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Mountainsmith Zerk 40 Backpack Review

Mountainsmith Zerk 40 Backpack Review

The Mountainsmith Zerk 40 is a multi-purpose backpack with vest-style shoulder straps that can be used for backpacking, thru-hiking, or long day hikes. I hesitate to call it a fast-pack because its utility extends far beyond that narrow style of use.

Weighing 30 oz, the Zerk 40 has a roll-top closure, a removable hip belt strap, and a removable foam framesheet. Numerous open mesh pockets on the shoulder straps and backpack exterior make it easy to store frequently accessed gear and food so you don’t have to stop and open the pack to access them. The pack comes with bear canister 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: 30 oz
  • Volume: 40L
  • Type: Frameless
  • Access: Top (roll-top)
  • Gender: Unisex
  • Hydration compatible: Yes, central hose port internal hang loop
  • Pockets: 9 (all open, mesh, and external)
  • Torso Range: 16-19″
  • Includes extras: Stretch bungee cord for the front pocket, bear canister straps
  • Bear canister compatibility: BV500 and BV475 fit inside vertically; BV450 and BV 425 fit inside sideways; In addition, external straps are also included for a top carry
  • Materials: 210-denier recycled nylon ripstop (UHMPE); recycled 210-denier nylon SD oxford; nylon 100-denier x Span 840-denier stretch mesh
  • Max recommended load: 20-25 lbs (manufacturer claims 30 lbs)

Backpack Frame and Suspension

The Mountainsmith Zerk 40 is a frameless fixed-length backpack with wide vest-style shoulder straps and a removable hipbelt made with webbing. 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 claims the pack can carry 30 pounds comfortably, but I think 20-25 lbs is a more realistic limit that you should shoot for.

The Zerk 40 is a frameless pack with vest-style shoulder straps.
The Zerk 40 is a frameless pack with vest-style shoulder straps.

If you’ve never had a backpack with vest-style shoulder straps, they can take some time to get used to because they provide a very different feel than more traditional backpack straps. For one, vest-style straps tend to lie flat on your shoulders and upper chest and don’t really rotate around the curves of your torso the way that traditional backpack straps do. Depending on the pack, you may find them too narrowly spaced or too widely spaced, which can also affect comfort. I happen to like the shoulder straps on the Zerk 40 because I think they favor a broad chest (size 46) unlike a lot of running packs that I find favor people with narrower torsos. In fact, I find many running-style vests and trail running packs unusable for just this reason.

While the Zerk’s shoulder straps aren’t heavily padded, they’re also not gossamer thin. They’re 3″ wide at the top and sewn directly into the top of pack-bag, widening to 4.5″ at the bottom, with a single adjustable sliding sternum strap between them (there’s no way to add a second sternum strap). The shoulder straps each have two mesh pockets on the front with gear loops along the side so you can secure items like an inReach 2 to prevent it from getting lost even if it falls out of a pocket. The straps themselves are J-shaped, which may be a consideration if you’re female or have a well-developed chest where S-shaped straps would be more comfortable.

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

The Zerk’s hipbelt is a webbing strap and not load-bearing. It’s provided to help keep the pack close to your hips so it doesn’t bounce around, 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.

There is a foam pad in a pad pocket behind the Zerk’s shoulder straps, its 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 pack bag from barrelling into your back if you overstuff it. You can remove the pad or replace it if you really want (it weighs 3.2 oz) but I’d still recommend using some kind of insert in 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 laid out like a typical ultralight pack with a roll-top, front mesh pocket, and side water bottle pockets. The roll top can clipped to itself on top or anchored using clips along the side of the pack, which is my preference because it provides better compression. The top of the roll top does not have a stiffener of any sort, so you want to make sure you turn it a few times to keep it from unraveling. There’s also a single webbing strap that runs from the front mesh pocket over the roll top to hold it in place.

The Zerk has a convention UL layout with a front mesh pocket and roll top closure.
The Zerk has a conventional UL layout with a front mesh pocket and roll top closure.

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 the degree of the organization they provide 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 or the Zerk’s enhanced side water bottle pockets.

The Zerk has side water bottle pockets and compression straps that can secure tall bottles.
The Zerk has side water bottle pockets and compression straps that can secure tall bottles.

The Zerk’s side pockets can hold water bottles, but what makes them unique is an additional stretch mesh pocket on the outside, which I liken to “hamster cheeks. These are good for storing extra snacks, fruit, or even trash wrappers so you don’t have to stop and open your pack to access/stow them. I much prefer these “hamster cheek” pockets to the mesh pockets that many ultralight backpack makers have started putting on the base of their backpacks. The latter makes no sense from a durability perspective since the highest abrasion on a pack occurs on the bottom when you put it on the ground.

Mountainsmith Zerk 40 Backpack


Highly Functional Fast Pack

The Mountainsmith Zerk 40 is a multi-purpose fast pack with a vest-style shoulder harness that can be used for lightweight backpacking, peakbagging, fast-packing, or travel. It has lots of strategically placed pockets to keep food and important gear close at hand throughout the day.

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While I’ve always preferred packs that let you add your own accessory pockets to daisy chains on the shoulder straps, I’ve grown to like the Zerk’s pocket layout. I can securely store snacks, my Garmin inReach2, sunglasses, a camera, and Smartphone on the shoulder straps, and more food and fruit in the hamster cheek pockets. The contents stay put too, which is handy on the trail, when I carry the pack around town, and for traveling. Much to my surprise, the Zerk 40 has become a favorite for this reason.

Close up of the extra stretch mesh pocket on the outside of the side pocket.
Close up of the extra stretch mesh pocket on the outside of the side pocket.

