Gear Review: Mountainsmith Zerk 40L Fastpack

The Zerk 40L Fastpack is a brand new pack from Mountainsmith designed in conjunction with backpacking athlete Tom Gathman, aka the Real Hiking Viking. It was built for a Fastest Known Time (FKT) attempt of the Colorado Trail, so it is made to comfortably and efficiently carry weight in a speedy manner. It has thick, padded shoulder straps that allow it to carry almost like a running vest and is covered in accessible pockets so that you don’t have to take the pack off to get your hands on anything you may need in a day of hiking. The pack was originally built for a small release online through Indiegogo, and will be available on a larger scale in July through Mountainsmith.

Mountainsmith Zerk 40L Fastpack At-a-Glance

Weight (with optional hip belt and bear canister straps): 28 ounces
Capacity: 40 liters/30 pounds
Materials: 100D Nylon HT w/ 200d Spectra Double R/S TPU
MSRP: $220

Circumstance of Review

I have hiked around 70 miles with the pack so far, most of those done on day hikes with a 25-pound load. The longest hike was an overnight trip of about ten miles in heavy rain. I experimented both with and without the optional hip belt, and ended up preferring to go without it. The most unique feature of this pack is definitely the beefy shoulder straps, and they function very well within the weight limit of the pack. I typically prefer to hike with a framed pack and hip belt, but if you’re going to go without that weight distribution, straps like these are the way to go. The pockets on this pack are ample and impressively functional.

Zerk 40 Features

  • Removable bear canister straps: There are multiple attachment points on the pack, making these rather versatile. They also work very well to strap a foam pad to the outside of the pack.
  • Removable hip belt
  • Stretch mesh front pocket: This pocket is about the same size as most lightweight packs out there, but it is on the stretchier side, meaning that it adds more space than average.
  • Reflective removable front bungee cord: Another versatile strapping option on the front of the front mesh panel.
  • Secure overtop webbing
  • Stiff atilon foam and breathable mesh back panel: Gives the pack a bit of a frame and keeps it comfortable. It isn’t enough mesh to significantly air out a hiker’s back, but it does definitely improve comfort.
  • Ultrarunning-inspired shoulder straps: The standout feature of the pack, these are what make the pack truly superior. They distribute weight extremely well for a pack that doesn’t have a full frame or hip belt.
  • Dual mesh shoulder pockets: These provide plenty of room for an any-sized smartphone as well as a few snacks for the day. There are even extra pockets on the front of these pockets that are great for food wrappers and headphones.
  • Low-profile side compression straps
  • Massive side pockets: Big enough for two Smartwater bottles each.
  • Outer mesh side pockets: That’s right, extra pockets on the sides of the already jumbo side pockets.


With hip belt: As with most frameless/light-framed packs, using the hip belt doesn’t actually affect how the pack carries all that much. It helps keep the pack more snug against the body, but it doesn’t make much difference in the weight distribution.

Without hip belt: This is the way the pack is built to carry. One of the biggest focuses in the pack’s construction is on the shoulder straps, which are wide and much more padded than the average pack. It wears almost like a vest and therefore carries impressively comfortably.

Gear Category Details

The Zerk 40 was designed for an FKT: Built for speed and efficiency, a pack like this needs to be lightweight, comfortable to carry for long stretches of time, and easily accessible. The Zerk 40 accomplishes this with its ample pockets, wide shoulder straps, and 28-ounce weight. This pack has enough pockets accessible while wearing it that you could easily load it up with food and water for the entire day and never have to take it off while staying hydrated and satiated. This pack is definitely ideal for anybody who wants to cover a lot of miles quickly and efficiently.

Here’s the deal with this pack: it is made to be worn all day with a fairly light load. You can pretty much put anything you want anywhere you want on the outside of the pack so you can load up in the morning and not have to take this thing off all day. And while that makes it ideal for something like an FKT, it’s also great for any backpackers who want to up their efficiency.


  • Carries extremely comfortably for a pack without a full frame.
  • Many large accessible pockets. You can pack snacks and trinkets all over the pack so that you can organize your things while keeping them accessible.
  • Lightweight.
  • Shoulder straps really do make this pack feel like you’re hiking in a big harness or vest. It’s a unique and comfortable option for lightweight backpacking.
  • Lots of removable straps make this pack efficient and flexible. You can strap things all over the outside of your pack or remove all the straps and save the weight.


  • Even though the Zerk 40 comes with a hip belt, wearing it doesn’t transfer much weight off the shoulders.
  • Bear canister straps are difficult to attach/detach.
  • Difficult to obtain: currently not available until July 2019.

Overall / Value

This is a fantastic pack, but it fits into a unique category. If you want the lightest pack available, this pack is nearly a pound above some of the competition. If you want something that weighs 28 ounces and carries as comfortably as possible, there are fully framed competitors. The places that this pack shines are its accessibility and unique shoulder straps. Even though this pack doesn’t have a full frame, it really is significant competition in terms of comfort. I think it ends up just coming down to preference: some just might not be able to abandon their framed packs. If you are wanting to try a frameless pack without a hip belt, I would highly recommend this one. At the end of the day, this pack is kind of a niche model, but in that niche it positively excels. 

Comparable Packs

Osprey Levity 45 – $250

Gossamer Gear Gorilla 40 – $215

Pa’lante Packs V2 Gridstop – $215


**This product was donated for purpose of review.

Affiliate Disclosure

This website contains affiliate links, which means The Trek may receive a percentage of any product or service you purchase using the links in the articles or advertisements. The buyer pays the same price as they would otherwise, and your purchase helps to support The Trek's ongoing goal to serve you quality backpacking advice and information. Thanks for your support!

To learn more, please visit the About This Site page.

Comments 3

  • Mike : May 2nd

    Interesting that the ULA Fastpack wasn’t listed as a comparable. They look to be very similar packs.


What Do You Think?