The Zerk 40L Fastpack represents Mountainsmith’s first stab at an ultralight backpack aimed specifically at thru-hikers doing big mile days with few breaks. Inspired by trail running packs, the Zerk is designed to give you access to food, water, and essential gear while being comfy enough to leave on all day. The design is centered around wide, thick shoulder straps and a removable foam framesheet.

CDD5523 Clean
Photo: Mountainsmith The Zerk 40L Fastpack

Mountainsmith designed the Zerk in collaboration with thru-hikers, specifically Mountainsmith athlete Tom “The Real Hiking Viking” Gathman. Such partnerships can sometimes feel like marketing gimmicks, but I had reason to believe that Mountainsmith takes these things seriously. I’m a professional photographer/videographer, and the Mountainsmith Tanuck 40L, designed with photographer Chris Burkard, is my all-time favorite camera bag because it feels like a backpack designed by a photographer.

Going into this initial First Looks review, I hoped that the same “on the front lines” design and execution would spring from the collaboration between Mountainsmith engineers and the thru-hiking community.

With all this in mind, I took the Zerk out for a quick two-day, one-night cross-country skiing excursion in the eastern Sierra Nevada.

Everything pictured here, minus Bothy the Cat and the trekking skis, found a spot inside or outside the Zerk 40L. The Zerk is a well-organized pack, particularly with regard to its external pockets and lashing systems.

Features and Specifications


  • Wide, cushy shoulder straps
  • Dual mesh shoulder pockets with drawstrings
  • Shallow, diagonally cut side pockets
  • Outer secondary mesh side pockets
  • Wide roll-top enclosure with over the top webbing
  • Large front stretch pocket
  • Elastic gear attachment points located high on shoulder straps
  • Integrated shoulder-strap safety whistle
  • Stabilized with a two-piece Atilon and EVA foam panel (no frame sheet or stays)
  • Removable non-load bearing waist belt
  • Removable bear-can straps with multiple attachment points
  • Removable reflective front bungee
  • Hydration system port and internal attachment


  • Dimensions: 18.5 x 11.75 x 5.75 in (47 x 30 x 14.6 cm)
  • Weight (including all straps and waist belt): 28 oz (793.78 g)
  • Recommended load: Up to 30 lbs (13.6 kgs)
  • Volume (main compartment, extended): 40 L
  • Volume (main compartment, compressed): 32 L
  • Pack Material: 100D Nylon HT w/ 200d Spectra Double R/S TPU
  • Foam Material: Atilon, EVA, and PE
  • Mesh Material: 201g / 320 g Stretch Mesh
  • Compression Straps: 10mm webbing
  • MSRP: $219.95

Product Strengths and Limitations


  • Superior external organization: with good planning, you can hike all day and never take this pack off.
  • Shoulder straps are wide, comfortable, and secure.
  • Shoulder strap pockets are well placed, large, and hold items securely.
  • Side pockets are cut diagonally, allowing you to reach water bottles easily.
  • Foam stabilizing back panel is comfortable and seems to breathe well.
  • The bright orange, reflective accents are a nice safety touch.
  • The “designed by a thru-hiker FOR thru-hikers” element feels genuine.
  • A mass-produced minimalist pack with cottage industry features.


  • Shallow, diagonal cut side pockets don’t hold bottles securely.
  • Removable bear-can attachment straps are hard to get on and off.
  • Compression straps seem to pull a bit “sticky”.
  • Stitching on one of the mesh pockets is already coming loose: worrisome for a pack that is built to last for thousands of miles.

Compared To

ProductMountainsmith Zerk 40LGranite Gear Crown2 38L
Dimensions18.5 in x 11.75 in x 5.75 in (47 cm x 30 cm x 14.6 cm)22 in x 14.75 in x 7 in (55.9 cm x 29. 84 cm x 17.78 cm)
Weight28 oz (793.78 g)33.5 oz (950 g)
Volume40 L (extended), 32 L (compressed)38 L
Suggested Load30 lbs (13.6 kgs)35 lbs (15.87 kgs)
Load Lifter strapsNoyes
FrameRemovable dual layer Atilon and EVA foamRemovable PP frame sheet
Pack Material100D Nylon HT w/200d Spectra Double R/S TPURobic High-tenacity nylon (100D and 210D)
Internal OrganizationOne compartmentOne compartment
External OrganizationFour side pockets, four shoulder strap pockets, one rear mesh pocket, removable bear canister webbing, removable lashing bungeeTwo hip belt pockets, two side mesh pockets, one rear mesh pocket, removable lid with pocket
Closure StyleRoll-top with over the top webbingRoll-top with over the top webbing
Hip BeltRemovable, non-load bearingRemovable, load bearing with pockets.

