Teaser: Sierra Designs Flex Capacitor 40-60 Pack

The Sierra Designs Flex Capacitor Pack atop Norris Mountain in Glacier National Park.

The Sierra Designs Flex Capacitor Pack atop Norris Mountain in Glacier National Park.

In news coverage this week from the Outdoor Retailer tradeshow, you may hear about two Sierra Designs products with which I’ve been deeply involved: the High Route Tent 1FL and the Flex Capacitor 40-60 Pack.

The High Route Tent 1FL (HR1) was my brainchild and is nearly old news if you regularly read this website. A 110-unit early production run is available now for pre-order directly from Sierra Designs, and will ship early this month. Next spring you’ll see wider retail distribution.

The Flex Capacitor 40-60 Pack was already in the works when I joined SD nearly two years ago. The original design by Casey, Chase, and former SD brand manager Michael Glavin had three things going for it:

  • Easily accessible interior using a zippered top lid;
  • Simple, lightweight, and stiff Y-shaped suspension made of tubular aluminum; and,
  • Adjustable volume using a unique front-side gusset.

My involvement was in perfecting these features (e.g. What zipper should be used on the top lid?) and in designing and making decisions about pockets, fabrics, padding, compression, fit and other details. I also extensively field-tested the prototypes, mostly in Colorado and Utah. Most notably, I used it to pack out an elk last October, making two trips with 69 and 60 pounds; my partner carried the rest.


A small production run (195 units, I’m told) will be available in early-October, directly from Sierra Designs. Bulk production will arrive in Spring 2017, and hopefully a retailer near you will carry it.

Product specs

  • $200 MSRP
  • 2 lbs 8 oz (size S/M)
  • Volume: 40 to 60 liters, using the adjustable gusset
  • Y-staked tubular aluminum suspension, anchored directly into the hipbelt
  • Generously sized pockets:
    • Two side pockets
    • Two hipbelt pockets
    • One shoulder strap pocket
    • One top lid pocket
  • Durable fabrics throughout:
    • Body fabric: 100d ripstop nylon with UHMW reinforcement, aka Dyneema
    • Bottom fabric: 420d nylon oxford
    • Side pockets: partial use of heavy-duty stretch mesh
  • EVA foam lumbar and scapula pods
  • Two removable compression straps
  • Removable hydration sleeve
  • Two ice axe loops


The small production run available in October 2017 will come in three sizes: S, M, and L.

Sizing for Spring 2017 bulk production will be consolidated: small/medium (17”-19”) and medium/large (19”-21”). I recognize that this is a limited size assortment, but I believe that SD wanted to validate the pack first before expanding the SKU count into lower volume sizes.

The hipbelt is interchangeable and, again, there are two sizes: 30″-44″ and 32″-46″.

Optimal uses

For most backpackers, the Flex Capacitor will be a one-pack quiver. Its load-carrying capacity and 60L maximum volume make it suitable for week-long 3-season trips with a bear canister, like a section-hike of the JMT or Kings Canyon High Basin Route.

But at just 2.5 pounds and with a minimum volume of 40L, it is entirely appropriate for a long weekend as well, like the 28-mile Aspen Four Pass Loop.

Regardless of trip length and base weight, I think all users will appreciate its feature set, durable construction, and $200 price.

Naturally, the FC has limitations. For overnights and gram weenies, it’s overkill. And for multi-week expeditions and dedicated meat-hauling, it’s small and skimpy.

The Flex Capacitor should perform well in hot and humid environments. Airflow through the back panel is on par with the “trampoline” packs that normally weigh 5+ pounds. But pick your tradeoffs: because it sits off the back some, it does not “hug” the user like, say, a well designed frameless pack.

Full Y-FLEX suspension, which anchors directly into the hipbelt, and the EVA foam lumbar and scapula pods.

Full Y-FLEX suspension, which anchors directly into the hipbelt, and the EVA foam lumbar and scapula pods.

Volume adjustment

A unique and beneficial feature of the FC is its adjustable front-side gusset that allows the pack volume to be expanded or collapsed, between 40 and 60 liters. This greatly increases the pack’s versatility, almost a two-packs-in-one proposition. Some photos:

When collapsed, the gusset folds inside the pack. It does not fold perfectly cleanly because there are no seams in the gusset, but I've never been bothered by it.

When collapsed, the gusset folds inside the pack. It does not fold perfectly cleanly because there are no seams in the gusset, but I’ve never been bothered by it.

