Sales Pitch: Seven reasons the Flex Capacitor 40-60 Pack stands out

Earlier today I filmed the product video for the new Sierra Designs Flex Capacitor 40-60 Pack, which with the High Route Tent 1FL currently constitutes SD’s “Skurka Series.” To develop my script, I had to prioritize and succinctly describe the pack’s must-mention specs and features. Later I’ll write a self-critique, as I did for the High Route, but for now I want to explain what makes the Flex Capacitor stand out.

1. It’s just $200 and 2.5 pounds (1.1 kg)

The Flex Capacitor is built well and has the right feature set. But we held down its price and weight by keeping its design simple, using reasonably priced materials, and pushing hard on our partner factory.

2. Volume adjustment

Using the adjustable gusset on its front side, the Flex Capacitor’s volume can be adjusted between 40 and 60 liters (2440 to 3660 cubic inches), which is the difference of nearly two BV500 bear canisters. Load-carrying performance is excellent throughout this range, essentially making the Flex Capacitor a “two packs in one” proposition. The gusset is a more functional solution to volume adjustment than conventional extension collars and cinch hooks.

3. Full suspension

Last year I carried out an elk from the Colorado Rockies with a Flex Capacitor prototype. The first load, which weighed 69 pounds (31 kg), was crushing. But the Flex Capacitor carried it shockingly well, and I used it again for the second 60-lb load, despite having a true meat-hauling pack in the car. The Flex Capacitor features a stiff Y-shaped tubular stay made by DAC; it anchors directly into the hipbelt for efficient load transfer.

The Y-Flex suspension adds little weight, but it greatly increases the Flex Capacitor's load-carrying performance. It was tested to 69 pounds. To take this photo, I turned the pack inside out.

The Y-Flex suspension adds little weight, but it greatly increases the Flex Capacitor’s load-carrying performance. It was tested to 69 pounds. To take this photo, I turned the pack inside out.

4. Back panel airflow

Designers face a mutually exclusive choice with pack panels: load-carrying or user comfort. Packs carry best when they are flush against the back, because the load is closest to the user’s natural center of gravity. But gappage between the pack and user helps to avoid SBS, or “sweaty back syndrome.” We chose user comfort, but did so without adding much weight (see #1) or giving up much load-carrying performance (see #3).

The gap between the back panel's lumbar and scapula pads is a wind tunnel. It was nearly a liability on this snow-dusted Colorado 13'er, but it's a definite perk in warmer and more humid conditions.

The gap between the back panel’s lumbar and scapula pads is a wind tunnel. It was nearly a liability on this snow-dusted Colorado 13’er, but it’s a definite perk in warmer and more humid conditions.

5. Six external pockets, including a shoulder strap pocket

Hipbelt pockets and side pockets should be standard on any backpacking pack. The Flex Capacitor has them, and they’re very functional. We installed a fifth pocket in the top lid, for small essentials. My favorite, however, is the shoulder strap pocket, which is sized for a 20-oz water bottle or bear spray.

The Flex Capacitor features six exterior pockets: two on the hipbelt, two side pockets, one top lid pocket, and a shoulder strap pocket.

The Flex Capacitor features six exterior pockets: two on the hipbelt, two side pockets, one top lid pocket, and a shoulder strap pocket.

6. Easy in/out top lid

Like me, do you find annoying the multiple buckles, straps, cinch cords, and lids that you must futz with in order to access your pack’s main compartment, or to close it up? How does a top lid with a single, big-toothed, easy-gliding zipper sound? Yeah, it’s awesome.

An easy-gliding #10 zipper offers annoyance-free access to the pack's main compartment.

An easy-gliding #10 zipper offers annoyance-free access to the pack’s main compartment.

7. Compression and utility

The expanding gusset could serve double-duty as a compression system, but we added two removable horizontal straps anyway. First, they decouple volume adjustment from load compression. Second, they are useful in securing trekking poles, ice axes, shovels, or even skis (no metal edges, please).

