Sierra Designs Flex Capacitor Backpack Review


Sierra Designs Flex Capacitor Backpack

The Sierra Designs Flex Capacitor (updated in Fall of 2019) is a top-loading backpack with a unique top lid and two side water bottle pockets. Designed by Andrew Skurka, it has wrap-around compression 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.

Sierra Designs has made a minor few changes to the Flex Capacitor this year. The pack is now offered in two new colors, white and black, replacing the original silver-colored version (which is now 20% off at for $159.99.) A second mesh pocket has been added to the shoulder pads to hold snack bars and gadgets. In addition, the internal mesh hydration sleeve can be hung on the exterior of the pack for storing wet items or gear you want to access frequently without having to stop and root around inside the backpack. I review these below.

The Flex Capacitor is also now available in two additional volumes, 25-40L and 60-75L, but the number of hip belt sizes available has dropped from four with the original silver-colored pack to two sizes with the new white or black colored version. While I haven’t had a chance to try the 60-75L version yet, my guess is that it will make a super winter backpack when you have to carry a lot more gear, food, and fuel. The Flex Capacitor’s internal frame is very stiff and it can carry 50 lbs or more so there’s no doubt in my mind that the 60-75L model can carry the extra weight.

Specs at a Glance (40-60L)

  • Gender: Men’s Only
  • Torso lengths (2): S/M (16″-19″) and M/L (18″-21″)
  • Hip belt lengths (2): SM/MD (29″-32″), MD/LG (32″-35″)
  • Pack Weight: 41.5  (size M/L)
  • Mfg. Max recommended load: 50 pounds
  • Bear canister compatible: yes
  • Fabric: main body is 100D Nylon-Poly ripstop, base is a 420D Nylon Oxford
Flex Capacitor Backpack Lid
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 two stretch mesh pockets on the shoulder straps. The main compartment has two fabric loops that you can hang a water reservoir from or 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 made with mesh, the bottom of the pockets and the perimeter around the mesh are reinforced nylon for increased durability.

Flex Capacitor Backpack Side Pockets
The side water bottles are easy to reach while wearing the backpack

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.

You can hang the internal hydration sleeve on the outside of the backpack

External storage was quite limited on the original version of the Flex Capacitor, particularly for wet items like rain gear, a wet tent, or a wet water filter. Sierra Designs added a feature to the new packs by making it possible to move the interior mesh hydration pocket to the outside of the pack and suspend it from four exterior gear loops, as shown here. It’s functional, but that’s the best thing I can say about it. The pocket is tiny and the mesh is flimsy and not designed for use on the outside of a backpack.

There are a lot of better ways that this external mesh pocket feature could have been implemented that would be more functional and durable. For example, look at the way that Seek Outside adds a Talon Pocket to their backpacks (video).

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 ax 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 ax shafts, or an avalanche shovel to the pack when you have to haul heavy gear.

The Flex Capacitor has numerous compression straps and external attachment points for carrying extra gear

The volume of the main compartment is controlled by four compression 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 ax 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, up to 50 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 lightweight 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 Y-shaped frame and padding work together to provide a large air channel behind your back through which air can pass, in order to reduce 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.

Flex Capacitor backpack ventilation
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 two hip belts lengths: SM/MD (29″-32″), MD/LG (32″-35″).

Limiting the hip belt size to a maximum of 35″ seems small to me and I suspect this is an error in the quoted specs on the Sierra Designs website. My contact there is looking into this and I’ll update this post when I hear back from them. I used a pack with a MD/LG hipbelt for this review and it fit me fine with plenty of extra slack, even though I have a waist size of 38″.

Sierra Designs Flex Capacitor Pockets
Sierra Designs added a second stretch mesh pocket to the shoulder straps.

Shoulder pads

The Flex Capacitor’s shoulder straps are simple straps without a lot of attachment points. Both the right and left shoulder straps have a stretch pocket that’s large enough for a small smartphone or snack bars (the second pocket was added in the latest model.) 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.

Comparable Backpacks


The Sierra Designs Flex Capacitor Backpack (2019) is a 41.5 ounce 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.

If the Flex Capacitor 40-60L appeals to you, I’d suggest grabbing one of the previous Silver versions of the Flex Capacitor, which are steeply discounted to make way for the new White and Black colored models. While Sierra Designs has made a few tweaks to the original pack, it really hasn’t been altered in any substantive way. In fact, the sizing options on the Silver model are better than the new White and Black versions, making this a good opportunity to pick one up on sale.

Summary of New Model Changes

  • New Colors: White and Black
  • One stretch mesh pocket added to shoulder straps
  • External attachment system for hydration pocket, providing external wet storage
  • Reduced number of hipbelt sizes: from four lengths to two

Disclosure: Sierra Designs provided the author with a backpack for this review.

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Last updated: 2019-10-21 12:58:03

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