Sierra Designs Flex Capacitor 40L - 60L | Review - Outdoors Magic

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Sierra Designs Flex Capacitor 40L – 60L | Review

U.S. brand Sierra Designs are now making moves into the UK. Here’s a good example of the kind of quirky backpacking kit available from them.

Why We Chose The Sierra Designs Flex Capacitor 40L – 60L: Versatile, clever design, lightweight

Sierra Designs make kit that is different. Most of the products across their line have something that’s fresh or unique about them, and we like that.

This, the Sierra Designs Flex Capacitor 40L-60L, is a great example. The designer behind it is none other than Andrew Skurka. In case you’ve not heard of him, he’s basically King of the Backpackers over in the States. It’s a pack that, as its name suggests, flexes. Thanks to its six compression straps and a big gusset across the front, what you have here is a carrier that can switch from a 40L to a 60L and cater for all the sizes in-between as well. There are a bunch of other useful features too.

Who Is The Sierra Designs Flex Capacitor For?

At 1.2kg, this fits into the lightweight category but only just. There are certainly much lighter packs out there. It’s still an option that’s well worth considering for any backpackers who are conscious of their gramme count though. You can see that it’s been made with long trails in mind, one like the Pacific Crest Trail and Appalachian Trail in particular, where the overall pack capacity is going to expand and shrink constantly.

Back System

The back system here is simple but supportive. It involves one lightweight Y-shaped frame that forks at the top, channeling weight down from the shoulders to the waist. A thick padded lumbar support and then two pads also up on the shoulder pads, keep the pack raised off your spine while also allowing a good amount of airflow as well.

It’s certainly durable enough to withstand the demands of life on the trail.”

Like most backpacks this comes in two different back lengths: S/M and M/L. Unlike most options out there, however, you also get a choice of hipbelt size as well, with four different sizes altogether. It’s interchangeable so you could potentially even swap sizes if you find your waist drastically changes for some reason. They say this happens on the trail but we’re yet to see evidence of that!

Prominent U.S. backpacker Andrew Skurka was behind the design. Photo: Mike Brindley
The bag's design channels weight from the shoulders down to the waist. Photo: Mike Brindley
Useful, easily accessible, stash pockets. Photo: Mike Brindley


Interestingly, the access into the main compartment of the bag is via a wide bucket-style zipped lid. This means quick and easy access to whatever is in the top but also that you’ll need to root around for anything stored at the bottom. There’s an outer flap covering this zip but nothing underneath. As the zip isn’t waterproof, there is potential for moisture ingress here.

The main fabric used throughout the bag is a 100D and 420D ripstop nylon/polyester. This is certainly durable enough to withstand the demands of life on the trail. It’s also got an impressive water resistance to it. Bear in mind it isn’t waterproof though; this coupled with the potential flashpoint at the top zip, means you’d be sensible to use dry bags for your kit inside.

The pack is designed by Andrew Skurka, ‘King of the Backpackers’. Photo: Mike Brindley

Lots of pockets. There are two stretch mesh ones on the sides which can each easily take a water bottle (not a sleeping mat though) and can be used while you’re on the move. The fact that the base and top cuffs on these have been toughened up is a nice tough. There are two other stretchy pockets on the shoulder straps which are very useful for keeping something like your phone or GPS. The hipbelt has two large zipped pockets either side of it and finally, you also have one small zipped pocket on the lid which is only really big enough to take items like a map, guidebook or phone.

Tester’s Verdict

Will Renwick, editor of Outdoors Magic

“What I like most about this backpack is that it does the job of two bags. On a three-day trek, I’d probably take a 40L backpack whereas on a trek that lasts over a week I’d normally want something with a slightly larger capacity. Because of this, I’ve got two big backpacks taking up space underneath my bed. I’m now considering passing those onto friends and just sticking with this for all my backpacking needs now. On a long distance trek, that ballooning volume is also going to really come in handy. It means you don’t ever have to put up with a sagging, unstable load when you’ve got plenty of supply points ahead, nor do you have to cram stuff in or hang it off the sides when you know you’ve had to fully stock up for a few barren days.

“The lumbar gives excellent support and the load management overall is impressive.”

“When I first saw pictures of this I thought it looked fiddly and unnecessarily complicated but the reality is that it’s not. It’s very straightforward and easy to use. I like the fact that you can use all those straps to hitch items of kit if needs be as well, things like an ice axe, trekking poles or even a folding mat.

“I’ve also found the pack very comfortable. The lumbar gives excellent support and the load management overall is impressive. It’s actually one of the better back system’s I’ve come across, in a way, it’s quite similar to the one found on one of my favourite packs, the Fjällräven Kaipak 58.

“Probably the biggest downside is the limited access to the main compartment. Some people who are organised packers will like this simplicity, but I’m the kind of person who usually makes good use of any bottom or side access that a pack might have.

“One last touch I really like on this bag is the fact that the internal mesh pocket that holds a hydration bladder can be removed and clipped onto the outside of the pack. I can see this coming in handy as a place to dry out any damp kit (like my pants after I’ve gone for a spontaneous dip) or even for just stowing a waterproof jacket while on the go. It’s not particularly big, so you couldn’t get, say, your flysheet in it, but it’ll be fine for a waterproof jacket.”

Sierra Designs Flex Capacitor

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