Two removable compression straps run horizontally around the pack. They improve load-carrying, and can be used to secure trekking poles, oft-needed clothing layers, an ice axe, and more.

Two removable compression straps run horizontally around the pack. They improve load-carrying, and can be used to secure trekking poles, oft-needed clothing layers, an ice axe, and more.

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30 Responses to Sales Pitch: Seven reasons the Flex Capacitor 40-60 Pack stands out

  1. Rob August 30, 2016 at 7:41 am #

    Those side pickets look a little puny. Can you fit two 1liter smartwater bottles in them?

    • Andrew Skurka August 30, 2016 at 9:22 am #

      Yep, they should. I only have one smartwater bottle around, but I have plenty of 20-oz bottles that are around the same diameter. The pocket should also fit the 80-oz Platypus SoftBottles, and can be secured using the lower compression strap.

      Flex Capacitor side pockets with 20-oz bottles. The hipbelt was removed from this pack for review; it would go on the right side.

    • CJ August 31, 2016 at 3:23 pm #

      I was thinking the same thing. They don’t look tall enough to hold the 1 liter Smartwater bottles securely. I like the rest of the design, but with the gusset system there’s no mesh front pocket to hold wet gear…can’t have both I guess.

      • Andrew Skurka August 31, 2016 at 3:43 pm #

        I’m a stickler on side pocket design, and I think these are satisfactory. I’d be very surprised if smartwater bottles popped out on you. The 20-oz bottles in the photo are 8 inches tall, which means the pockets are about 6-6.5 inches tall.

        A 34-oz smartwater bottle is 12 inches tall, and begins its taper at about 9 inches.

        No, there is no mesh pocket for wet gear. I’ve carried more wet gear than I would like to have, and I find that an equally good solution is to put it inside the main compartment but outside a trash compactor bag pack liner.

        • Fowler August 31, 2016 at 10:07 pm #

          welcome to retail, for whatever reason smartwater bottle users seem to think any pocket under 24″ tall will not secure a bottle. Everyone that isn’t worried that it is too short to secure a bottle will worry about it being too tall to reach with the pack on.

          • Andrew Skurka August 31, 2016 at 10:50 pm #

            That is the way it seems. Tradeoffs, tradeoffs tradeoffs.

          • Rob September 1, 2016 at 7:30 am #

            I do use smartwater bottles, but wasn’t concerned about their security, only the approximate volume. I actually carry a REI flexlite chair (don’t judge me!), and like to put it in the side pocket so I can get to it easily at breaks. I figured that if two smartwater bottles could fit, then the chair would fit as well.

            If I were looking for a new pack, this would be on my short list.

        • CJ September 1, 2016 at 7:23 am #

          Thanks for the clarification. It is hard to tell from pictures how tall the pockets are, so it is probably just the view/ angle or the photo that makes them look smaller than they are. I do like the shoulder strap pocket for the bear spray…I always worry that mine is hard to get to if I should need it!

          This packs seems to be a massive contender to compete with the SMD Fusion Packs, especially the previous year models…good weight carrying capacity while still being light.

  2. Dale August 30, 2016 at 8:35 pm #

    This summer I’ve been rethinking my whole system, aiming to go lighter. Your blog has been a great help. (Thank you!) The one thing I’ve been putting off is getting a new pack. My current one is 3.5 lbs, 60 L and all the ultralight packs either have a very low load rating and / or are very expensive. This pack is very interesting 🙂

    Can you comment on the sizing? I’m 6’1″, waist about 38″. I recently tried the largest Osprey Exos, which looks great on paper, but was just too small for me.

    • Andrew Skurka August 30, 2016 at 8:59 pm #

      What about the Exos was too small? Torso length, hipbelt?

      The October release will have packs in three sizes, for both torso length and hipbelt. I don’t have the specific inch sizes, but those three sizes should cover most everyone, including you at 6’1″. The hipbelt is interchangable, so if you’re not a medium-medium, you can it out for what you need.

