Preview: Sierra Designs Flex Capacitors 25-40L, 40-60L and 60-75L

The original Sierra Designs Flex Capacitor backpack has performed and sold well since it was released three years ago. So Sierra Designs recently expanded the Flex Capacitor series with two new editions, while also updating the original.

I was involved with the design and launch of the original Flex Capacitor. But I haven’t been affiliated with the brand for nearly two years, and did not help with the development of these new and updated models. Last spring I visited the local Boulder office to get an early preview.

Preview: Flex Capacitor 25-40L, 40-60L, and 60-75L

The three-pack lineup now consists of:

  • New: Flex Capacitor 25-40L ($180, 2 lbs 7 oz/1.1 kg), best for thru-hiking and overnights/long weekends;
  • Updated: Flex Capacitor 40-60L ($200, 2 lbs 9 oz/1.16 kg), still the sweetspot of the line, and best for 5- to 10-day trips, bear canisters, and colder/stormier conditions; and,
  • New: Flex Capacitor 60-75L ($220, 2 lbs 10 oz/1.19 kg), best for winter backpacking and the family pack mule.

All three models are now available. It’s difficult to launch a summer product in October, but at least they’ll be available for holiday shoppers and thru-hikers this winter.

The new Flex Capacitor lineup consists of a new 60-75L (left), the original 40-60L (center), and 25-40L (right).
The basic aesthetic and core feature set of the original Flex was extended to the new models.

Sizing

The new Flex Capacitors will be available in just one or two sizes, with each size accommodating a 3-inch range of torsos. (To measure your torso, watch this video.) The limited sizing will compromise sales, but may have been necessary if sales forecasts did not exceed factory minimums. If the packs catch on, Sierra Designs can add sizes later.

  • 25-40L: S/M (16″ to 19″)
  • 40-60L: S/M (16″ to 19″) and M/L (18″ to 21″)
  • 60-75L: M/L (18″ to 21″)

Each model is available in two hipbelt sizes:

  • S/M (29″ to 32″)
  • M/L (32″ to 35″)

What’s the same, what’s new?

The basic aesthetic and core feature set of the original Flex Capacitor was extended to the new models, and is largely unchanged. Expect:

  • Gusseted front panel that can be collapsed or expanded to uniformly adjust the pack volume;
  • Durable all-nylon construction;
  • Multiple and functional exterior pockets;
  • Two horizontal compression straps; and,
  • Robust suspension system with a comfort capacity much greater than the average 2.5-pound backpack.

The updates to the Flex Capacitor are mostly small or behind the scenes:

  • Two color options: Birch, an off-white, and Peat, a dark gray;
  • A second shoulder strap pocket was added, bringing the total number of exterior pockets to seven.
  • The internal hydration sleeve is double-duty: it can be used as an exterior mesh “shove it” pocket, the lack of which was a common complaint (among non-owners, at least).
  • The pack bottom fabric is the same 420d nylon oxford, but the body fabric was switched to a silicone-coated 100d nylon-polyester rip-stop with better tear-resistance.
  • The anchor stitching for the volume adjustment straps was improved, to prevent tear-outs.
  • The hipbelt was redesigned for simpler manufacturing, but its performance should be about the same.
The chief complaint of the shoulder pocket on the original Flex was that there was only one. So a second has been added.
The internal hydration sleeve now doubles as an external mesh pocket. The functionality looks questionable, but it could be useful sometimes.

I was disappointed that the harness was not more fundamentally designed, so that it more consistently wraps around the body. Currently, there’s a “wedge” between the hipbelt and lumbar pad. The problem is not exclusive to the Flex Capacitor — my ULA Catalyst shares the same flaw. This redesign would have required more time and testing.

Note that the 25-40L and 40-60L use the same 8-mm aluminum Y-shaped stay, while the 60-75L version was spec’d with a stronger 9-mm version. Given the higher expected loads for this model, that seems appropriate, but I wonder if a tougher pack body fabric should have been used, too (e.g. 200d instead of 100d).

The Flex 60-75L uses a slightly thicker aluminum stay than the 25-40L and 40-60L.

Leave a comment

  • What questions do you have about the Flex Capacitors?
  • What do you think of the updates and the new models?

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32 Comments

  1. Steven on October 29, 2019 at 8:49 pm

    I like that both shoulders have a pocket now.

    I wonder if the 25-40 would be overlooked for the 40-60 since the listed difference is 2 oz. Since I already have the 40-60 and love how it carries, I just kind of wish that the 25-40 was lighter, but I could see how that might involve too much redesign if SD slimmed the belt, shoulders, and chose lighter fabric. I think parts/construction commonality allowed an attractive price for the series.

