Sierra Designs Flex Capacitor Backpack Review

Touting the ability to expand from 40 to 60 liters in capacity, the Flex Capacitor transforms from a medium sized overnight pack to a large multi-day hauler. This is my review.

The flex capacitor features one large primary compartment. This compartment makes up the bulk of the backpack and can be expanded from 40 liters, all the way up to 60 liters, or any size in between. This is thanks to the integrated gusset system, with dedicated straps that simply wrap around and through the body of the bag, pulling the pack in on itself to reduce the internal volume. Inside, there is a robust “Y” shaped frame built from DAC Pressfit aluminum poles that terminate at the spine of the wearer, as well as a removable hydration sleeve that can hold bladders or bottles. The packs has two large zippered hip pockets, a small chest pocket, a medium sized lid pocket and two stretchy water bottle pockets. The pack is constructed primarily from 100D Nylon-Poly Rip stop, with 420D Nylon Oxford used in high stress areas. The pack comes in various sizes, weighs about 2 lbs 11 oz as tested, and retails for $199.

I recommend packing the heaviest gear low, for maximum stability.

What I liked

Starting with what is most important in any pack, comfort, the Flex Capacitor has very much impressed me. From the very first time I slung this work horse onto my back it was immediately clear that every aspect was designed by someone who has carried a lot of backpacks and suffered from their shortcomings. Most importantly, the weight of the pack and its contents is transferred entirely and directly to the padded hip straps by the robust “Y” frame, which is essentially a network of tent poles. The frame extends from shoulder to shoulder, meets at a hub, and travels all the way to the bottom of the pack. This puts all the pressure that would normally bear down on the shoulders directly down onto the hips, which are built to hold weight. Thanks to this, the shoulder pads sit gently across the shoulder, only serving to balance and stabilize the pack against the chest, which helps prevent shoulder, neck and back pain. The shoulder and hip straps and pads are both well padded, firm, and pre-curved to fit the natural contours of the human skeleton. This means everything wraps onto and fits snugly in place, which makes the pack feel more like it is being worn than carried. The back panel is padded where it contacts the body, but only does at the shoulder blades thanks to the outwards curving frame. This curve prevents gear from bulging into the spine while also providing extra ventilation across the back. Carrying weight up to about 40 pounds, the pack feels light, nimble, and above all, supremely comfortable.

Organization with the Flex Capacitor is also quite good. While it isn’t overly zealous with pockets and compartments like many, what is included is all the essentials. The main compartment does most of the work, housing tents, sleeping bags, pads, food, cooking essentials and most other gear. The two zippered hip pockets are great for stashing snacks or even a small camera or phone, while the zippered top lid provides quick access to things like bug spray, headlamps, car keys (with included key clip) or other items one would need to access less often. There is a small chest pocket that I honestly never used, and the two water bottle pockets easily hold 1 liter Platypus bottles with room to spare. The internal hydration bladder sleeve is handy to keep water suspended and easily accessible, be that a bladder or otherwise, but can be removed to shed a little weight if desired. The organization provides everything that is actually needed, and nothing that isn’t.

The key clip is a welcome addition.

Build quality and durability are both great. Being constructed of rugged 100D rip-stop nylon for most of the body, it holds up quite well to being drug around, stuffed to the brim and repeatedly compressed and tugged on. High stress areas are 420 denier, which is basically indestructible within normal means. The stitching is impressively consistent and robust with reinforcements being applied in high strain areas, such as shoulder and tension straps. The zippers are burly, slide easily, and really haven’t snagged up on me yet, while the compression straps are all equally as rugged.

I love not only the texture of, but durability of the fabrics.

Having the ability to expand the pack, while seemingly a small detail, actually makes a big difference. It’s one pack that can compress down tight and stable with a huge range of gear inside, making it equally viable for weekend trips, multi-day excursions, or just snow treks where a lot of gear will come in handy. Instead of having a loosely packed backpack flapping around with a light load, which creates instability, it snugs down tight and secure. It even makes a decent day hike bag with enough gear inside, although it definitely feels like overkill. This also allows for more flexibility when packing, as gear can be more easily positioned and shifted, with the straps happily expanding and contracting to fit different orientations and positions of whatever is inside. I use this to pack all my heavy gear at the bottom, while stacking lighter gear up top.

