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Sierra Designs Flex Capacitor Backpack Review

Sierra Designs Flex Capacitor Backpack



The Flex Capacitor Backpack is good for multi-day trips or thru-hikes with an excellent frame capable of carrying heavy loads. It's also quite durable making it suitable for on-trail and off-trail use.

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The Sierra Designs Flex Capacitor is a top-loading, minimalist backpack with a unique top lid and two side water bottle pockets. Designed by National Geographic’s 2007 Adventurer of the Year, Andrew Skurka, it has volume adjustment straps that let you shrink or expand its capacity from 40L to 60L so you can fit more food and gear inside when you go on longer trips. This is complemented by a stiff but lightweight frame that makes it possible to haul heavy loads, far exceeding those that can be carried by similarly sized ultralight or internal frame backpacks.

While the Flex Capacitor is a radical departure from other lightweight backpacks that weigh 3 pounds or less, it provides many of the same benefits such as excellent back ventilation, interchangeably sized hip belts, side water bottle pockets, and large hip belt pockets, making it a tough and durable backpack that’s well suited for harsh and mountainous terrain.

Flex capacitor specs:

  • Sex: Men’s Only
  • Torso lengths (2): S/M (16″-19″) and M/L (18″-21″)
  • Hip belt lengths (4): XS/SM (29″), SM/MD (29″-32″), MD/LG (32″-35″), LG/XL (>35″)
  • Pack Weight: 41.2 oz – 43.2 oz (depending on torso length and hip belt size)
  • Mfg. Max recommended load: 35-50 pounds
  • Fabric: main body is 100D Nylon-Poly ripstop, base is a 420D Nylon Oxford
The main compartment has a top hatch that pulls back to provide access to the packs contents. It has a shallow pocket good for storing maps.
The main compartment has a top hatch that pulls back to provide access to the pack’s contents. It has a shallow pocket good for storing maps.

Internal Storage and Organization

The Flex Capacitor has a large main compartment and two side water bottle pockets. There are two large, solid-faced pockets on the hip belt and a small stretch pocket on one of the shoulder straps. The main compartment has a reservoir two fabric loops which you can hang a water reservoir from and a separate reservoir sleeve, with a single hydration port behind the neck for a hydration hose.

The main compartment is crowned by a top lid with a U-shaped zipper that flips up to provide access to the inside of the pack. The top lid contains two shallow pockets, sized to hold maps and other thin objects like a cell phone or snack bars. When flipped open, the top of the pack is large enough to swallow a large Garcia bear canister, so no worries there. Both of the zippers on the top lid, the one to the pocket, and the one that runs around in a U-shape around the top of the main compartment are protected from the rain and dust by solid fabric flaps. The U-shaped top zipper has two sliders but has the potential to compromise the pack’s usability if it ever fails. Still, worse comes to worst, you could pin it shut with a safety pin.

The two side water bottle pockets are reachable while wearing the backpack and it’s easy to pull bottles out or put them back. The side pockets are sized for one 1L bottle, although they stretch enough to also accommodate tent poles or a Tenkara rod that’s lashed under the compression straps and rests in the side bottle pocket. While the outside of the water bottle pockets is a very heavy duty mesh, the bottom of the pockets and the perimeter around the mesh are reinforced nylon for increased durability.

The only place to put wet stuff, like a water filter, is in the side water bottle pockets
The only place to put wet stuff, like a water filter, is in the side water bottle pockets

The Flex Capacitor hip belt has two large hip belt pockets which are large enough to store snacks, water purification drops, and electronics. Both pockets are hard-faced with fabric and quite durable, even when hiking off trail through scrub, thorns, and brush

External storage is quite limited, however, especially for wet items like a saturated water filter or rain fly. Your only alternative is to stuff them into a side water bottle pocket if they’ll fit or put them inside your backpack, preferably under a pack liner, since there’s no external stuff pocket for shoving layers or wet gear.  It’s not something I’m very keen on, but an issue that you’ll probably need to come to grips with on the Flex Capacitor if you backpack where there’s a lot of precipitation.

External Attachment and Compression System

The Flex Capacitor has numerous external straps including two tiers of compression straps, four volume adjustment straps, and two ice axe loops. Despite this, the straps never feel like they’re overwhelming the pack and they don’t “get in the way.”

The compression straps run continuously across the sides and back of the pack and can be used to compress your load or attach gear to the outside of the pack. They’re long enough and spaced far enough apart so that you can attach snowshoes, skis, packraft paddles, a sleeping pad, secure ice axe shafts, or an avalanche shovel to the pack when you have to haul heavy gear.