      • Dale August 31, 2016 at 2:47 pm #

        The Exos was too small on the hipbelt. It’s different from other hipbelts, it kind of has a shape to it–a rigid arc–and if your hips don’t fit in that arc a longer belt strap won’t help because the left and right ends of the arc will dig into your back. The guy at REI knew what I was talking about, he’s smaller, but had to size up to the large just to fit in the hipbelt. Torso length seemed fine, though.

        • David October 22, 2016 at 9:36 pm #


          I have owned a 35L Osprey for over 5 years. I love it. It is almost indestructible and I find it to be a perfect fit. I need a bigger pack so had decided to purchase a Exos 58, which I feel is a good “compromise”, volume, weight, quality and features. Speaking of features, I don’t know that I can live without the large elastic pocket on the back-body of the pack. I stuff my hat, buff, and any other light item that I want to access easily there. I am however very interested in your pack Andrew due to it’s obvious careful-thoughtful design, it’s quality and flexibility in volume. I have a few questions please.

          1. I am 80kg, 179cm and have a small, “imperial” 32-33 inch waist. (My current pack is a L, but I am actually a M). It is my small waste that seems to work really well with the Osprey belt. How does the flex capacitor compare or work for me?

          2. Andrew, you mention placing a trash bag in the pack to aid in waterproofing. I find that my pack, now getting old admittedly, soaks up rain water like a sponge. I need to don a cover quickly to prevent water adsorption. Do you advise NOT using a cover with the flex capacitor? If so, why?

          3. At 60L the pack looks ungainly. The bace looks to narrow to be stable at this volume. At 60L, is it as stable as a 60 pack carrying 40 pounds?

          • Andrew Skurka October 22, 2016 at 10:09 pm #

            You are not the first person to be nervous about it lacking a shove-it pocket. I wouldn’t be. You can open and close the main compartment so quickly that there is no need for one. If it took as long to access the main compartment as it does with a conventional roll-top or cinch-and-lid, I would agree that the lack of a shove-it pocket would be a serious omission.

            1. 179 cm = 70.5 inches, or a half-inch more than 6′. The pack has two sizes: torso and waist. For this first production run, you need to fit both, since SD is not offering substitutions; that will change in the spring, when they will have extra hipbelts available in the event that your torso length and waist are of atypical proportions.

            Your torso length is related to not but not perfectly correlated with height. Some people are leggy, and have a relatively short torso, or the reverse, or about average. Whatever the case, you need to measure your torso.

            If your waist is 32-33, a Large pack might be too big, since the hipbelt is for 34+ inches. Technically, it depends on where the pack sits on your hips, since that may or may not be the same place where you keep your pants.

            The Medium size should be fine. The low end on that is 31 inches.

            2. Besides weight gain, there is no penalty in having a wet pack. It’s more important to keep dry the rest of your gear, like your sleeping bag and clothing. The trash compactor bag does an excellent job in waterproofing these items. My experiences with pack covers never ended well — in heavy rain, water runs down the shoulders and wicks through the back panel, or it soaks everything while I’m digging through the main compartment in search of something.

            3. I agree, in the expanded mode the pack looks really big in the product photo. Not sure if the distortion is due to the photo angle/zoom or to the side-by-side comparison with the pack in collapsed mode.

            I have used the pack across its entire spectrum, and I think it carries evenly throughout. Of course it’s more agile when in 40L mode than 60L mode, but that’s a function of the load more than the pack.

          • David October 22, 2016 at 11:15 pm #

            thanks for the detailed reply Andrew.
            I agree with your water-rain fact of life, deal with it approach, weather pack or clothing related. (I still hate a wet pack). Yes, water pours down your back with a cover on the pack. It can also wet from the bottom up as water seems to find it’s way in there too.
            I work part time as a hiking guide, often get wet and I don’t own any rain gear at all. 🙂 One will get wet, there is NOTHING on this earth that can prevent it. I will use an umbrella strapped to my pack, as I use hiking poles, if the conditions allow. Preventing hypothermia is obviously critical. Keep moving fast enough to keep warm, once this is not possible, stop, get dry and shelter. I keep dry clothing in a drysack.
            As for boots, I love them and love to hate them. After a rain soaked 6 hour day hike at altitude in Mongolia this summer, (cold temperatures), my boots went from 850g a piece, to God knows what. That was the last time, never again. Not just because they were heavy, but because it took two days of warm sunny weather on my patio to dry the clodhoppers out. If I had to keep hiking in them life would have been slow painful and miserable.
            As for your FC pack, thanks so much for the reply. Unfortunately I live in China and don’t order products from overseas due to the variances of China customs. I will find another way to get one on my back as it seems to be perfect for my needs.

