Mountainsmith Zerk 40 Ultralight Backpack Review

Bottom Line

The Mountainsmith Zerk 40 carves out its own niche as a tactical/ultralight hybrid pack. It appeals to the ultralight hiker who appreciates the organization and functionality of a traditional pack. 

The vest-style shoulder straps with large mesh pockets, abundant external storage, and highly modular layout make it one of the most versatile frameless packs we’ve tested. We put the Zerk through hundreds of miles of mountain travel and dozens of nights in the backcountry – from the high glaciers of the Olympic Mountains to the Wonderland Trail around Mount Rainier – and consistently appreciated how intuitive it was.

The biggest drawbacks are poor comfort under heavy loads and limited weather protection. Though the harness and semi-rigid back panel are surprisingly supportive for a frameless ultralight backpack, a lot of weight still falls on the shoulders, and the non-sealed construction requires a pack liner in sustained rain.

The Zerk may feel overbuilt for minimalist backpackers, but it’s a strong option for experienced fast-and-light hikers who will actually use all the features. It’s affordable without sacrificing quality, which is an enticing combination for many.

How We Tested

We put this pack through hundreds of miles of rugged mountain travel and dozens of nights on the trail. From the high glaciers of the Olympic Mountains to a lap around Mount Rainier on the Wonderland Trail, we got a clear sense of where it shines – and where it could improve.

Comfort

Frameless packs aren’t typically known for their comfort, but Mountainsmith went the extra mile to make carrying the Zerk 40 feel as natural as possible. The first things we noticed were the ultra-wide, running vest-inspired shoulder straps and two adjustable sternum straps. It’s hard to overstate their size – these things are huge. If you have a particularly small torso, the straps might seem like a bit much, but for most, they are a welcome feature that effectively supports and disperses the load.

With frameless packs, your entire load sits on your shoulders, so strap design is everything. Here, the wide, cushy padding does its job well. We didn’t experience any pressure points – a common issue with frameless packs. The Zerk carries up to 30 pounds very well, but anything more that will overwhelm it.

The Zerk 40 also features a slightly more structured back panel than most frameless designs. It doesn’t transfer substantial weight to the hips, but it takes just enough of the edge off to be meaningful over time. The soft material sits comfortably against your back and helps wick sweat. An optional webbing waist belt is included to help stabilize the pack and keep it close to your body during tricky scrambles. 

Functionality

With their most recent update, Mountainsmith doubled down on the Zerk’s already impressive functionality. The pack is loaded with organizational features and attachment points, many of which are removable so you can fine-tune the setup to match your style.

The main compartment uses a wide roll-top closure with a double-V top strap that accommodates a range of bear canisters. Up front, there’s a stretchy mesh pocket for quick access to gear, which is flanked by durable daisy chain webbing for attaching items externally. The roomy front zippered pocket serves as a secure spot for valuables.

The elastic side pockets are double-layered and easy to reach on the move, with an additional upper mesh pocket on one side. Four side compression straps, along with ice axe loops and bungee tie-offs, give you plenty of options for carrying trekking poles or ice axes externally.

The real highlight, though, is the shoulder strap design. Each strap features two oversized mesh pockets that can easily swallow water bottles, a phone, or a full day’s worth of snacks. They keep essentials right where you want them. If that’s not enough, the Zerk 40 is compatible with Mountainsmith accessories like a brain or chest rig for additional storage.

Water Resistance

The Zerk 40 is more water-resistant than it looks, but it’s still best to protect any sensitive gear with a pack liner. In our testing – which simulated a several-minute torrential downpour – the inside of the pack took on a moderate amount of moisture, primarily from the top and around the seams. The 150-denier, high-density polyester liner keeps some moisture at bay, but rain will find its way in through the unsealed seams.

We’ve used this pack in the field without a liner in humid, misty conditions and the occasional shower. It kept our gear dry for the most part, but we wouldn’t rely on it for protection. The fabric will eventually soak through, and the lack of seam sealing means there are vulnerabilities regardless.

Construction

So far, the Zerk 40 has handled everything we’ve thrown at it like a champ. The main body uses 100-denier Robic nylon X-Grid Extreema fabric, with tougher 210-denier Extreema reinforcements in high-wear areas like the bottom and back panel. The stitching is solid throughout, and the daisy chain webbing helps bolster overall durability.

It’s worth noting that the Zerk has been around awhile and gone through many iterations. It’s the result of years of refinement and tens of thousands of trail miles. The design covered here feels more dialed and durable than ever.

Should You Buy the Mountainsmith Zerk 40?

The Zerk 40 is aimed at ultralight hikers who want a highly functional, feature-rich pack without moving into heavier framed systems. It’s a logical fit for experienced backpackers, thru-hikers, and fast-and-light users who are comfortable managing weight on their shoulders and dialing in their own packing system. The vest-style harness and abundance of accessible storage make it especially useful for long days on trail where efficiency, access, and movement matter more than structured support.

It’s not the best choice for newer backpackers, heavier loads, or anyone expecting traditional, rigid-frame comfort or full weather protection. Although it’s more supportive than most frameless packs, it works best when your base weight stays within a reasonable range.

What Other Ultralight Backpacks Should You Consider?

Backpacks are one of the most personal pieces of gear in your kit. Make sure to check out our full ultralight backpacks gear guide before pulling the trigger.

Atom Packs The Pulse EP50 Review: If you’re looking for a little more support, the Pulse EP50 could be a good fit. It doesn’t have the same level of organization or functionality as the Zerk, but it offers better weight transfer with a minimalist frame – and can still be carried as a frameless pack if you prefer.

Nashville Pack The Cutaway 40 Review: The Cutaway is arguably the strongest competitor to the Zerk 40. Both packs feature a ton of customization, organization, and functionality, but The Cutaway is more of a typical frameless pack with lighter materials and slightly less support.

Evolved Supply Co. The Ranger Review: Similar to the Zerk 40, The Ranger was designed by a veteran thru-hiker. It’s a frameless pack with sturdy shoulder straps but features a much more stripped-down, minimalist design. For seasoned ultralight backpackers with a dialed-in kit, it’s worth a look.