The Flex Capacitor has a gusseted bottom and back that expands when more capacity is needed. It's controlled by four volume adjustment straps, located in the center of the pack's back.
The Flex Capacitor has a gusseted bottom and back that expands when more capacity is needed. It’s controlled by four volume adjustment straps, located in the center of the pack’s back.

The volume of the main compartment is controlled by four volume adjustment straps. When loosened, they release the gusseted back of the Flex Capacitor and increase the diameter of the pack’s main compartment, expanding the volume of the pack from 40L to 60L, or whatever fraction you want in between. This is particularly useful if you need to pack a large bear canister in your pack and want to use the same lower volume pack you use for peakbagging or weekend trips. The reverse is also true.

There are two ice axe loops on the bottom of the Flex capacitor, but no shaft holders…you just secure the shafts using the other webbing straps on the back of the pack or thread your own using cord and some cord locks.

Backpack Frame and Suspension

The Flex Capacitor has a Y-shaped, aluminum frame stay that slots into the pack’s hip belt, providing an excellent load transfer to the hips when carrying really heavy loads, even in excess of 40 pounds. It’s really a remarkable frame, given how lightweight and simple it is. The secret sauce that makes it so supportive is the tension in which it’s held in place inside the backpack. While removable, it takes some elbow grease to remove and replace the aluminum stay: this is necessary if you want to change the hip belt size.

The Flex Capacitor Frame consists of three aluminum tubes joined by a hub and held in place under strong tension.
The Flex Capacitor Frame consists of three aluminum tubes joined by a hub and held in place under strong tension.

However, the back of the Flex capacitor very stiff and less form fitting than other light weight packs. While there is mesh covered padding on the back, it’s quite firm and it takes a while to get used to the pressure it exerts on the shoulder blades and lumbar area of your back. While any discomfort fades into the background eventually, it’s a very different sensation than most multi-day backpacks.

The padding and the Y-shaped frame work together to provide a large air channel behind your back through which air can pass, in order to reduce clothing perspiration buildup. Sweat dripping down your back and into your underwear is a key cause of chafing, which is why good back ventilation is important.

The gaps between the pads and the curve of the Y-frame stay create large air channels to full air flow behind your back
The gaps between the pads and the curve of the Y-frame stay create large air channels to channel airflow behind your back and dry perspiration.

Interchangeable hip belts

The Y-frame stay is removable so you can switch hip belts if you need to change sizes, although it takes a bit of elbow grease (and cougar screams) to take it out and reinsert it properly. If you order the Flex Capacitor at Sierra Designs you can specify the torso length and hip belt size of the pack you want. There are two torso length sizes, ranging from 16″-19″ and 18″-21″, and four hip belts lengths: 29″, 29″-32″, 32″-35″, >35″. Why so many different hip belts sizes? Sierra Designs told me they wanted to really dial in the fit for customers.

While you can order a pack configured to your exact torso length and hip belt specifications on the Sierra Designs web site, it’s not clear whether other retailers will have the ability to sell and assemble the different torso length and hip belt size combinations for you. Instead, they’re likely to pair the S/M:16″-19″ torso length pack with the 29″-32″ hip belt and M/L: 18-21″ torso length pack with 32″-35″ hip belt to make it simpler for them to stock. My advice would be to purchase the Flex Capacitor through Sierra Designs if you can, unless they’re out of stock. Switching the hip belt isn’t actually that hard once you do it a few times. :-0

Sierra Designs also sells just the interchangeable hip belt for $40 if you can’t obtain your size any other way.

The shoulder straps have very few external attachment points for attaching accessory pockets or electronic devices
The shoulder straps have very few external attachment points for attaching accessory pockets or electronic devices.

Shoulder pads

The Flex Capacitor’s shoulder straps are simple straps without a lot of attachment points. The right shoulder strap has a stretch pocket sized for a small squeeze bottle or snack bars. While both straps have an elastic band for managing a hydration hose, there aren’t any daisy chains, plastic rings, or good anchor points for attaching other accessory pockets or electronic navigation tools. If you want to keep those tools handy, you’re best off storing them in the pack’s large hip belt pockets.

The Flex capacitor can be used for a wide variety o fhikes and backpacking trips ranging from peakbagging to multi-day backpacks and thru-hikes.
The 40L-60L Flex Capacitor can be used for a wide variety of hikes and backpacking trips ranging from peakbagging to multi-day backpacks and thru-hikes.

