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.
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 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.
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 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’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.
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.
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.
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 / Model||Weight||Volume|
|Mountainsmith Zerk 40||30 oz||40L|
|Gossamer Gear Fast Kumo 38||21.5 oz||38L|
|Six Moon Designs Swift X||38.9 oz||50L|
|Six Moon Designs Minimalist||39.8 oz||50L|
|Nashville Packs Cutaway||10.6 oz||33L|
|Ultimate Direction Fastpack 40||25.2 oz||40L|
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.