Disclosure. This post contains affiliate links, which help to support this website.

Posted in on August 3, 2016


  1. PStuart on August 3, 2016 at 8:56 am

    Looks like a great product. I’m accompanying my son’s scout group to Philmont next year, and this looks like it could be the ticket, if there are tie-on options, as the Philmont-issued gear seems to be on the bulky/heavy end.

    Just not sure if I can wait until then with all the practice hikes and trips coming up in the fall.

    • Andrew Skurka on August 3, 2016 at 2:02 pm

      I have good news for you: we’ll have 195 units available in early-October of this year. Can you wait that long? Like the High Route Tent, they will be available for pre-ordering a few weeks beforehand.

      • PStuart on August 3, 2016 at 2:38 pm

        Excellent! I think I could last that long. I was on the notification list for the HR1, so hopefully I’ll see the notification for the Flex Capacitor, too.

  2. Terrance Glover on August 3, 2016 at 9:47 am

    Like PStuart, I was thinking this would be a good Philmont pack for next summer. The ability to expand capacity for resupply days or hikes into dry camps would come in handy. The timing will be tricky though.
    If there is any chance for an early batch (like with the HR1), I’d be onboard for 1.

  3. J on August 4, 2016 at 12:01 am

    Looks a good pack fo u know if there will be any UK/EU distributors in the spring?

    • Andrew Skurka on August 4, 2016 at 11:43 am

      We obviously hope so. We are showing it at the trade show this week and will know more certainly in the next few months when stores and distributors place orders.

  4. Bill Olsen on August 4, 2016 at 10:01 am

    Andrew, I have been using a North Face backpack for the past 30 years and I’m finally ready to upgrade. This new backpacklooks like it will do the job nicely. In the initial run ready in October will the Medium/Large size be available?

    • Andrew Skurka on August 4, 2016 at 11:41 am

      Yes. Both sizes will be available.

  5. Rex on August 4, 2016 at 11:56 am

    Excited to see one of these in person (I know it will be a while). Recently got a great deal on a pack with a trampoline suspension. It’s quite comfortable, but over twice as heavy and a little washy above 50lbs (I never pack that heavy for trips, but I do for training).

    I’d be interested in seeing some more detail on how the gusset system works.

    • Andrew Skurka on August 4, 2016 at 1:30 pm

      Rex – Yes, the trampoline system adds a lot of weight. Comfy but heavy. I just added some additional photos of the gusset system. Basically, you tighten four webbing straps to close the gusset, or loosen them to open the gusset. The gusset folds up inside the pack. In the photos I removed the two compression straps so that you can better see just the gusset system.

  6. Shawn Donnelly on August 4, 2016 at 2:47 pm


    This looks like a great pack for my needs. I currently use an old REI Flash 65 (mustard yellow color version) which was like 2.8 lbs new; heavier now with dirt and repair stitching. Your pack seems to improve on the things I don’t like about my Flash like more robust belt pockets and easier expansion for loading. I really like zip top pocket. One of my complaints with the roll top packs is the elimination of the top pocket. It looks like your combines both features nicely. I will definitely purchase one.

  7. Sam A. on August 4, 2016 at 2:52 pm

    I’m very excited for this pack – I’m looking to upgrade my old hiking pack with something newer, and I think this will be it. I also like the gray color with red accents, but will there be other color options once the pack is released?

    • Andrew Skurka on August 4, 2016 at 3:06 pm

      Definitely not for the first release in October, and probably not for the bulk production in spring.

      The color is safe but not exciting. That is recognized internally and I think it will be the subject of conversation before the sign-off on the spring production.

      • Sam A. on August 13, 2016 at 4:51 pm

        Hi Andrew, another question regarding the pack: Will it have a built-in whistle into the chest strap buckle? I can’t quite tell in the photos.

        Congratulations on your 1st place finish in your recent 100-miler!

        • Andrew Skurka on August 13, 2016 at 10:43 pm

          I can’t recall, but I don’t believe so.

  8. Tom on August 4, 2016 at 4:16 pm

    This pack looks interesting! I am gradually entering the market for a new pack and have been settling pretty comfortably on the ULA Circuit. I know you’ve spoken highly about that pack in the past. How does this compare to it? Any notable improvements or differences here? I’m graduating from 3-season walk-in camping to more backpacking and thru-hiking– 3-season, usually not longer than a week, most often 1-3 day long-weekend-type trips. Whatdya say?

