First Look: Expanding ‘Flex Capacitor’ Backpack From Sierra Designs

A new take on backpack design, the Flex Capacitor 40-60 Pack offers lateral expansion to open wide (or cinch tight) to manage gear. We tested an early model for review with one of its designers on a tough Rocky Mountain trek.

Sierra Designs calls the to-be-released backpack the “Flex Capacitor 40-60” because it transitions between sizes — from 40 liters of capacity with straps tight, up to 60 liters with the quick release of a couple buckles.

It has a unique frame system made of aluminum tubing similar to tent poles. Empty, the mid-size pack weighs 2.5 pounds, which feels light for a pack with a beefy frame that handles a hefty load.

The Sierra Designs Flex Capacitor 40-60 backpack comes to market in a limited batch this fall for $200 and ships to the mass market in early 2017.

Designed With Renowned Backpacker

Andrew Skurka leads the way while hiking in Colorado
Andrew Skurka leads the way while hiking in Colorado

Andrew Skurka, best known for long-distance hikes through extremely remote locations (as in around the state of Alaska), got into the gear-design game in this collaboration with Sierra Designs.

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The Flex Capacitor is one of his first projects. It’s not his original design, but the company hired Skurka to refine the pack and help bring it to market. (It bears his name as a part of the branding.)

I was lucky enough to spend a night and day with Skurka, cruising an exceptional high-route in the Colorado Rockies. The 15-mile journey, loaded with high winds, chilly weather, and some light rain, put the new pack (and tent system; review coming soon) to a great test.

Review: Sierra Designs Flex Capacitor Backpack

A tip: If you go hiking with Skurka, lace your shoes up tight — he moves fast over rugged mountains.

We picked a “high route” near Alice, Colo. The 15-mile, largely off-trail route followed ridge-lines and passes, tagging five 13er peaks along the way. It was a perfect proving ground for the new pack.

Poles serve as frame for support (they are internal on production model, shown here for illustration)
Poles serve as frame for support (they are internal on production model, shown here for illustration)

Overall, this is a sweet-carrying, versatile backpack. The frame system and the expanding back make it stand out in a crowded category.

My pack was pretty light for this overnighter, probably around 15 pounds of gear and a lot of water. But the weight carried almost entirely on my hips and lower back, right where I wanted it.

Skurka said he used a prototype of the pack to carry out an elk during hunting season last year, noting that it carried the boned meat loads of around 60 pounds with little trouble.

Expanding Pack: As noted above, the Flex Capacitor stands out thanks to a clever expanding design that lets it carry between 40 and 60 liters while still acting “full.” The pack expands outwards, not up and down, by tightening or loosening a series of compression straps.

This gives the pack more versatility for those who want a single pack to do a lot of jobs. Even though my pack was carried fairly empty, it didn’t shift on my back and I had tons of room to expand if I needed more gear or food for longer excursions.

Design notes: The pack has a useful water bottle pocket on the right shoulder strap. It looks a little goofy at first glance, but dang if it isn’t helpful. You don’t need a buddy to reach your bottle!

The author testing the Flex Capacitor. Note the water bottle carrier; photo by Andrew Skurka
The author testing the Flex Capacitor. Note water bottle carrier; photo by Andrew Skurka

The pack has a zippered top opening, and the top flap has a nice-size zippered pocket for storage of small items. The hip belt also has large pockets for a phone, keys, or snacks.

The pack has solid but standard features. Stretch mesh side pockets provide additional storage. Ice axe loops give carry options on back, and an interior water reservoir sleeve can be removed or used to carry other small items like maps.

Overall, this is a well designed, functional, light backpack that can carry significant weight if needed. It will provide a versatile solution for hikers and others who want a backpack for many uses on high mountains and trails anywhere you may roam.

–Want to learn more? Check out Andrew Skurka’s pitch on the pack on his website.

Sean McCoy

Editorial Director Sean McCoy is a life-long outdoorsman who grew up hunting and fishing central Wisconsin forests and lakes. He joined GearJunkie after a 10-year stint as a newspaperman in the Caribbean, where he learned sailing and wooden-boat repair. Based in GearJunkie's Denver office, McCoy is an avid trail runner, camper, hunter, angler, mountain biker, skier, and beer tester.