    • Andrew Skurka on October 30, 2019 at 9:10 am

      The suspension of the 25-40L is WAY overbuilt for what you could actually put in there. Consider that they’re using the same stay as in the 40-60L, and the same hipbelt, shoulder straps, and lumbar pads as they use in the 60-75L. But, as you said, it would have taken a lot of work to downsize everything, and then SD would have to charge you more for that work and for the increase in SKU’s.

  2. Joseph Seago on October 29, 2019 at 9:34 pm

    I like the concept of the pack. What I dont like was the lack of a external stuff it pocket, which seems to have been solved, and second, the zipper top lid. I dont see the benefit of having a zipper top lid. Maybe its because I am so use to the closure styles that most packs have. I do like that SD has added two additional sizes and a second color option. The pack is still on my radar and one day I may purchase one.

    • Andrew Skurka on October 30, 2019 at 9:07 am

      This concern has been expressed since the original Flex was released, but you only hear it from people who don’t actually own the pack. I have used extensively the Flex and roll-top packs, and I prefer the Flex overall.

      A roll-top adds compression into the closure system, and it creates a more water-tight seal. But compression can be provided in different ways (e.g. horizontal compression straps, which also have more general utility like for a shovel, rope, or snowshoes), and the water-tightness is only a valid benefit if the pack fabric is waterproof, which is usually not the case.

      Meanwhile, the zipper top lid gives you extremely easy access to the main body of the pack, and closes up much more quickly. It takes seconds. With a roll-top, it has many more parts: roll/unroll the closure, engage or release two or three side-release buckles, and tighten the straps.

      With a roll-top, I would say that a front mesh pouch is essential, because getting into the main body is a relative pain in the ass. With a zipper closure, you really don’t need it, and you won’t miss it.

      But then what will you do with all your wet stuff? The very easy solution is to put wet stuff inside the main compartment but outside the plastic pack liner. This way, your dry stuff will stay dry, and the volume of your wet stuff does not need to be less than the volume of your external mesh pocket (e.g. if you have a wet 2-person fly + body or a wet tarp + rain jacket + bear canister, your mesh pocket won’t fit all of that anyway).

  3. Brad R. on October 30, 2019 at 1:39 pm

    Instead of the second sewn-in shoulder strap pocket, I would have rather seen them have a modular design with the option of a waterproof pocket. I modified my Flex Capacitor to allow me use a shoulder pocket from Hyperlite Mountain Gear. Works way better for a phone than the mesh water bottle pocket.
    (I also posted this comment on Andrew’s Facebook page with a photo.)

  4. Alex Rodenberg on November 13, 2019 at 4:00 pm

    How well does the 60-75 compress? I could see myself going above 60l in winter times, but would be cool if it still could compress down to 40-50 liter without problems

    • Andrew Skurka on November 13, 2019 at 6:52 pm

      Using just the bellow, minimum volume is 60 liters. If you engage the compression straps as well, I think you could get it to 40L, though it will become misshapen the smaller you go, i.e. the bottom retains its 60L shape, but the mid-section is equivalent to 20L.

      • Alex Rodenberg on November 15, 2019 at 7:03 am

        Cool thanks for the info, the volume is the actual volume inside the pack ? Not with additional pockets etc like some manufacturers do.
        From reading the website it seems to be actual volume inside.
        If thats the case I might just settle for the 40-60L 🙂

        • Andrew Skurka on November 15, 2019 at 8:24 am

          I believe that the spec’d volume refers to the total pack volume, not just the main body. However, if you look at the pack, you’ll realize that about 90 percent of the volume is the main pack body.

  5. Johan on November 14, 2019 at 4:41 pm

    Would you recommend the new version over the old one? The stronger fabric of the new version is a strong selling point for me, but they have a great sale on the old one at around 100 USD.

    • Andrew Skurka on November 14, 2019 at 5:31 pm

      That’s a very attractive price.

      So long as you’re aware of the tear risk on the volume adjustment straps, and treat those carefully (i.e. don’t use them as compression straps, and don’t try to adjust the volume of the pack when it’s loaded), I think you’ll be fine.

      My demo gear library includes four original Flex packs, and they just finished their second season, maybe 50-75 days of careless use per pack. Some have been stressed as I’ve described, but the others look fine. These packs will go at least a third season for me.

      Bottom line: At that price, I think I’d pick up the original.

  6. jerry on November 29, 2019 at 7:36 pm

    what’s your opinion on frameless packs for a load out of less than 20lbs?

  7. David M. on December 7, 2019 at 8:15 am

    Hi, is the backpack strong enough to carry snowshoes attached to its compression straps without any significant impact on its durability? Have you tried it?
    I also wonder why are there only two compression straps, theres clearly enough space at the bottom part of the pack for a third one.