Flex Capacitor, expanded out to allow for more gear.

The ability to compress the pack down wouldn’t make a lot of sense if it was heavy (using a heavy pack with a light load is a waste of weight after all). Luckily, the Flex comes in at just 2 lbs 11 oz, making it one of the lightest packs I’ve tested, but also one of the lightest that I would actually consider viable and appropriate for long distance packing with any kind of substantial weight inside. Considering its ability to easily haul 30+ lbs while staying comfortable is really impressive on its own, but considering the weight makes it doubly so.

The main compartment opens up wide for easy packing.

at just under 200 bucks, the value here is really great. With packs getting more and more expensive, while also getting lighter but more fragile, it’s excellent to see a pack that is durable, light, comfortable, and affordable all at the same time.

Ample padding on the hips, where it is needed the most.

What I didn’t like

The pack can be a bit noisy at times. While the “Y” frame feels great, the single point termination at the bottom leaves room for the pack to flex and move around a bit, which creates a plasticy creaking sound. It’s not loud, but hiking alone in a quiet forest, it can really start to stand out. So far, I’ve not really found a way to prevent it, but it isn’t the noisiest pack I’ve carried either. It’s not bothersome most of the time, but when it gets eerily silent, it’s hard to not notice.

The internal “Y” frame, unique, sturdy, but a little creaky.

The load strap (the red one designed to hoist the pack) for me sits directly against my neck, joyfully licking me up and down as I hike. Luckily, I generally wear collared shirts these days (bug and sun protection FTW), but when it sits directly against the skin it can be annoying. Thankfully, it can be easily pinned back or even removed if you’re desperate, but I wouldn’t recommend the destructive approach. A clever camper could even attach a magnet to it, snapping it to the pack itself.

Rain covers are starting to be standard on packs these days, and there isn’t one included. At 200 bucks though? Maybe I’m just begging at this point. In its defense, it does a pretty decent job of shedding water, but I still recommend one.


It is rare these days that a backpack really stands out to me. Most brand name packs are fine, being light enough, comfortable enough, or packing neat features that may or may not actually useful in the long term. That being the case, I blissfully surprised when I first put on this pack. It immediately felt exceptionally comfortable, feathery lightweight, and stable. Yet, the pack was still constructed in a way that felt rugged, dependable and of high quality. This is not a combination that I come across often. While scrambling, climbing or just lugging an unusually heavy weekend load down a narrow trail, the pack always felt like it was a part of me, moving when I did without causing me pain or discomfort. I rarely even realized it was there (aside from the fact that I just gained 30 lbs). Checking off all the important boxes, the Flex Capacitor stands out even before taking in its most unique feature, which is the expandability of the pack itself. Having a single pack that can be used for nearly any backpacking trip simplifies packing, or even saves one from having to make a second backpack purchase to accommodate a wide variety trips. It’s priced aggressively, has held up extremely well, and has taken the crown of being my new go to backpack after 3 years of repeatedly going back to my previous favorite. It is the entire package, and for this I cannot recommend this pack enough.

The Highest of Recommendations

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I want to extend a huge thanks to Sierra Designs for providing this product for review. We couldn’t do it without their help.

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4 thoughts on “Sierra Designs Flex Capacitor Backpack Review

  1. My relatively new(3 months old Osprey Atmos AG 65 squeaks so badly I’m considering getting it repaired by Osprey or returning it to REI. I’m wondering if you’ve ever tried that bag before and if you’ve heard the squeak in it and how the sound volume is in comparison to the squeak you noticed in the Flex Capacitor?

    1. Hi Jan,
      I haven’t tried that exact bag, but Osprey bags do tend to squeek. Honestly, just about any backpack with a thin wire frame will. It’s just a matter of the thin bar shifting around in the fabric guides. There really isn’t a lot that can be done to avoid it…
      It’s not an Osprey specific issue either. I’ve ran into this from most manufacturers.
      There is a little bit of it pressent in the Flex Capicitor, but it didn’t bother me too much.

  2. Great review! I really liked how you also described how the pack felt when you wear it – most don’t seem to bother with that.

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