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 on sale at REI for $150.) 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

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

Recommendation

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.

Editor's note: Help support this site by making your next gear purchase through one of the links above. Click a link, buy what you need, and the seller will contribute a portion of the purchase price to support SectionHiker's unsponsored gear reviews, articles, and hiking guides.

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28 comments

  1. Philip – I am assuming a bear canister would have to go in vertically – or does the Garcia fit in horizontally?

    • Yeah, the Garcia fits horizontally (went back and checked), so that means bearvaults will too.

      • A BV500 will fit horizontally with the “silver” 40-60 pack expanded to 60L.
        It is a snug but not tight fit on mine. A Garcia fits about the same. The given pack sizes are is 28.5/30″ * 13 ” * 10″ and BV 500s are ~ 12.75″ * 8.5″ so that seems about right. The 13*10 seems right but 30 on my M/L seems optimistic.

        I tried looking at the new packs specs to compare but the given measurements are confusing and inconsistent and it is unclear if they are with the pack expanded or what.

  2. I have the original version of this pack and really like it. It hit a sweet spot I couldn’t quite get with other packs. It was just the right combination of price, weight and functionality. The adjustable capacity gave me flexibility that I wouldn’t have if I chose a single size and saved me quite a bit of money from buying 2 or even 3 different size packs. I bought it on sale for $150. Fixed capacity packs may save a bit of weight but this is perfectly reasonable. This pack does everything well and I haven’t once felt I’ve compromised anything.

    I didn’t have the same experience as Phil; the pack never felt odd to me. Once I had it fitted, it disappeared and carries the load beautifully (that frame is genius simplicity). The extra weight capacity for this pack isn’t something I’ll typically need, but will be welcome on the occasional extended food or water carries, where many other packs in this class begin wilting around 35 lbs.

    Now that I’ve owned the pack for a little over a year, here are a couple of notes to some comments I’ve seen online and that Phil touched on. The first thing was a lot of people griped at the lack of external pocket. I’ll say that this is way overblown. In terms of convenience, if the pack were a roll top then perhaps it would be more problematic. But the top lid is so quick that there’s no reason you can’t get to anything you need if you were smart enough to pack such items toward the top. And the slim pocket within the lid is perfect for small items and very quickly accessed. Between the hip belt pockets, lid pocket and easy access to the main compartment, I never once wished I had an external pocket. (Not to mention, those are usually the first places to wear out on packs so equipped.) Like Phil and others, I also took note of possible zipper failure with the top lid, but this really doesn’t concern me since this is not under any strain like panel loaders; it’s just sitting on top. Further, a husband and wife team both using this pack covered 1400 miles on the PCT including all of the desert sections and dirt or dust never phased the zippers.

    Similarly, people complained about where to put wet items like a tarp or tent fly. With so many straps, if one can’t figure out how to lash it to the outside, perhaps the outdoors isn’t for them. Furthermore, I also appreciate Skurka’s response where he noted that if you’ve packed your sleep system and clothing in waterproof bags, then shoving wet items into your pack, outside those bags, presents zero problems. You’ll have to air them out anyway since they really won’t dry even if outside the pack. He’s right, of course.

    The only thing I’d care about from the new version is the second should strap pouch, but that’s very minor, especially since the side water bottle pockets are easily reached and hip belt pockets are big enough to be useful.

    I get the impression this pack is a bit of a sleeper. I think there are a lot of people that would be well-served by this one. Mostly anybody that doesn’t want to spend a lot and could use a good jack of all trades. At around $150, this is a seriously good deal.

    • Question. I have an Ospery pack and LOVE the suspension system. However, despite the admittedly good ventilation my lower back still gets sweaty. How do you think that the central lumber pad of this pack compares with other packs in this regard? I am considering the levity because as I said, I love the suspension, and I want a simple pack. At 5,10 I have a narrow waist at around 32″, so the small belt size of either pack suits me. The weight difference and for that matter, the extra fragility of the levity is not a major consideration for me as with the latter, lifetime warranty.

      Thanks in advance.

