Mountainsmith Zerk 40 Backpack Review

Mountainsmith Zerk 40 Backpack


High Volume Fast Pack

The Zerk 40 is a high volume fastpack with pocket-rich, vest style shoulder straps and the capacity to haul more overnight gear than other fast-and-light backpacks.

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The Mountainsmith Zerk 40 is a speed hiking backpack designed for ultralight thru-hiking. Weighing 25 oz, it has a roll-top closure, removable, hip belt strap, a running vest-style shoulder strap system, and removable foam framesheet. Numerous open mesh pockets make it easy to store frequently accessed gear and food on the exterior of the pack so you don’t have to open it during the day. The pack comes with bear can straps so you can lash a canister on top and includes numerous external gear loops so you can attach additional gear to its exterior.

Specs at a Glance

  • Weight: 25 oz
  • Gender: Unisex
  • Hydration compatible: Yes, central hose port
  • Bear canister compatible: Yes, vertical; External straps are also included for a top carry
  • Pockets: 7 (all open, mesh, and external)
  • Frame: Foam Backpanel
  • Volume: 40L
  • Includes extras: Stretch bungee cord for the front pocket, bear canister straps
  • Materials: 100d Nylon HT w/ 200d Spectra Double R/S TPU, 210g Stretch Mesh
  • Max recommended load: 20 lbs (manufacturer claims 30 lbs)

Backpack Frame and Suspension

The Mountainsmith Zerk 40 is a fastpack for hikers who are out to set speed records or just want to cover a lot of ground fast. Although it has a soft foam back panel, the lack of a load-bearing hip belt means that all of your pack weight will rest on your shoulders. Mountainsmith clams it can carry 30 pounds comfortably, but that’s doesn’t jive with my experience. I think 20 lbs is a more realistic limit that you should shoot for.

The Zerk 40 has a front stretch mesh pocket and side water bottle pockets
The Zerk 40 has a front stretch mesh pocket and side water bottle pockets

The shoulder pads are J-shaped unisex straps that flare widely over the chest and have open mesh pockets sewn on the outside for gadgets and snacks. The top of the shoulder straps is 3 inches wide where they’re sewn to the pack bag and 4 inches wide over your chest, under the mesh pockets. The straps’ wide flare is designed to hold the pack close to your torso, so it doesn’t bounce around if you’re running or hiking. However, the pack’s weight still rests on the tops of your shoulders and upper back, not your chest.

Each shoulder strap has two mesh pockets, one layered on top of another, which makes it easy to organize your snacks, electronics, and navigation aids and keep them close at hand. There’s also an adjustable sternum strap that slides up and down on a rail. When adjusting the sternum strap, you want to tighten it as snug as possible to keep the flared portion of the shoulder straps away from your underarms where they can interfere with your arm motion when walking or running.

Each shoulder strap has two overlapping mesh pockets so you can keep items accessible

The Zerk’s hipbelt is a webbing strap and not load-bearing. It’s provided to help keep the packbag close to your hips, but it can also be removed. The hip belt is attached to the pack with gatekeeper clips that you can pop open and remove. You’ll need a pair of pliers to compress the buckle enough to open the gate, which is too firm to do by hand.

The Zerk’s hip belt is a simple removable strap and not load bearing

While there is a foam pad in a pad pocket behind the Zerk’s shoulder straps, it’s primary purpose is to protect your back from being poked by the pack’s contents and isn’t structural like a proper frame.  It’s also very soft, and won’t prevent the packbag from barrelling into your back if you overstuff it. You can remove the pad or replace if you really want (it weighs 3.2 oz) but I’d stll recommend using some kind of insert its place because it gives the pack bag a shape, making it easier to pack.

The foam pad protects your back from pointy objects in the main compartment

Backpack Organization and Storage

The Zerk 40 is designed with a lot of external pocket storage so you have everything you need to get through the day within easy reach without having to stop to find something in your pack’s main compartment. While this packing philosophy isn’t that different from any other lightweight or cottage-made thru-hiking backpacks, the location of the Zerk’s external pockets and and the degree of organization they provides sets them apart from the hipbelt pockets on more conventional backpacks. Everything you could possibly need from snacks, gel packs, water bottles, bug dope, sunscreen, maps, a Smartphone, etc can be stored within easy reach in the shoulder strap pockets described above or the Zerk’s enhanced side water bottle pockets.

The top compression strap can be used to secure a foam pad

While the Zerk 40 is laid out like a typical ultralight pack with a roll-top, front mesh pocket, and side water bottle pockets, its side pockets are unique in both form and function. Each side pocket is surrounded by a stretch mesh pocket (like hamster cheeks) which can be used to store additional items, including sunproof clothing, a fleece hats buff, extra food, or even the trash generated by eating snack bars, so you don’t have to stop and dispose of wrappers. Both side pockets are also reachable so you can remove and replace bottles with ease.

