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. Newly updated, the Zerk 40 (34 oz) has a roll-top closure, a new zippered front map pocket, a removable hip belt strap, and a removable foam framesheet. The external mesh pockets are more durable than before; the external attachment options are more flexible, and new pockets expand the pack's already considerable external storage. If you like to carry a lot of gear on the outside of your pack for easy access, the Zerk 40 is worth a close look.

Finally, the range of packs and accessories under the moniker “Zerk” has been expanded: there is now an optional Zerk 7L top lid compatible with all Zerks, a higher-capacity Zerk 55 model, a new Zerk UL 20 daypack, which replaces the Zerk 25 (which is my favorite daypack), a Zerk 13 daypack w/ 2L hydration system, and Zerk 3 lumbar pack.

• Weight: 34 oz
• Volume: 40L
• Type: Frameless
• Access: Top (roll-top)
• Gender: Unisex
• Hydration compatible: Yes, central hose port internal hang loop
• Pockets: 12 (11 open, 1 closed)
• Torso Range: 16-21″
• Bear canister compatibility: BV500 and BV475 fit inside vertically
• Materials: 210-denier Robic nylon X-Grid with Extreema (UHMWPE) R/S UTS; 100-denier Robic nylon Extreema (UHMWPE) R/S UTS; 150-denier high-density polyester liner; YKK zippers; EVA foam
• 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 webbing hipbelt. 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.

The shoulder straps have highly functional pockets

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 the pack-bag, widening to 4.5″ at the bottom, with dual sliding sternum straps between them. The shoulder straps each have two mesh front pockets and gear loops along the sides, so you can secure items like an inReach to prevent them from getting lost even if they fall out of a pocket.

The Zerk 40 has a non-load-bearing, removable hip belt.

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. The foam backpanel in this latest Zerk 40 is slightly different from the previous model and slightly lower weight. You can remove the pad or replace it if you really want (it weighs 2.9 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 internal foam back panel

Backpack Organization and Storage

The Zerk 40 is laid out like a typical ultralight pack with a roll-top, front mesh pocket, and side pockets. The roll top can be clipped to itself at the top or anchored with 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 (even then it will still unravel). The lack of this stiffener is a negative because it makes the pack hard to pack unless it's bursting at the seams. There's also a new Y-strap in this model for securing gear, such as a foam pad, to the top of the pack.

The front stretch mesh pocket is made with much more durable fabric.

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 backpack, the location of the Zerk's external pockets and the degree of 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.

A one-liter bottle will interfere with the top side pocket.

There are three side pockets on each side of the Zerk 40: two upper mesh pockets and then four lower pockets, two on each side, that overlap. The upper pockets are sized for a one-liter cook pot. The innermost lower pockets can be used to store a water bottle, while there's a second pocket covering it,  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.

Another example of how the top pockets interfere with lower pocket use

There's one downside to the upper pockets, which are new on this pack: they interfere with the contents of the innermost lower pocket if you try to put a one-liter soda or Smartwater bottle in it because they're too low. So if you're going to use water bottles with the Zerk 40, use short ones, a hydration bladder, or small 500 ml bottles in the vest shoulder strap pockets.

There's also a new map pocket on the front of the pack, above the front mesh pocket, that is handy for storing maps and personal items you want to keep track of.

The new front map pocket (yellow interior) provides extra secure storage

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, plus a new hydration pocket.

The front mesh pocket is large and ideal for carrying wet or loose items such as rain gear, a water filter, or an ultralight tent, and is made with the same heavy-duty mesh used on the side water bottle pockets. It is surrounded by black daisy chains 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 can be moved to different anchor points on the daisy chains that run up and down the side of the pack. If you like to have hack backpack attachment points, this is the pack for you. The flexibility is almost overwhelming.

Top Y-strap lets you secure items on top of the pack

There's a new Y-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. There's also an ice ax loop, though no shaft holder is provided. If you need one, just roll your own with an elastic cord and a cord lock.

If you need to carry a bear canister, you'll want to store it vertically in the Zerk's main compartment. The Zerk doesn't have much body, even when it's packed full, so trying to strap it under the Y-strap doesn't work all that well.

Comparable Vest-Style Hiking Packs

Recommendation

The Mountainsmith Zerk 40 is a multi-purpose backpack with a vest-style shoulder harness and a non-load-bearing hip belt. While it's large enough for ultralight backpacking, it can be used in so many different ways from peakbagging to commuting or traveling without breaking stride. Weighing 34 oz, 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.

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