  3. I also own the original and have just one complaint that was apparently addressed with the newer version. The volume adjustment straps (not the compression straps) made slight tears where the strap was connected to the body of the pack. I guess I was pulling too hard on the volume adjustment straps and treating them like compression straps which I’m sure many would do. Anyway, SD replaced the pack without issue. On the replacement pack I applied some seam sealer or other viscous goop at these seams and have had no issue at all. So if you buy the old mode on sale, you’ll be fine if you secure those seams.

  4. Did you ever hear back on the size of the hip belt? I agree that 35″ seems really small and hope that this is incorrect.

    • I did. They’re sticking with that sizing, but I think their specs are wrong.

      • I agree. I have a 40-60 Flex Capacitor with a Md/Lg hip belt. My waist (measured around the iliac crest) is 36”. If my waist was even an inch smaller, I would not be able to cinch down the belt tight enough. The Md/Lg hip belt can fit people with waists well in excess of the advertised 35”.

      • That’s my experience too, that their hip belt specs are way off. I lost a little weight and my Md/Lg hip belt (on the 40-60L “jet” color Flex Capacitor) is now too loose.

        If anybody wants to trade a Sm/Md for my Md/Lg, let me know!

      • Hey guys, thanks for all the info about the old/new Flex, I’m going to take the plunge and get one. Quick query and I know it’s sort of been covered here already, but – I’m almost bang on 32″ hip measurement, I sent an email to SD asking about belt sizing and if I do go for the S/M hip belt, would there be enough belt webbing to go over layers and wet weather gear (live in the Cairngorms,Scotland). Their reccomendation was to go for the larger belt size, but Steve’s point (in this thread) almost sounds like there would be enough extra play if I went for the smaller size? Anyone got the S/M belt that could give me the actual length of the hip belt?

      • I first bought the old pack with the md/lg hipbelt and I measured 33 inches around my hips and found I had to cinch it all the way up and it only felt good once I was wearing 3 layers (base, fleece, and rain jacket). I contacted sierra designs and they sent me a sm/md hipbelt for the new version since the sm/md hipbelt for the old version is sold out everywhere and it works great. I would say get the sm/md size.

  5. Could you provide the spacing between the side compression straps (i.e., top of upper side strap to top of lower side strap? I have emailed Sierra Designs and haven’t got a response.
    Thank you

  6. Hi Phil. Thinking about getting either the 60-75L SD Flex Capacitor or the 70L Cold Cold World Chaos, primarily for longer outings year round. Which do you feel carries and packs better? Thanks!

    • The Choas is a winter-specific pack without any side pockets or hydration sleeve. It also has an exterior crampon pocket. If you wanted one pack for year-round use, I’d go with the SD,

  7. So I picked up one of the silver ones to see if it will work for me as an alternative to my comfortable but heavy 2008 Baltero 70. If so, it shaves 3# off my base weight which is pretty good for ~$130. I’m intrigued by the different design. I go where the bears are and where they are not and like to keep the bear can inside when carrying it so the capacity variation seems like a good idea. But… I think the lack of brain and external pockets will drive me crazy.

    Generally I unload the main bag of my pack when setting up camp leaving water filter, raingear trowel etc, microspikes and a “brain” full of miscellany in the pack since I don’t need those things. With this single bag pack I have to empty it all then throw the things I’m not using back inside. Then get them out again to pack out so they can be near the top …Hmm!

    Hate to add the weight back with stuff sacks…but I keep these things in external pockets for a reason…

    I would really prefer to keep the trowel etc in a separate pocket (particularly the etc if packing it out). I guess I can be OK keeping this in the pack in its own bio-hazard stuff sack but…

    I like to keep rain gear in its own pocket to prevent punctures and tears from getting ruffled around. A stuff sack would work as well I guess.

    Water filters leak after use so it’s better in its own pocket where it can drain and evaporate and very handy to get when it is needed. This is going to be slightly annoying.

    The microspikes have their own bag but if used tend to be wet and dirty. I don’t have a problem storing them with the trowel etc in an outside pocket where they can tuck in the bottom of the pocket but they will need to be separated if rattling around in the main bag.