Comparable Backpacks

Make / ModelPriceWeight (oz)Type
Gossamer Gear Silverback 55$32543.4Roll top, Top lid
Gossamer Gear Mariposa 60$27030.5Speed flap
Granite Gear Crown 2 - 60L$20036.7Roll top, Top lid
Granite Gear Blaze 60$27048Roll top, Top lid
Hyperlite Mountain Gear SW 3400$34532.11Roll top
Osprey Exos 58$22043Top lid, speed flap
Gregory Optic 58$21043.35Top lid, speed flap
Zpacks Arc Blast 55$32521Roll top
Mountain Laurel Designs Exodus 57$21518Roll top
Mountainsmith Scream 55$16045Roll top
Seek Outside Gila 3500 (57L)$33947Roll top, side zipper
Sierra Designs Flex Capacitor$20041.2Top lid
Elemental Horizons Kalais$27037Roll top


The Sierra Designs Flex Capacitor Backpack is a 41 ounces lightweight backpack with a rigid, but lightweight frame that is capable of handling heavy loads of 50 pounds or more. That alone makes it a unique offering among lightweight backpacks weighing 3 pounds or less. But the novelty of this backpack doesn’t stop there. It can expand from 40L to 60L of capacity if you need to carry a lot of gear or food, including a bear canister. It’s also easy to attach a ton of gear to the exterior of the pack, which is durable enough that you can take it off trail without worrying about it being ripped to shreds by thorns and aggressive vegetation. The biggest question you need to ask yourself if you’re considering the Flex Capacitor is whether you need the extra load carrying capabilities it provides. If you need to “go heavy,” then the Flex Capacitor is one of the best lightweight packs under three pounds that can get the job done.

Who is most likely to benefit from the Flex Capacitor? I think backpackers who want to switch out of a heavy 4-6 pound backpack for something lighter weight, but who aren’t quite ready to purchase an ultralight backpack from one of the cottage manufacturers. For example, if you carry a Kelty or Deuter backpack, or even one of the heavier Osprey Packs, and want something lighter, but just as durable and capable of carrying a 40+ pound load, I’d encourage you to consider the Flex Capacitor.


  • Solid faced hip belt pockets that are large enough to be useful
  • Side water bottles pockets are easy to reach
  • Backpack is capable of hauling heavy loads in excess of 50 pounds
  • Durable fabric exterior
  • Interchangeable hip belt length provide a custom fit


  • Difficult to securely store wet items outside of the main compartment
  • Shoulder straps lack good external attachment points
  • Frame and back padding is quite rigid

Disclosure: Sierra Designs provided the author with an extended loaner backpack for this review.

Updated 2019.

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  1. Nice pack, but can it travel back in time? Oh no, wait, that’s the flux capacitor.

  2. I almost pulled the trigger after Skurka reviewed this pack at its release couple months ago. I’ve been debating between this and a GG Crown 2, here’s my use case, I think the Flex Capacitor sounds like a better fit:

    We do “beginner backpacking trips” with our Scout troop a few times a year. They are quite short, typically 5-7 miles out, set up camp, dayhike a bit, cook, eat, sleep, head back home. I always take extra water (like 4-5 liters total load) because of the SoCal drought and some people don’t bring enough or drink too much or whatever. If some kid is overloaded (that never happens! :) ), then I take part of their load and have the “leave it at home discussion” (I am not going to tell a 12-year-old newbie “you brought it, you carry it” if that is going to turn them off to backpacking on their first trip). My Gossamer Mariposa has enough volume for this, but is woefully inadequate for the weight (can get over 40 lbs on a bad day), so I’ve been looking for a “guide” pack, and the flexible volume that Skurka designed into this guy seems just the ticket.

    What do you think?

    • For a load carrying perspective the FC is the better pack, but I think the Crown 2 has a better external attachment system, what with the floating lid, it’s higher volume including the extension collar, and a bigger opening to swallow gear. It’s a lot like the Mariposa in that respect. But if the weight of the gear and water is your chief concern I’d go with the FC.

      • I can’t compare it with the FC, but I did buy a Crown2 and I like it. As Philip says, it’s got a wide, easy-packing opening. The main compartment is really easy to pack because of this and its nice volume. And I really like the floating lid. I found that in principle, I like the compression straps, too. (I had mistakenly assumed each of the two tiers was one continuous strap. Nope. Each tier has a strap on each side and one across the back stretch pocket.) There are some things I don’t like. One, if the bottom side straps are routed through the side pockets, I find them difficult to tighten. Second, all the straps, and there are many, are pretty thick and at least on a new pack, difficult for me to pull through the buckles. But I’ll take that over, say, Gossamer Gear straps that are so flimsy they twist frequently and annoyingly in the buckles. Third, the Crown2 doesn’t carry equal weight (in my subjective impression) nearly as well as my Gossamer Gear Gorilla (2015 model modified to the new hip belt). And finally, unfortunately a deal breaker, I can’t reach the side water bottle pockets. They’re too tall for my “shot” shoulders. I ordered it through REI so I can take it back, but I am almost in mourning over water bottle pockets.