  3. Shawn K. August 31, 2016 at 7:55 pm #

    Good looking pack. Some MYOG folks would probably like to get their hands on that DAC frame.

    Speaking of which, is it easy to remove the frame and break it down? If so, that’s another advantage the FC has over many packs, since it could be easily stowed in luggage for flights, or shipped in a small package.

    • Andrew Skurka August 31, 2016 at 8:32 pm #

      Yes, it can be broken down and fully removed, fairly easily. Hadn’t thought of that advantage for travel, but, absolutely, that’s a huge advantage if trying to use it as carry-on.

  4. Terrance Glover September 22, 2016 at 11:18 am #

    Just pre-ordered. I’m hoping this is a perfect pack for Philmont next summer. I don’t normally need the flexibility – on most of my trips, my pack gets lighter and smaller after I leave the trailhead as food and water are consumed. But, at Philmont, we’ll be dealing with variable amounts of (bulky) food during the trip. This pack looks to be the perfect solution.
    I was happy to see a Large frame size option (as I have a 21.5′ torso length).
    Once it comes in, I’ll get it out on the trail and report back. Backpacking season here in Arizona is just getting started, so I should be able to get 2 or 3 trips in before the cold month (!).

    • Andrew Skurka September 23, 2016 at 7:35 pm #

      Looking forward to hearing your opinion of it. I think you’ll like it.

  5. Noor hamdi September 28, 2016 at 10:59 pm #

    Backpack with capability to expand its capacity is something thoughtful to eliminate the needs for multiple different size & capacity backpack for various expeditions. May work or not is personal preference but as backpack is design by experience adventurer Andrew Skurka possibility to hit the sweet spot is might as well spot on…?

  6. Ryan Verma October 22, 2016 at 3:56 am #

    Hi Andrew, I’ve been pretty happy with my exos for camping but I need to get something a little more durable for traveling and hunting. I really like the ability of the flex capacitor to be able to change volumes and the weight is comparable to my exos which is impressive considering the FC’s load carrying capacity you mentioned. I’m debating between the FC and an osprey atmos and my main concern is comfort while carrying loads up to 50lbs. Can you comment on the comfort of load carrying of the FC against the atmos at these loads? Thanks!

    • Andrew Skurka October 22, 2016 at 8:50 am #

      I have not used the Atmos so I have no first-hand comments about it. My understanding is that many people love it, but the fit is particular and it does not fit everyone perfectly. No pack will. It’s also 4.5 pounds and $260 (for the 65L), versus 2.5 pounds and $200.

      The FC carries well up to 50 pounds, although 50 pounds still sucks. I have made two very heavy carries with it — 69 pound and 60 pounds — and regularly have put up to 40 pounds in it on casual glamping and guiding trips. I’ve written more in-depth about those heavy loads.

      Perhaps the greatest evidence of its load-carrying comfort is that I’m taking it out again next month on my hunting trip, when I could again be presented with these huge loads. I have a true meat-hauling pack, a Kifiru, but I feel like the FC is better for the hunting part, and then it’s sufficiently capable for the pack-out.

      • Ryan Verma October 23, 2016 at 12:45 am #

        Thanks for the prompt reply, Andrew. I’ve always appreciated that about your blog! Do you have any experience to make comparisons between the FC and the ula catalyst? Or even any speculations or educated guesses?

        • Andrew Skurka October 23, 2016 at 9:15 am #

          I can speak much better to the Catalyst, since I have used it personally, since I have loaned it out to a lot of clients, and since I have used extensively the Epic, which has the same back panel.