    • Andrew Skurka on December 7, 2019 at 7:38 pm

      So long as you put any sharp metal points towards the outside, it should be fine. I would be cautious with any kind of backpack if something sharp was going to be in direct contact with the pack fabric for extended periods.

      Two compression straps Will give you all the utility you need to carry snowshoes, trekking poles, even skis, although again I’d be careful with the latter because of the metal edges.

      • David M. on December 9, 2019 at 11:44 am

        Thanks!

  8. NC on January 31, 2020 at 5:58 am

    Hi, I used the original flex (revision model) on the pacific crest trail in 2019 and I loved the overall flexibility. Nice to see additional colors/sizes and 2nd shoulder pocket!

    My main complaints would be about the side pockets. I always had to keep a 2nd empty bottle in the pocket otherwise a lone bottle would just come flying out because it was too loose. At times, even with two bottles in placed they would eventually work their way up whenever i’m running down hill, requiring constant attention to push them back down every few minutes (wasn’t fun having to retrieve a bottle a few times on steep terrain). I wish there would be a way to tighten the side pockets similar to how the shoulder pockets are. In combination with a tightening method, I wish the side pockets were BOTH taller. I think bringing the height of the side pockets up to about 1 inch below the bottom compression strap buckles would make it more useful as a shove pocket when not storing bottles and help mitigate the issue of water bottles coming out (especially taller water bottles). I promise this won’t take away from the silhouette of the pack (lol I think I remember reading this somewhere as a rebuttal to the side pocket requests). Not as bad as loose excess straps anyway! (see below) 🙂

    A separate thing that isn’t related to functionality but more of an aesthetic complaint. I wish each of the straps had come with some type of excess strap management system (whether velcro or elastic loops). Whenever the pack is reduced to its smallest form, all the the hanging excess straps really take away from how “clean” the pack could look.

  9. Ron Jones on February 2, 2020 at 5:21 pm

    Hello, I recently purchased a new version of the Flex 40-60. Since it can’t be found in stores near where I live, I had to order two different sizes and return the one that was too big. Will be headed to the GC for short vacation to include a couple nights camping at the bottom. Using it as a shake down for the flex. It is 23” high so is technically one inch to long for Alaska Airlines carryon. I’ve had hit and miss with luggage a bit too big so like to be within spec. I figure if I remove the Y bar it will shrink the inch. I know it’s possible to swap belts which must require pulling the Y bar. But I can’t figure out how to remove it. Any help is appreciate.

    I’ll offer that I ordered the old gray model in m/l first then the newer version in white s/m. I like the subtle improvements SD has made. I have been torn between the Flex and HMG Southwest. I really like the simplicity of the HMG, but as someone who is working to lighten loads, l felt having suspension might be wise. I also like the accessible side pockets with pack on.

    Planning on the WRHR in late Aug. or possibly one of the sections. I want to say that I ordered your guide and it’s awesome!

    • Brad R on February 2, 2020 at 6:00 pm

      Loosen the Velcro on the webbing that wraps over the top of each upper section of the frame, then slide the webbing to the side (this might be somewhat difficult). The two frame sections can then be slid upwards, separating them from the round connector. The lower frame section can then be slid upwards out of the sleeve it fits into on the hipbelt. The hipbelt can then be removed after separating the Velcro.

      • Ron Jones on February 2, 2020 at 7:33 pm

        Thank you. I was exploring that option but it is sooooo tight. The Velcro tabs are partially sewn into place. I didn’t want to proceed and ruin the pack. I’ll give it another try. Thank you!

        • James Johnston on April 28, 2020 at 8:20 pm

          Yeah I had to replace the hipbelt as part of a warranty replacement. I did it the way Brad said. The key was realizing that the sewed velcro was only on one side. So you can partially undo the velcro and then slide out the pole.

        • Alan Laidler on June 6, 2020 at 5:06 am

          Do I really need another pack??. I currently use a montane grand tour 55 for summer and lighter winter camps and an osprey Aether 70l in winter if more space is needed. I take a tripod and camera and filters etc as I do a bit of landscape photography also sometimes. So would a flex capacitor replace both of these. I’m thinking the 40-60l but would I need that extra in the winter? I’m not a thru hiker. Normally just a night or a weekend in the UK or Scotland.
          I use hillieberg tents not ultra light tarps. So is it worth trading to get this and 40-60 or 60-70l..
          Any advice greatly received.

  10. Brad R on February 2, 2020 at 9:03 pm

    Yeah, the first time I took mine apart I was having similar thoughts.