      • Try this simple trick with the water bottle pockets. Don’t pack the bottom of the pack bag on the pocket sides super tight. It’s easier to get the bottles back in if the exterior mesh isn’t pulled taught.

      • The problem I’m having is getting a bottle out. Though I see how your suggestion would help with replacing it. I just don’t have the range of motion. Short bottle, tall bottle. Makes no difference. It’s not the fault of the pack. I’m open to suggestions on how to make this work.

      • Ha! I think you’re right! I will be on a section of the Ozark Highlands Trail the rest of this week and really wanted to take the Crown2, but the guys I’m going with had to keep giving me my water bottle two years ago on an AT section hike. That was with an Exped Lightning 45. If I had to ask them AGAIN to take care of me, I might need another kind of 45!

      • RE: The problem reaching the water bottle pockets on the Crown2, I’ve found a workaround. Since I usually only carry a liter at a time, I found I can store a bottle within easy reach in the top pocket! I get to keep the pack!

    • Take a look at the granite gear blaze ac. It’s got an awesome frame hip belt and shoulder straps for big loads, but is otherwise set up like a light weight pack with stretch mesh pockets. It also has a huge extension collar so you can carry 60 liters if you want to.

      Personally, I find all my packs that are tall and narrow, and slightly banana shaped so that the top of the pack is over your shoulders carry loads much much easier than wide squat packs with heavy loads. I think it’s because that shape makes it easier to walk upright and doesn’t pull sideways when you walk as much as wider packs.

      • Good suggestion John. The blaze is very similar to the crown 2 and has a stiffer and adjustable frame.

      • Thanks, I saw it when I read Philip’s “How to choose” article, but had forgotten about it. The fact that it is laid out like my preferred packs means that I don’t have to change my loading strategy, so the Blaze has moved to the top of my list…

      • Another vote for the Granite Gear Blaze. For the type of loads you’re carrying, weight transfer to the hips is vital. The Sierra Designs single pole to the mid lumbar looks really questionable?? I use Zpacks Arc Haul and love it… but in the winter or when my dog is coming and I’m carrying larger/heavier loads, I sometimes return to my Granite Gear Leopard AC (same suspension as the Blaze). It’s has remarkably generous padding and the solid frame sheet distributes the weight nicely. I’m amazed at how light it is for as substantial as it is.

    • Sounds like you need to have pre hike pack checks!

  3. Philip,

    Thanks for your review.

    Would you mind comparing (or ranking) the Flex Capacitor, Paragon 48 and Crown2 in terms of ventilation and freedom of movement/torsional flexibility?

    And do any of these three get close to the ventilation offered by trampoline-style packs?

    • The flex has the best ventilation, the paragon the next, and lastly the Crown 2.
      You can move easily wearing all of them, but the flex and paragon are stiff, and the Crown 2 will move with you, like other UL backpacks.
      The flex has as good or better ventilation than a trampoline backpack. But it takes a while to get used to, since the stiffness is comparable to that of an external frame backpack.

  4. Thanks Philip!

    About the Flex Capacitor, in what dimensions do you find it stiff? Does it allow your hips and shoulders to move in opposition? How does it compare with the Seek Outside Divide in stiffness and freedom of movement?

    Also about the Flex, in photos it appears that the frame is curved such that the shoulder blade pads are tilted forward a little. My suspicion is that this would tend to promote a flexed (or slightly hunched) torso posture. Did you find that to be the case?

    • The Seek Outside Packs have more give because the shoulder straps hang form the frame and are not fixed to the back of the pack.
      No you don’t turn into a hunchback if you use the Flex. You’re going to have to try them at some point.

  5. Yeah, definitely going to have to try. Just trying to narrow it down. Thanks much for your help.

  6. I’m waiting for the women’s model. They said it would be in production by now. Does anyone know a solid date for distribution?

  7. Heather MacDonald

    Any more word on the women’s model? I’m have a tough time deciding between this and an ULA Circuit..

  8. Hei Philip,

    could you please tell me the length of the Y-shaped frame / back panel of the pack?


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