          When it’s expanded the Flex Capacitor has about as much volume as a Catalyst. I know that the Catalyst’s official spec is 4600 cubic inches, but 600 of that is the shove-it pocket (where you would never want to put 600 cubic inches of dense items because it’d throw off your center of gravity) and another 600 is the extension collar, which is not supported by the frame and which thus does not carry weight well. If you subtract 1200 from the official 4600 = 3600 = 59 liters.

          The packs serve a similar niche: lightweight packs that have convenient features (e.g. generous hipbelt pockets, external storage), that can carry a decent amount of weight, and that are built to last.

          The Catalyst is made of heavier fabric, has dimensional side pockets (rather than stretch mesh), and is available in more sizes and colors. It’s also USA-made.

          The FC is a few ounces lighter and is more versatile, because of it’s adjustable gusset. When compressed, it carries like an Ohm. It’s also $60 less.

          The two packs feel quite different when on. The Catalyst has a flat mesh backpanel and rides close to the body. The FC has three mesh/covered foam pods (which BTW will not absorb pine needles) that allow for more airflow but that result in less body contact. The stays of the FC are longer, and thus it can better transfer weight off the shoulders. The Catalyst’s shorter stays may result in a better “body wrap” but ultimately it cannot carry as much weight.

          Overall, I think both packs are really good choices. If you pick the FC, it’s probably because you want a one-pack quiver. If you pick the Catalyst, it’s probably because you already have a small-volume pack or because the FC is not yet available in your size or in a color that you like.

  7. Jacob Finger December 13, 2016 at 7:18 pm #

    Hey Andrew, recently I have learned a lot from watching your videos and reading you blog, I am an avid back packer looking to lighten my load. I currently have the Atmos AG 65, great pack, but too heavy. I actually have recently been using a Osprey Stratos 36 for overnighters in places where I don’t have to have a bear can, but I need something bigger than that for multiple nighters and when I have to lug along the bear can. So, I stumbled upon the flex capacitor. I really like it. I am torn on it and the Exos. One question I have, if I am using a water bladder, is there a hole in the pack fabric to run the hose to the exterior of the pack? Thanks for the time!

    • Andrew Skurka December 13, 2016 at 7:44 pm #

      Yes, there is a reservoir sleeve inside the main compartment and a shingled hole just above so that you can thread the hose through the pack body. The reservoir sleeve is full removable if you don’t want it; it weighs less than 1 oz.

      Your preference for a hydration reservoir may be completely valid, but I would add that I generally discourage this approach, in favor of conventional hard- or soft-sided bottles. Easier and faster to fill, and no effect on the volume or compression of the main compartment.

  8. Kate April 29, 2017 at 11:28 am #

    I am super excited to try this pack. Can you expand a little more on how you keep it affordable by pushing hard on the partner factory? I know SD has its American Rec Code of Conduct, but paying the factory less almost always means paying the workers less. I trust you; please reassure me anyway.

    • Andrew Skurka April 29, 2017 at 5:50 pm #

      Ultimately, SD wanted a fair price for the manufacturing. The original cost estimate was more expensive than it should have been relative to market norms, so SD haggled for a fairer price.

      The relationship between what SD pays and what workers receive is probably not as correlated as you seem to think. Wage negotiations between workers and factory owners happen separately. When the factory owners give SD a cost estimate, the owners account for the labor costs, which are fixed and known. If SD pays more than they should, it just means more profit for factory owners, not higher wages for workers.

      • Kate April 30, 2017 at 8:34 am #

        Sold! Thank you!

  9. Alex May 9, 2017 at 1:43 pm #


    For someone with a bigger build (powerlifter here) would you recommend going on the bigger shoulder strap size? I know you mentioned that they allow a wider chest fit.


    • Andrew Skurka May 9, 2017 at 4:48 pm #

      Watch this video to determine your size,

      If you are on the cusp between the S/M and M/L, I would recommend that you size up given your build. If you are firmly a S/M, give that size a shot, because the longer torso probably will not work for you.

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