Mountainsmith Zerk 40L Review: Ultralight Pack, Vest-Style Straps

Brett 'Grandpa' Kretzer
Mountainsmith Zerk 40L Ultralight Pack Running Vest Shoulder Staps Review GGG Garage Grown Gear

During the past hiking season I’ve used the Mountainsmith Zerk 40L Pack for about 300 trail miles on the PCT in Washington and Oregon; in the many ecosystems of Olympic National Park; and on the Wonderland Trail around Mount Rainier. Not included in those 300 miles are the numerous shakedown hikes I took the Zerk on in my home state of Colorado.

This was my first experience with a frameless pack and I was pleasantly surprised. Being that the Zerk 40L Pack was designed by a human about three times my size (Jaba aka The Real Hiking Viking), I was legitimately concerned with how the pack would carry on my small frame. Luckily, the Zerk’s features seem to translate well to different body types. 


Mountainsmith Zerk 40L Ultralight Pack Running Vest Shoulder Staps Review GGG Garage Grown Gear

  • Wide mouth roll-top access with opposing buckles
  • Wide, ultra-running inspired shoulder straps
  • Dual mesh shoulder pockets (2 pockets on each shoulder strap) 
  • Stiff atilon foam & breathable mesh back panel
  • XXL side pockets
  • Outer mesh snack & trash stash pocket
  • Secure overtop webbing compression strap
  • Side compression straps
  • Stretch mesh front pocket
  • Removable bear & gear canister straps with multiple attachment points
  • Reflective & removable bungee cord
  • 1.5” removable waist belt


  • 100d Nylon HT w/ 200d Spectra Double R/S TPU
  • Atilon Foam (Backpanel)
  • 210g Stretch Mesh
  • EVA & PE Foam

Volume: 32L (compressed) // 40L (at capacity)

Weight: Packed Weight - 28oz // Trail Weight (straps detached) - 25oz

What I Liked

Mountainsmith Zerk 40L Ultralight Pack Running Vest Shoulder Staps Review GGG Garage Grown Gear

Comfort and Carry

One of the first things you’ll notice when you look at the Zerk is the massive running-vest style shoulder straps. Being that this is a frameless pack, so virtually all of the load is taken on by the shoulder straps, it’s quite nice that they are so incredibly supportive. As a small guy, I felt like my entire torso was embraced by the pack and that the weight was evenly distributed over my whole core rather than just my shoulders.

Furthermore, the back panel doesn’t have a traditional internal frame, but it does include a “removable interior laser cut frame sheet (EVA Atilon)”. This tiny frame sheet provides a modicum of rigidity that is just enough to make the hip belt useful. It’s not going to be the same as having a frame but I noticed, in a pinch, I could tighten up the hip belt and transfer some weight to my hips.

A note on the weight this pack can carry: Mountainsmith rates the Zerk 40L with a maximum carry weight of 30lbs. I'm sure that’s nothing for the Real Hiking Viking, but I found anything more than 20-22lbs to be uncomfortable for me.


This pack has some pretty smart storage features. Since you lose any hip belt pocket storage with the minimalist webbing belt, they have consciously made up for it by adding some intelligently placed elastic pockets. The shoulder straps have 2-layered elastic mesh pockets on both sides. I have used them to hold my phone, on-the-go snacks, occasionally a map, and even a 1-liter smartwater bottle.

The side pockets are also double-layered. They’re big enough to hold two 1-liter smartwater bottles, and still be able to slide a thin item like a pocket-sized guidebook or some protein bars in the second layer. This is the first time I’ve seen these layered side pockets on a pack and I think it’s pretty clever.

Mountainsmith Zerk 40L Ultralight Pack Running Vest Shoulder Staps Review GGG Garage Grown Gear



Mountainsmith generously provides a couple of optional features such as the front panel attachment bungee system, and some extra overtop webbing straps for a bear canister or big bag of potato chips. The hip belt is also removable.

Personally, I enjoyed having the bungee system and the hip belt. I didn’t use the extra overtop straps, since the pack includes a single, centered overtop strap, as well.


At $219.95, it’s a pretty great deal assuming that you have your ultralight kit pretty dialed in. But then again, if you’re serious enough about backpacking to own expensive ultralight gear, do you care about saving an extra $100 on a pack? Either way, you’re getting a lot for the price — a thoughtfully designed pack conceived by experienced hikers.


I’ll admit, I’m not a fabric expert, but I am hard on gear and so far this pack is holding up nicely. I have yet to put any major wear on the fabric, seams, or mesh of this pack. I even saw a few well-seasoned Zerks on the backs of PCT thru-hikers in Washington, and they all seemed to be holding up well. 

The main body is constructed with 210D EXTREEMA Recycled Nylon Ripstop. Although the EXTREEMA fabric sounds kind of like Dyneema, it is not related. EXTREEMA is water resistant, but not completely waterproof. However, it will probably keep your stuff dry in a light or quick rain, and doesn’t carry the high price tag of Dyneema. 


What I Didn’t Like

No Buckle on the Side Straps 

I’ve gotta say, this annoyed me multiple times on trail when I wanted to quickly stow away my trekking poles. Instead of being able to stow them mid-stride, pack still on, I needed to stop, take my pack off, and slide my trekking poles through the top of the straps down into the side pocket. If you don’t carry poles or don’t stow them often, it’s not a big deal; but I think if Mountainsmith addressed this issue, it would significantly increase the functionality of the pack.

The Hip Belt Could Be More Padded

I love that the back panel has a thin frame sheet, but I think it could be better utilized by making the hipbelt a hair beefier. 

Good to Know

Mountainsmith Zerk 40L Ultralight Pack Running Vest Shoulder Staps Review GGG Garage Grown Gear

The folks over at Mountainsmith seem to be putting some serious practical thought into their gear designs. I’ve met some of them in person, and we shared backpacking stories, and bounced gear preferences and ideas back and forth. It’s nice to know that their gear is being designed by people who are actually getting out there and putting it to the test.

With any frameless pack I would say to know your physical limit on how much you’re willing to carry on your shoulders. For me, this pack maxed out around 22lbs, but you may be comfortable carrying more or less. 

Bottom Line

All in all, this is a comfortable ultralight pack with a lot of handy features that aren’t widely available elsewhere. It strikes a beautiful balance between simplicity and practicality. I’ll still keep my framed packs around, but the Zerk 40L is now my go-to pack for fast-packing and ultralight, warm-weather backpacking trips.



Zerk 40L Backpack by Mountainsmith



Continue Hiking with 'Grandpa': @bert.kertzer

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