    • Andrew Skurka on August 4, 2016 at 4:41 pm

      The FC is like a combined Catalyst/Circuit due to its volume adjustment, or maybe a Circuit/Ohm depending on how you count up the volume. Specifically, I try not to load up the extension collar or the front mesh pocket on the ULA packs, so in effect they are smaller for me than the spec would suggest.

      In terms of features, durability, price, and weight, they are very comparable — a little bit better here, a little better over there. The Circuit is lighter after you remove stuff, $35 more but sewn in USA, has a front mesh pocket but no shoulder strap pocket, etc.

      The suspension and back panel are probably most different. The FC has stiffer suspension that is anchored directly into the hip belt, so better load transfer than the ULA packs. And foam pods that create some airflow between pack and back. ULA has flat mesh panel that hugs better, but is sweatier.

  9. Nate on August 4, 2016 at 7:40 pm

    Hi Andrew,
    Looks good man. I have a ULA Circuit and don’t like spending money unless I need to so I likely will not be purchasing this pack for sometime, but we’ll see. Anyways a question came to mind related to bear canisters. Does the BV500 fit vertically or is there room to fit horizontally?

    Especially like the feature to “shrink” the pack size. Thank you

    • Andrew Skurka on August 4, 2016 at 8:05 pm

      BV500 can fit horizontally, but barely, and I would recommend vertical so that you don’t compromise the usefulness of the pockets or have a funny-looking pack.

  10. Andrew on August 5, 2016 at 5:05 pm

    Hey Andrew,

    I’m really interested in this pack but I have one question. Do you think that this will be able to carry a pair of backcountry skis? Would the compression straps be strong enough and is the fabric durable enough to handle possible abrasion from the metal edges? Thanks!

    • Andrew Skurka on August 7, 2016 at 8:31 am

      I would not regularly carry skis without reinforcing the area with a heavier nylon. I have carried metal edge skis on 210d Dyneema packs, and it did okay except when there was a hard object, eg pot, immediately inside.

      The compression straps and buckles are plenty strong. We did not go stupid light on them. However, you may find that the skis migrate (“may”) because of where the housing is. It is not a problem with trekking poles or ice axes, but skis are considerably heavier and longer (so more torque).

  11. Vince on August 9, 2016 at 10:33 am


    Can you tell me what the load limit is on this pack?

    • Andrew Skurka on August 9, 2016 at 11:43 am

      Define “load limit.”

      As a maximum, I carried 69 pounds of gear and meat for 2+ hours. It did well enough that I used for a second trip with 60 pounds, despite having a dedicated meat-hauling pack in my car made by Kifaru. Would I recommend this much weight? Absolutely not — it’s a crippling chore, and absolutely no fun.

      Assuming that you’ve put some thought into it, it’s hard to imagine a 3-season load weighing more than 50 pounds, including 7-10 days of food, a bear canister, and a packraft. The FC will be fine with this type of load. But regardless of the pack, you’ll be happier if you keep it lighter.

  12. Dan on August 26, 2016 at 9:20 am

    Intriguing, would like to check it out in-person. Will this be available in stores (ie; Boulder/Denver area) or online only?

    • Andrew Skurka on August 26, 2016 at 9:24 am

      The limited run in October will be available online only from Sierra Designs.

      In Spring 2017 it should have wider retail distribution. But the exact retailers that have it will not be known for another few months.

  13. Henry on September 21, 2016 at 11:54 am

    You mentioned three sizes in the initial run, what are the torso measurements for them and also, what is the accompanying hip belt size? I am a tall guy and your M/L will probably not work for me.

    • Andrew Skurka on September 23, 2016 at 7:51 pm

      The sizing specs are on the Flex Capacitor product page.

      The hipbelt is interchangable. If you need a different size than the default option listed, contact Customer Service. I believe they can ship your pack with the right hipbelt, to avoid mail-tag later.

  14. Allison Wagoner on October 6, 2016 at 8:10 pm

    My husband got me this backpack for my birthday! Love the features, color, pockets.
    Now I need a hiking trip to take it on……

    • Andrew Skurka on October 6, 2016 at 10:10 pm

      Awesome! Please report back after your trip, or post a review on the SD website.