The rest of the Zerk 40 is fairly conventional. 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 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 an 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 long extension collar that extends up past the shoulders and provides extra storage space.

Comparable Vest-Style Hiking Packs

Make / ModelWeightVolume
Mountainsmith Zerk 4030 oz40L
Gossamer Gear Fast Kumo 3821.5 oz38L
Six Moon Designs Swift X38.9 oz50L
Six Moon Designs Minimalist39.8 oz50L
Nashville Packs Cutaway10.6 oz33L
Ultimate Direction Fastpack 4025.2 oz40L


The Mountainsmith Zerk 40 is a multi-purpose backpack with a vest-style shoulder harness and non-load bearing hip belt. While it’s large enough for lightweight backpacking, it can be used in so many different ways from peakbagging to commuting or travel without breaking stride. Weighing 30 oz (minus optional cords and webbing straps) the Zerk is a surprisingly comfortable backpack provided you don’t overload it to try to carry more than 20-25 pounds. 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 and side pockets provide a level of organization and access that’s unavailable on many other packs.

Disclosure: The author purchased this backpack.


  1. How does this pack compare to Fast Kumo 36 Fastpack
    I can’t find the reviews here for the fast Kumo have you had a chance to use it?

    • The Fast Kumo just came out. I’ve only used the regular Kumo and quite extensively. But I can tell you some of the key differences right off the bat. The kumo requires the use of an external sit pad, it doesn’t have a roll top, and it doesn’t have the extra hamster cheek side pockets I refer to in the review. In other words, it’s just a Kumo with vest straps. You kind of have to love those things I just listed to like it and you really might not. For example, a lot of people find the over-the-top lid to be kind of annoying and the sit pad doen’t really remove the need to carry a sleeping pad if you plan on backpacking.

    • I own both, I almost always prefer fast kumo due to sizing, weight and intended purposes.

      #1 sizing
      as noted from in review for the zerk it only comes in one size and it is built for larger folks, it was co-designed with ” The Real Hiking Viking” and it shows. If you have proportions like him (check the mountainsmith website) the pack may fit great. For me at 17″ torso, 38″ chest, 30″ waist, with the pack straps fully tightened, feels like it sits too low on my back and the “chest” pockets end up being abdomen or high waist pockets. I got the fast kumo size in “s/m” and love it, the pockets are where they should be up high on my chest, hi[pbelt actually sits above my belly button allowing mobility to do knee highs, it sits high up on my back as well which means when I have gone through my consumables at the end of the trip I can literally run to the car comfortably with minimal bounce or awkwardness.

      #2 weight
      there is a pretty big weight difference, for the same weight as the zerk you can get a framed pack that can carry 10+ lbs more for example the gossamer gear pack mariposa. That being said the fast kumo is FAR less durable than the zerk so the weight does not come for free. No failures yet but I anticipate needing to repair the chest pockets very soon after only 10 nights out, stitching is coming loose and anything that is strechy is staying expanded.

      #3 intended uses
      In my experience they both carry < 20 lbs comfortably and could start a longer hike with 25 lbs assuming your consumables are ~5+ lbs. Looks like the review confirms this as well.
      The zerk is a durable frameless pack that has a lot of volume when needed if you have bulkier gear or longer resupplies or need to carry a bear can, these features are great if you are on a long distance hike and can only have one pack for hundreds of miles while most of time carrying < 20 lbs total (comfortably) but you may need to carry 25-30 lbs in a pinch (very uncomfortably) for a day or two. It makes a pretty good travel pack, I wouldn't be afraid to toss it around an airplane or the let it slide around a van. Again for my torso this pack feels too cumbersome to prefer it as a day pack.
      The fast kumo is a crossover pack that was designed to be a fastpack (used for intermittent running/jogging) or just as a typical backpacking pack and is an amazing day pack as well I may even try taking it on the bike. I wouldn't travel with it as I will guarantee some of the mesh or pockets will be damaged in transit.

      I only own both because I got zerk new 60% off

  2. really like my zerk also, tons of organizational pockets are great. really wish mountansmith would offer it in Ultra, I’d buy that in heartbeat.

  3. Dreaded Bohemian

    appreciate the effort. are you AI?
    how does this pack shine/differ?
    what is unique to this potential purchase…

    • I’m not an AI. The thing that’s unique are the mesh pockets on the outside of the side pockets. They’re high utility. The shoulder straps also work with broad chests, unlike most vest-style packs which are designed for thin scrawny people. And lastly you can actually carry gear on the outside of this pack because it has enough body to hold its shape. Most small volume frameless packs fail on that dimension.

  4. I keep going back and forth between this and the Virga 3. This is heavier, but the pockets seem great. I’m not sure the vest straps will be a good fit for me, and the Virga has a lot of adjustability. Tough choice! You’ve worn both, which feels more comfortable on you with about 20lbs?

    • I’ve used both packs pretty extensively last winter and this spring. I like the Zerk better. The shoulder straps are just so much better in terms of comfort than on the Virga 3.

  5. You probably address this elsewhere in the site, but it’s mildly annoying how companies assess capacity of packs in a non-standard way. Cottage companies count main compartment + mesh pockets, whereas mainstream companies typically only count the main covered compartment.

    This gives cottage brands an added advantage in comparison tables like the one you have above. For example, the Fast Kumo 36 is actually just a 28L main compartment + mesh pockets, whereas the Zerk 40 has a 40L main compartment + mesh pockets.

    If possible, it’d be great to use standardized capacities when you make tables comparing pack capacities and weights (or just put a disclaimer). The only place I’ve seen this done is Outdoor Gear Lab — they actually measured total capacity for every pack, which I appreciated.

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