The Zerk’s features are most commonly found on cottage industry packs (the shoulder strap pockets and diagonal side pockets remind me of our recent review of the Atom). Likewise, the price point and weight are similar to the classic Gossamer Gear Mariposa. For the upcoming Gear Review, I’ll be sure to do in-depth comparisons of the available cottage industry frameless and minimalist packs. But for this short review, I wanted to compare the Zerk to another mass-produced pack of a similar weight, volume, internal organization, and load capacity – the Granite Gear Crown2 38L.

The manufacturers advertise both packs as slimmed down options aimed at the ultra-light crowd. The Crown2’s volume sits squarely between the Zerk’s compressed 32L volume and its extended 40L volume. It does so at the cost of about 5.5 oz (155.9 g) of weight. The Crown2 has a recommended load of 35lbs (15.87 kgs) as opposed to the Zerk’s 30 lb (13.6 kg) capacity. The Crown2 achieves this with a thick hip belt and a plastic molded removable frame-sheet.  Removing the frame sheet gets Crown2 down to 25.6 oz (754.1 g), 2.4 oz (68 g) lighter but significantly less stable than the Zerk.

The Crown2 makes a nod towards accessibility with large mesh hip-belt pockets, but other than that has roughly the same accessibility as many other mass-produced packs. The shoulder straps and hip-belt are industry standard designs.

These features are why the Zerk stands out among mass-produced packs: trail running inspired shoulder straps and a foam frame sheet that give it killer comfort-hauling chops for its weight class. Those same features allow for excellent and unusual (for mass-produced packs) pocket placement.


The Zerk 40L Fastpack is many things, but it isn’t a marketing gimmick. It feels like the Zerk was designed by a foot-sore, achy-shouldered thru-hiker with nothing to do but daydream about how he would change his pack if he could. One thing to keep in mind though: At 28 oz (793.78 g) the Zerk isn’t the lightest minimalist pack available. It wasn’t made to be. It was designed to sit at a crossroads of comfort, accessibility, durability, and weight.

This First Look will focus on two of these elements: accessibility and comfort. An extended, more comprehensive Gear Review is forthcoming in summer 2019. In that review, I’ll elaborate on the factors discussed here. At that point, I’ll also be able to speak to the long-term durability of this pack, as well as make a more in-depth comparison to other minimalist frameless packs.

The Zerk 40L Fastpack’s main strengths are comfort and accessibility. Both are enabled by the wide trail running inspired shoulder straps.

To start with, the Zerk gives you easy access to frequently used pieces of gear. Single compartment roll-top enclosures are common in minimalist packs, but where the Zerk shines is its external organization. It has four well-placed shoulder strap pockets, two on each strap, with the upper pocket nesting inside of the lower pocket. The top pocket cinches with elastic cord, making it ideal for securing electronics or small water bottles. The bottom pocket is cinched permanently with elastic, and it’s roomy enough to hold snacks or other small pieces of gear.

On my testing trip, I fit my Sony Rx100vi, my iPhone 6 and case, a can of bear spray, 800 calories worth of snacks, a small pocket knife, headphones, and spare camera batteries in the shoulder strap pockets. All these items felt secure, and I especially appreciated knowing that my camera and phone were easy to reach but safe behind an elastic drawcord. Hikers with larger phones (or marginally larger cameras) should have no problem: the pockets swallowed both those items with room to spare. If needed, the shoulder-strap pockets could easily fit chapstick, a small tube of sunscreen, a lighter, a contact lens case, or other sundries on top of all the other items I’ve mentioned.

Wide shoulder straps give plenty of space for deep, stretchy pockets. In this photo, mine are holding a camera, extra batteries, 800 calories of food, bear spray, my phone and wallet, extra batteries, a pocket knife, headphones, and chapstick.

