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
- 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
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).
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.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.
All told, I put nearly 400 miles on the Zerk 40, much of which occurred on high mileage and/or long-duration days.
Member's Only Content
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- Performance Assessment: Accessibility and Organization, Comfort, Durability, Stability, & Weight
- Product Strengths & Limitations
- Compared To: Ultimate Direction Fastpack 45L
Member's only version is 6,030 words and includes 34 photographs.
The 40 L storage capacity and 30 lb recommended load mean you need to have your kit well dialed in to make this pack work for you. The Zerk 40 is a pack for experienced backpackers with low-volume, low-weight gear who like to keep moving, do long days, and store needed items in quick access locations. It isn’t an expedition pack. I’ve used it in the winter, but volumetrically I think its sweet spot is for spring, summer, and fall hiking.
Viewed through that lens, the Zerk 40 excels in every regard. There’s no question that it is rugged and well-built. It’s comfortable on long days, stable across uneven footing, and allows you to access all the food, water, and gear needed to blast out high miles. The materials are well-chosen and well implemented. The foam is thick but always paired with breathable fabric that allows perspiration to pass through. The webbing and body fabrics have yet to show wear after hard use.
The details are just right. From the pocket size and shape to the modular webbing, from the generous attachment loops to the black, red, and grey styling, you can tell that each factor was adjusted and dialed in. The flaking lettering is a rare misstep, but one that will be adjusted in future models, and has no bearing on the performance of the pack.
I can confidently give the Zerk 40 a Highly Recommended review rating. It will be my thru-hiking pack of choice for the next few years.
Where to Buy
- You can purchase the Mountainsmith Zerk 40 here.
- Here's an article that describes how to analyze whether a frameless pack can actually carry some pack weight or not.
- Going frameless? Here’s some advice from our forum.
- You can read more about Ultimate Direction’s Fastpacks here.
- How we acquired these products: Product(s) discussed in this review were either acquired by the author from a retailer or otherwise provided by the manufacturer at a discount/donation with no obligation to provide media coverage or a product review to the manufacturer(s).
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