The Sierra Designs Flex Capacitor is a top-loading backpack with a unique top lid and compression straps on the sides and front that let you modify the pack’s volume or strap bulky gear to the outside. 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. Updated in 2023, the Flex Capacitor Backpack has several new features that make it better than ever. If you’re looking for one pack for year-round backpacking, the Flex Capacitor is hard to beat.
Specs at a Glance (40-60L)
- Gender: Unisex
- Mfg Pack Weight: 3 lbs 2 oz (size M/L)
- Actual Tested Pack Weight: 2 lbs 15.5 oz (size M/L)
- Load lifters: Yes
- Pockets: 9 + main
- Torso lengths (2): S/M (16″-19″) and M/L (18″-21″)
- Hip belt lengths (2): NA
- Max recommended load: 50 pounds
- Bear canister compatibility: BearVault BV500 and all smaller sizes fit horizontally
- Fabric: Body is 100D Nylon-Poly ripstop, Base is a 420D Nylon Oxford
Originally designed by Andrew Skurka, Sierra Designs made a few significant changes to the Flex Capacitor in this latest update.
- There are two large stretch mesh pockets on the sides of the pocket situated above the side mesh water bottle pockets. They really enhance the usability of the backpack.
- The side compression straps can be (optionally) buried in fabric channels rather than running over the new large stretch mesh pockets on the sides of the pack.
- The number of volume-reduction straps was reduced from four to two.
- The internal mesh hydration sleeve in the previous version has been replaced by a solid hydration sleeve that can be used as a summit backpack for short hikes.
- While the hipbelt is still technically interchangeable, it appears that Sierra Designs has stopped offering hipbelts in different lengths.
- The larger 60-75L model of the Flex Capacitor has been replaced by one with 60-80L of capacity.
Internal Storage and Organization
The Flex Capacitor has a large main compartment, two side water bottle pockets, and two large stretch mesh pockets above them. There are two large, solid-faced pockets on the hip belt and two stretch mesh pockets on the shoulder straps. The main compartment has a removable hydration pocket (1.7 oz) with a single hydration port behind the neck for a hydration hose (along with a hang loop if you want to suspend a hydration bladder without using the hydration pocket). The hydration pocket can be transformed into a summit pack by repurposing the pack’s compression straps as shoulder straps but it has so little volume, you’d probably never use it that way.
A top lid crowns the main compartment with two burley zippers and flips up to provide access to the inside of the pack. It’s much faster to use than a roll-top closure and far less annoying if you have to do it a lot. The top lid contains a shallow top pocket with a key fob, sized to hold thin objects like a Smartphone or snack bars. When flipped open, the top of the pack is large enough to swallow a large Bearvault BV500 bear canister horizontally, so no worries there. All of the zippers on the top lid are protected from rain and dust by solid fabric flaps.
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 Nalgene 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.
The Flex Capacitor hip belt has two large hip belt pockets which are big enough to store all of your daily 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 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. This new model has two large stretch pockets on the sides above the water bottle pockets that provide abundant external open storage and compensate for the lack of the front stretch pocket that is found today on most lightweight backpacking packs. These new pockets are large enough to hold lots of gear and clothing and provide the ability to segregate your gear by function or “wetness” in a way that is even better than a single front stretch mesh pocket. They’re a really great enhancement.
The placement of these new mesh pockets required a refactoring of two of the pack’s compression straps, which are now buried in fabric channels along the top and bottom of the mesh. At first, I thought this would reduce the pack’s ability to carry snowshoes along the pack sides, but on further inspection, you can pull the compression straps out of the fabric channels to use them for this purpose. That’s a relief because the 50lb load-carrying capacity of the Flex Capacitor makes it an excellent option for winter backpacking and expedition travel.
External Attachment and Compression System
The Flex Capacitor has numerous external straps including two tiers of compression straps, two 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 compression straps also close with buckles which makes them much easier to use in winter, particularly when wearing gloves.
The volume of the main compartment is controlled by two compression straps and two volume reduction 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 want one backpack that can be used for multi-day backpacking or long day hikes and not two.
There are also 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 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.
However, the back of the Flex capacitor is 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.
The Flex Capacitor’s shoulder straps are simple J-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 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 hipbelt is heavily padded and quite firm, providing excellent load transfer in conjunction with the pack’s rigid frame. I didn’t experience any hip belt slippage at all, even with heavier loads. The hipbelt is tightened using push-forward straps, now standard on many backpacks. It closes with a beefy center buckle which is much easier to use in winter with gloves. Hip control straps are also provided to pull the pack bag closer to your hips.
Comparable Lightweight Backpacks
|Make / Model
|Zpacks Arc Haul 60L
|20.9 oz / 593g
|Hyperlite Mountain Gear Southwest 55
|34.9 oz / 989g
|Granite Gear Crown 3 60L
|32.6 oz / 1040g
|Osprey Exos Pro 55
|34.6 oz / 981g
|UHMWPE Nylon Ripstop
|ULA Circuit 68L
|37.3 oz / 1038g
|Gossamer Gear Mariposa 60L
|30.5 oz / 865g
|REI Flash 55L
|45 oz / 1276g
|Gregory Focal 58
|41.3 oz / 1171g
|Waymark Gear Lite 50
|34.8 oz / 987g
|Atom Packs Mo EP50
|32.1 oz / 910g
The Sierra Designs Flex Capacitor Backpack is a lightweight multi-day 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.
Disclosure: Sierra Designs donated a backpack for review.
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