The Zerk has a removable webbing system for bear-canisters or other bulky items. I used it to attach my Nemo Switchback to the pack. I like this system for three reasons. The first is that if you don’t have any bulky items you can leave the webbing at home without having to cut it off permanently. The second is that there are multiple attachment loops all over the pack, giving you the flexibility to attach gear where you want to attach it. The third is that the webbing (along with the side compression straps, removable bungee, and logo) are bright red-orange. The logo and bungee are reflective. Not only will this help you find your pack in the dark, but they also add an extra layer of safety during hunting season.

Removable bear-canister webbing can attach at multiple points (orange loops) or be removed completely. I used the webbing to lash my CCF pad to the back of my pack.

The removable bungee is designed to give you more lashing options for the outside of the pack. I can’t speak to its effectiveness as my sample pack seems to have shipped without one. Once Mountainsmith sends me a bungee I’ll be able to address its functionality in my longer review.

The dual side pockets are also worth mentioning. The upper pockets are fashioned from the same Spectra material as the rest of the pack. They are shallow and diagonally cut, with the upshot being I could easily reach and replace my 1L Smartwater bottles on the go. The downside? The bottles were not secure. They consistently fell out when I bent over to adjust my ski bindings or fetch a dropped trekking pole. I even lost a bottle entirely in the deep snow through which I was skiing.

I find the trade-off worth it. But if your trek involves talus scrambling, bushwacking, or snowdrifts, keep an eye on those bottles. You can always use the side compression straps to secure gear that doesn’t need to be reached on-the-go.

Shallow, diagonal cut side pockets give you easy access to your water bottles. The trade-off is water bottles that fall out when you bend over.

The lower side pockets are roomy, with an elastic band across the top for more security than the upper side pockets. I used these pockets to hold lunch, trash, and a small bag of sundries and, like the larger side pockets, was able to access these items without breaking my stride.

The back stretchy mesh pocket is large, the entire width of the pack. Other than that it is pretty standard — handy to store items in, but not accessible while walking.

For this First Look, I ate all my snacks and lunch on the go, never taking the Zerk off during my eight-to-ten hour skiing days. I’m happy to report, without any exaggeration, that this is the most comfortable mass-produced pack I’ve ever worn. Well designed, extra-thick shoulder straps and a comfortable foam internal structure seem to be the contributing comfort factors. It will be worth seeing how all that thick foam breathes once temperatures warm up.

I was testing a few other pieces of gear, so I had some extra items. My total weight (including food and water) came in between 24 and 28 lbs (10.88 to 12.7 kgs). Mountainsmith recommends a load no heavier than 30lbs, but even at these upper ranges, the pack was comfortable and secure. For my upcoming Gear Review I’ll overload the pack to determine its comfort at greater-than-recommended loads.

I noticed two drawbacks to the Zerk during my initial testing. The first is that I struggled with attaching and removing the optional bear-canister webbing, even with warm hands in the comfort of my home. Specifically, only one of the attachment loops gave me problems. In a similar vein, the compression straps seem a bit “sticky” to tighten. We’ll see if both of these things loosen up with prolonged use.

For some reason, this particular loop gave me trouble. Time will tell if it loosens up or not.

The second issue I noticed was one of durability: a small amount of stitching around the elastic on the upper-right shoulder strap pocket seems to have come loose. A minor thing, so minor that it was difficult to get a good picture, and I ultimately decided not to. That being said, it’s a tad worrisome after two days of use on a pack designed to last thousands of miles per year at 20, 30, and 40 miles a day (a paraphrase of the Mountainsmith marketing copy). Time will tell if this is a one-off fluke or something that will happen to stitches elsewhere on the pack.

All in all, I’m highly impressed with this pack. Within the first hour of my test, I shortlisted the Zerk as my pack of choice for my next long-distance hike. It’s comfort, security, and well-designed exterior organization are all top-notch, especially for the price point. I look forward to testing its durability and comfort over a more sustained period.

Where to Buy

Related Content

  • The Zerk has similar features to the Atom by Atom Packs. You can read an in-depth review of that excellent pack here.
  • Going frameless? Here’s some advice from our forum.


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