One thing I would caution you about is the mesh on these pockets. While it has a dense weave, it’s not especially strong and is prone to ripping. It’s also not protected on the bottom by nylon fabric and will come into contact with the ground or rock ledges, making it susceptible to abrasion and tearing.

There is another mesh pocket on the outside of the water bottle pockets

With the exception of the pockets reachable by your arms, the rest of the Zerk 40 is fairly conventional. The main compartment closes with a rolltop, which is secured by webbing straps to the sides of the pack above the side pockets. The pack is hydration compatible with a center hose port between the shoulder straps, hose keeper loops on both shoulder pads, and a hook inside to hang your bladder but no separate hydration pocket.

The Zerk 40 has a large front mesh pocket good for storing bulky items like a UL tent or wet ones like a water filter or rain gear. The bottom of the pocket does not have drain holes however, so moisture will leak into the base of the pack or drip down onto your butt.

The front mesh pocket is large and ideal for carrying wet or loose objects such as rain gear, a water filter, or an ultralight tent, but is also made with the same lightweight mesh as used on the side water bottle pockets. It is surrounded by red gear loops so you can run an external elastic cord over it or attach additional items to the exterior of the pack.

Compression and External Attachment Points

The Zerk 40 has side compression straps that you’ll want to use to bring the load inside as close to your center of gravity as possible, particularly if you’re carrying a heavy food load. The straps are configured in a zig-zag shape and a little awkward to tighten, but don’t interfere with reaching or replacing the water bottles in the side pockets.

While the pack comes with bear canister attachment straps, they’re really not that secure, and the canister can slip laterally through them.

There’s a webbing strap that loops over the roll-top to provide top-down compression, which can also be used to secure a foam pad to the top of the pack, as well as an ice ax loop, although a shaft holder is not provided. If you need one, just roll your own with elastic cord and a cord lock.

The Zerk also comes with straps for securing a bear canister to the top of the pack. The bear canister straps are two webbing straps that attach to four gear loops on top of the pack bag. While you can make them really tight or even crisscross them, the canister has a tendency to slide laterally through them and dangle precariously off the side of the pack. I’d avoid using them. If you need to carry a canister, you can store it vertically in the Zerk’s main compartment and just pack around it. The pack has a 12″ extension collar that extends up past the shoulders and provides extra storage space.

Comparable Speed Hiking Packs

Make / ModelWeightVolumePrice
Mountainsmith Zerk 4025 oz40L$220
Six Moon Designs Flight 40 FKT39 oz44L$215
Six Moon Designs Minimalist34 oz48L$235
ULA Fast Pack26 oz45L$135
Ultimate Direction Fastpack 4526.43 oz46.8L$200


The Mountain Zerk 40 is designed for ultralight speed hiking with a vest-style shoulder harness and non-load bearing hip belt. Weighing 25 oz (minus optional cords and webbing straps) its a surprisingly comfortable backpack provided you don’t overload it to try to carry more than 20 pounds. While the Zerk 40 is designed for fast packers, it also makes a nice one or two-night backpacking pack if you can keep your load light and compact. Granted, the experience of carrying a backpack without a load-bearing hip belt or a rigid frame takes some getting used to, but the Zerk’s shoulder strap pockets provide a level of organization and access that’s unavailable on other packs. If you carry a lot of gadgets or like to nibble all day, the Zerk 40 will be easy for you to get used to using.

Disclosure: The author purchased this backpack.

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  1. For some reason (probably related falling on my head a bunch as a child) this thread takes me back to my first lightweight backpack, a GoLite Speed. I used it a number of times solo and then packed it with thirty pounds (14 kg) of gear to take a four year old backpacking in relatively mild winter conditions. It didn’t handle that load very well, prompting me to purchase a GoLite Pinnacle, which actually weighed less and carried much more comfortably. I still use the Speed as my motorcycle pack.

    The Speed had an hourglass shape so that the wearer wouldn’t hit his elbows on the pack while trail running. My brother said it was a great relief not to worry about bruised elbows the last time he ran across the Bob Marshall Wilderness… yeah, right!

  2. Like them shoulder strap pockets !

  3. Great review, Phil,

    I know you love the GG 38 as your “go to” smaller pack but I’m intrigued by this pack due to it’s lighter weight and your belief it’s a comfortable pack with loads up to 20lbs. Do you have any other recommendations for a frameless pack weighing less than 25 oz, 38 or 40L capacity that is comfortable to carry with 20 or more lbs of weight? I want a lighter weight pack than my GG Mariposa for overnight of 2-3 treks during the summer when my base weight is closer to 10lbs. Thanks!!

  4. Thank you, Phil,

    Now I have several lightweight packs to choose from.


    • Phil. How do you get a pack to ride on your chest and not your shoulders and back only?

      • It rides on your chest and shoulders. You put really wide chest straps on it like a running backpack and remove the frame/backpanel, which is what this pack and others (especially Six Moon Designs packs) try to do. I can’t say I prefer it but runners and fastpackers seem to like it.

  5. How do you feel about the updated version?

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