no structure)
  • Removable bear-can straps with multiple attachment points
  • Removable reflective front bungee
  • Hydration system port and internal attachment


  • Dimensions: 18.5 in x 11.75 in x 5.75 in (47 cm x 30 cm x 14.6 cm)
  • Weight (including all straps and waist belt): 28 oz (794 g)
  • Weight (straps detached) 25 oz (709 g)
  • Recommended load: Up to 30 lbs (13.6 kgs)
  • Volume (main compartment, extended): 40 L ( 2441 cubic inches)
  • Volume (main compartment, compressed): 32 L (1953 cubic inches)
  • Pack Material: 100 denier Nylon HT w/200 denier Spectra double ripstop TPU
  • Foam Material: Atilon, EVA, and PE
  • Mesh Material: 210 g Stretch Mesh
  • Compression Straps: 10mm webbing
  • MSRP: $219.95

Review Context

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The Mountainsmith Zerk 40 has thick, ultra-running inspired ergonomic shoulder straps to transfer load away from the shoulders and towards the torso. This design is meant to minimize having to take the pack off during breaks (Photo: Beth Voss).

The Mountainsmith Zerk 40 is specifically designed to appeal to thru-hikers and anyone else who wants a small-volume pack with lots of easily accessible external storage. Opening up your pack and digging around takes time, and you only have so much daylight. You can reach eight of the nine external pockets while wearing the Zerk 40 (the ninth is a large stretchy mesh pouch at the rear of the pack).

With ultra-running-inspired shoulder straps, the Zerk 40 is meant to be left on all day – comfy, stable, and breathable.

Finally, the Zerk 40 is supposed to be durable – Mountainsmith hammers this home on the product’s webpage. They say, “Co-designed with The Real Hiking Viking, a professional hike-really-long-distances-really-quickly-er, the ZERK 40 was born out of a need for an ultralight pack that can withstand the thousands of miles per year that The Real Hiking Viking already covers. We wanted to make sure it would allow him to go farther, to hike longer, and to work more efficiently on the trail.”

I kept all of this in mind while testing the Zerk 40 for nearly six months – on desert day trips, in Sierra thru-hikes, and Montana off-trail traverses. I wanted to know if the Zerk 40 was comfortable enough for repeated 30+ mile days. I tried to find out if it was durable enough to handle bushwhacking and talus hopping. I also explored if it was accessible enough to keep the essentials close at hand while remaining voluminous enough to support five to six days of hiking between resupply points.

Description of Field Testing

My first test with the Zerk 40 was a short three-day, two-night late-winter trip in the Sierra Nevadas (described here).

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Winter backpacking with the Zerk 40.

After that, the Zerk 40 was my go-to day hiking pack for the rest of spring and early summer. Next, I brought the Zerk 40 on a thru-hike of the Tahoe Rim Trail, during which I covered 176 miles in six-and-a-half days.

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Testing the Zerk 40 in its native habitat – on trail, high mileage days.

After some more day hikes, I finished my official testing with a six-day, five-night off-trail trek across the valleys and plateaus of Montana’s Absaroka-Beartooth Wilderness.

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A close fit, wide shoulder straps, and capable compression made the Zerk 40 stable while navigating talus fields (Photo: Beth Voss).

All told, I put nearly 400 miles on the Zerk 40, much of which occurred on high mileage and/or long-duration days.

Performance Assessment

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My performance assessment of the Mountainsmith Zerk 40 is based on the following considerations:

  • Accessibility and Organization
  • Comfort
  • Durability
  • Stability
  • Weight

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