    • Linda Bootz Hubbard Lalande on June 30, 2017 at 9:55 pm

      You are missing an audience here. I’ve scoured the web looking for info on whether this pack is good for WOMEN – remember US? No mention anywhere about whether gender would make a difference in the fit of the pack. I finally see an Allison Wagoner saying she loves it… FINALLY it does look like it is for either men or women. Don’t make us work so hard! Since SD only seems to make ONE pack I think it’s probably a pretty good bet its a top quality one.

      • Allison Wagoner on July 1, 2017 at 12:27 pm

        Hi, i just left some feedback on my pack! Have used twice now on backpacking trip. Good luck!
        It is under my previous comment.

      • Andrew Skurka on July 4, 2017 at 7:36 am

        You’re right, we’ve made you work too hard. The reality is that it was designed as a unisex pack, but nearly all of our testing was done by males. That’s simply a function of who was involved with the project and who we could get involved from other brands in the office (difficult to do). The pack should fit women IF they have torso lengths between 16 and 21 inches, and hips between about 26 and 38. Refer to the product page for measurement instructions. Curvy women may struggle to get a good fit, but that will be the case with most other packs as well.

    • Allison Wagoner on July 1, 2017 at 12:24 pm

      Hi Andrew, i wanted to get back with you about my new pack! My husband and I have done 2 backpacking trips since I got the pack! I do love it but would like to share a couple of possible improvements… i think each strap should have the mesh water bottle holder. I like to have a water bottle and I carry my bear spray on my strap also. Last trip, i ran the bear spray through the little red elastic strip on the left shoulder strap but with that mesh holder not weighing much….i think both straps should have the holder. Second, the top zipper pocket…i am not a fan of the pleated top. I am unable to get stuff in there. Like only a couple of things fit in there once i pack my bag and zip the main pack compartment. Maybe a half moon zipper compartment. I like to unzip and see what is in my pack easily and with this zip pocket it is small and not easy to put the items i like to have in that pocket readily available. I LOVE the back and lumbar padding! I have had no back pain with this pack when i have used. Last trip in May, my pack was about 21 lbs. In March, maybe a couple ponds more.
      Oh, i hiked in a tank top in May…pretty warm. Anyway, the straps rubbed on my shoulders, so a touch more cushion at the straps if possible. Women yes like to be comfortable. Overall, love the pack! Can’t wait to use more but just wanted to give fedback as I said I would once had a chance to use!! Thanks so much!

      • Andrew Skurka on July 4, 2017 at 7:31 am

        Everything noted, thanks for the feedback, and will send it along.

        1. Re the shoulder strap pocket, I would have thought everyone would be gracious for one, since it’s a very rare feature, but you’re not the first person to ask for a second.

        2. What are you trying to keep in the lid? Its size is intentional. It’s big enough for toiletries and a few personal items that I like to keep within reach (e.g. TP & hand sani, light, pocket knife); the hipbelt pockets hold a few of these items too, e.g. camera, lip balm, sunscreen. But anything bigger and we’d have to rebuild the lid (for additional weight and cost).

        3. Shoulder straps and tank tops are a tough combination. But we’ll look into the fabric on the next round.

  15. CJOttawa on October 16, 2016 at 11:45 am

    Hey Andrew!

    I read the sizing info on the SD site and would order the Flex Capacitor in a heartbeat but I’m on the border of two sizes. (20-20.5-inch torso)

    Osprey pack sizes have a one inch overlap (M 18-21, L 20-23) and, after extensive fitting, I am more comfortable with the smaller. I’ve heard this is a “general rule” with pack fitting: go with the smaller of compatible sizes.

    Any suggestions on the Flex Capacitor? If necessary, I’ll wait until I can try one in person.

    • Andrew Skurka on October 17, 2016 at 12:21 pm

      I see your email contains “Ottawa,” and so I suspect you may be from Canada. If so, I understand why you want to get it right, to avoid shipping things back across an international boundary. But if you are in the US, it might be best to simply order both and keep the one that fits best. You can also order one at at a time and hope you get it right the first time, but you won’t have any way to compare the sizes.

      Assuming you are normally sized, it’s probably best to go smaller than bigger. If the pack is too big (too tall, really), it will ride too low on your hips, or not rest on your shoulders. Whereas if it’s too small, it just rides a little higher on your waist, and/or puts a little bit more weight on your shoulders.

      The shoulder strap length and hipbelt are proportional to the torso sizes. So if you are a girthy person you might size up if you are on the cusp. Whereas if you are narrower all around, you probably are better sizing down.

