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, its signature feature are 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 (a Women’s model is under development)
- 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
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 to shallow pocket, 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 it is ever fails. Still, worse comes to worse, 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 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 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 a 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’s takes some elbow grease to remove and replace the aluminum stay: this is necessary if you want to change the hip belt size.
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.
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 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 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 is 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 to 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.
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