    I’m going to need a solution for all keeping all the brain junk (first aid kit, repair kit, headlamp, buff, gloves, map, compass etc etc.) accessible. Does anyone know of a less expensive version of the something like the Hyperlight Mountain Gear Pods. Seems like having one of those on top of everything could serve the same purpose as the brain pocket, particularly if I could attach it inside but $50-60 starts to make this pack look less of a bargain.

    Anyone have any suggestions or is it just bung it all in and hope?

    • It always takes a while to adapt to a new pack architecture. Give it time and get yourself some mini-biners to hand stuff off the compression straps.

      Six moons copied the HMG pods and they’re a less expensive option.
      https://www.garagegrowngear.com/products/pack-pods-by-six-moon-designs

      • Cool. Thanks. Pack pods look like just the thing though one would be sufficient. More too good to throw away stuff in the gear box…

        I’ll consider biners but I don’t like things hanging from the outside unless there is good reason (eg: a z-light pad (which I so far don’t use), trekking poles if not in use (only in the car), sandals (obviously) are about the only excuse for strap on items in 3-season imo). Things tend to fall or get scraped off and are annoying if they wave around plus the extra windage is not helpful.

        Still it will provide some entertainment seeing if it works.

      • Also for anyone missing the back mesh pocket there is the Dutchware pack-back…
        https://dutchwaregear.com/product/pack-back/
        …basically a large mesh sack with shock cord attachments in the corners so you can attach it to, say, the load lifters at the top and around the shoulder strap attachment points at the bottom.

        It is not quite the same thing as a dedicated pocket but seem like it would serve most of the same purpose and apparently only adds less than 3 oz.

        Plus you can remove it when bushwhacking is on the cards or for use on a different backpack. Possibly a way to get someone else to carry your stuff without them noticing.

        You can also use it as a light weight pack for who knows why or attach it to a tree so you can forget all your stuff when you leave camp.

        For $30 it needs some justification. Not sure about the choice of .9 NoSee-Um mesh since that is not stretchy as far as I know…but maybe that is a good thing.

      • Or you could just use the hydration pocket they provide which hangs on the outside of the pack and get the same thing for free.

  8. For the new version yes, although it’s not all that as far as I can tell. A good idea but a bit of a lazy effort. Also I the old one and it doesn’t. The pack-back is a bigger and seems more generally useful…more what the new version should be.

    • Spending $130 (or even $200) to save 3# and still be able to carry 40#+ was a no brainer, but it seems I am between sizes on this pack and it doesn’t work for me so they are both going back.

      Briefly, My torso length is ~19′ which is supposed to be the top end for S/M and the bottom end for M/L. The shoulder straps on the S/M are obviously too short because the pouch is in an unusable position and the torso length on the M/L is too long because the shoulder strap attach to the bag too high up. Walking my 1.5 test hill with 30#, the hip belt rides down because of that probably not helped by its flat and floppy design (or my washtub abs). I do like the idea of the Y “frame” and it seems like it could carry well but with a brief test and incompatible sizing it is hard to say how well it works on a real hike.

      The hip belt pockets are very generous which somewhat offsets the lack of other closable exterior pockets but I’m not so keen on the rest of the bag. Having played around with it I think the zipped lid is just a bad design, especially for a pack that purports to accommodate varying volumes of gear. Its smallish pocket expands down into the pack which means you can’t put anything hard or bulky in it and be sure you can close the lid zip. I wasn’t all the impressed with the flex sizing which just fussy to deal with. The bag is actually a bit small once you put a BV500 in it. I can make it work but since that is “normal” for most of my trips, having to making it work is not really good enough for me. It needs to be a bit bigger or have some auxiliary pockets or it needs to be smaller and provide a way to attach an empty bear can on the top (which is not really my preference).

      Since it won’t fit there’s no point is trying the new 60-75 but I’m not convince that pack takes things in the right direction for me anyway since I’m not sold on the bag design. Back to looking at quilts I guess but I’m having a hard time getting excited about a $500 bag of feathers just to save 1.5#.

  9. Did you ever get an update on the hip belt size issue? (A typo saying the max hipbelt size is 35″?) I’ve ordered one of these, but I sit right around 34-36 depending on time of year and my training

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