      • CJOttawa on October 17, 2016 at 12:50 pm

        Thanks for the useful comment Andrew. I suspect I’ll end up with the smaller size, with a larger hip-belt.

        You called it: I *am* in Canada and international shipping is a factor. I’ll wait for domestic availability (or a road-trip to the USA).

        The Flex Capacitor looks like it would fit my needs flawlessly.

        Data-points for you:

        I’m a certified hike leader often acting as gear mule with an Osprey Xenith 75 (BPW still sub-19lbs despite heavy first aid/foot care kit etc).

        The Flex Capacitor would be half the weight of the Xenith 75 and offer much more capability and flexibility than Osprey Exos 58, ULA Circuit, and HMG Windrider series that are currently on my radar.

        Looking forward to seeing FC in person. 🙂

  16. Gregory on October 29, 2017 at 4:02 am


    I was wondering, how easy would it be to add a pocket to the bottom of the bag just for my sleeping bag. I don’t like to dig everything out of my backpack to get to my sleeping bag. Seems to me that 1 u-shaped zipper and mesh divider would do the trick. I have been comparing the Flex Capacitor and the Osprey kestrel 48. I am a Military man and have two small boys that i take on backpacking trips twice a year, and I have to carry more weight to allow them to have a easier time on the trails. I also travel a good amount, and the 40L backpack would be perfect as a carry on bag. I think that this pack would just about be perfect for each situation that I would find myself in, except, the sleeping bag compartment. I need one. Do you think that Sierra Designs would make me one? If not I’m thinking about buying one and having it altered.


    • Andrew Skurka on October 30, 2017 at 7:52 am

      The upper volume limit of this pack might be stretched if you need it to carry your stuff plus additional stuff for two small boys. For context, I have tapped out this pack on 9-day solo summer trips in the Colorado Rockies (with canister and ice axe), 5-day guided trips in the High Sierra (with canister, some group supplies like first aid, and a considerable portion of group food), and 5-day hunting trips in Colorado in the fall (with a warmer kit and a bunch of hunting items). When I have gone backpacking with my wife, I carry a different pack, because the Flex will not fit my stuff in addition to her bag and pad, and a bigger stove, real tent, etc.

      The Kestrel 48 definitely won’t make the cut.

      As far as the sleeping bag compartment, I think you’re making life more complicated than it needs to be. You could probably install one, but I don’t see the value-added. When you pull into camp, you need to take out your shelter, pad, stove system, food bag, and warm clothing. At that point, there is not much left in your pack, besides your sleeping bag.

  17. Fred Bar on March 28, 2018 at 9:32 am

    This seems to be targeted for thru-hikers with rather lighter-than-usual base weights who need to re-adjust their pack size as their food supply dwindles down and then as they re-supply. The interesting part is that instead of adding extra fabric vertically at the top for expansion, as seen on some bags with minimal weight penalty, it is added horizontally for better distribution and to prevent the “Weebles wobble, but they don’t fall down” tipsy tower-effect. On the section-hiker side, it is also probably a great choice for someone who is forced to carry a lot of stuff, such as a parent carrying everything for their smaller kids, a strong hiker carrying the weight of a weaker hiker or when food re-supplies will not be available for 1-2 weeks for the solo hiker. The fabric specs seem to be of the rather durable kind. Would be interesting to know how it performs on a thru-hike.

    BTW I’ve been reading on the new Osprey Levity/Lumina offerings, and they seem to be good only up to a max of 20-25 lbs, depending on the reviewer, so it may be a good fit for some SUL AT Hikers, but not for those seeking a bit more comfort (as in weight addition).

  18. Gary Peterson on July 10, 2020 at 6:58 pm

    I’m hoping to one day hike the PCT and was wondering if the 60-75 liter version of this pack would be suitable. I’m a bigger guy, though I have it as a goal to loose a bunch of weight before I think about trying the hike, and so my cloths and sleeping bag would be bigger by default as well. You are an inspiration on going lightweight, I just don’t know if that is an option at my size. Of coarse I am also waiting till the whole Corvid 13 thing runs it’s course and we hopefully get a vaccine before I will try backpacking as well so that just gives me more time to loose weight. I also need to convince myself that I won’t need to take spare copies of items with me as I currently have a bit of the two is one and one is none mentalitym which is counterproductive to lightweight hiking (at least I assume so).

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