  11. Murali Chinnakonda on February 17, 2020 at 6:32 pm

    Thanks for the info about better 100d body for better tear resistance. I wish manufacturers would provide such information when they provide updates to packs. I carried the original one on the JMT and loved it. The frame provides awesome load carry – like spinal cord of the human body. I see the details on the new packs on Sierra Designs website is pretty minimal – for a while, it didn’t have torso size info on the s/m etc and I think it is slowly getting updated….great pack though! I am going to get one more in the 40-60 which is I think ideal size for me.

  12. Amber B on September 9, 2020 at 5:05 pm

    I’m interested in the 60-75L pack for a winter expedition, but the sizing is not consistent with what I’ve been able to find on the SD website, given no torso size options as described for the 40-60 size. I’m an average size female at 5’5″, 120lbs, and a 17″ torso. The size information I can find is putting me in the M/L size hip belt (hips 33″), which seems like it might be large on me. Thoughts on sizing? Thanks so much for the input!

    • Andrew Skurka on September 9, 2020 at 8:05 pm

      The sizing options for very limited. Unless you have a particular size torso with a proportional waist, these packs are probably not going to fit you. You would be better off getting a pack that is more custom fitted, such as from ULA.

      • Mark on September 11, 2020 at 5:03 pm

        x2 on what Andrew said: My GF is 5ft 118lbs, 16-17in torso, and couldn’t stand how it fit on top of the waist belt sagging. She prefers the MassDrop collab Granite Gear Crown2 60L. I met another 5ft hiker on the PCT with the GG version, which is why I even entertained the idea. Both are/were using the regular torso versus the small, and never more than 28lbs max loaded with 3L water.

      • Steven R Johnson on September 17, 2020 at 2:47 pm

        I ordered a 40-60 M/L (with M/L hipbelt), and while the torso size was fine for me, the hipbelt was on the cusp of too small — snugged tight with a proper load, I had less than an inch of spare webbing on either side of the buckle, so if the padding packed in at all, it would be too small.

        No worries, I thought, I’ll just swap the hipbelt for the S/M hipbelt! So I called SD, only to be told they don’t do exchanges, period. The best I could do was return the entire pack and order a new one. Since I ordered the pack during their 30% off Labor Day promotion, this sucks. (Seriously, when you have a product that is unavailable in retail stores, you should be *more* generous about exchanges due to fit!)

        I ended up just returning it and not reordering a new one. If I see a similar discount on the pack again in the future, I might re-order with the smaller hipbelt, as there was a lot I liked about the pack otherwise. But the customer service (or lack thereof) left a bad taste in my mouth.

        • Andrew Skurka on September 18, 2020 at 6:16 pm

          I understand where SD is with this, but it’s really difficult to justify that system when a half-dozen other pack manufacturers offer fully customized sizing and will still get it to you in 3 days.

  13. Mark on September 11, 2020 at 5:10 pm

    I picked up another 40-60L to go with my 2017 model. After a few years, I was finding my waist belt was too large on my original, especially after a few days on the trail and losing a tiny bit of weight, so I picked up another for testing. I like the 2nd water bottle pocket, but that’s about it. I’ve put about 150 miles on it, and it just doesn’t seem to want to break in like my old one. The mesh appears to be flimsier, also, and I’ve torn it twice already on the side pockets. I used the water bladder as a rear pocket for testing in the Snowmass wilderness, but I could take it or leave it — I’d rather have a stretchy stuff pocket if I’m going to have something at all, but I’ve not required this feature due to the amount of straps on the Flex. I really wish there were 2 more eyelets on the lid so I could throw some bungie up top, but it hasn’t really been necessary. I’m currently looking into a Blaze60 or a ULA Catalyst for my gear hauling to weigh against my 2017 Flex and it’s new, for some reason, less comfortable hip-belt…

  14. Calvin Chow on September 25, 2020 at 2:36 pm

    According to Sierra designs website:

    – the 60-75’s dimensions (H x W x D): 28″ x 11″ x 12″ / 71 x 28 x 30.5 cm
    – the 40-60’s dimensions (H x W x D): 27.5″ x 11″ x 10″ / 70 x 28 x 25.4 cm (M/L torsop)

    So the 60-75 should be 1/2″ taller then the 40-60… but the 2x pictures with all 3x packs show the 60-75 as being much taller then the 40-60?

    With my very long torso, I find the original grey 40-60 much too short, where the load lifters does nothing to pull the pack closer against my back.

    I’m wondering if this 60-75 would better suite me? How much higher is the attachment points for the load lifters relative to the hip belt… betwwen the 60-75 and 40-60?

  15. Lowell Kleinman on September 27, 2020 at 5:28 pm

    I am hoping someone might be able to help me. I have purchased the M/L pack from a private party with the M/L belt and the belt is too tight. I tried to purchase an L/XL belt separately from Sierra Designs but it is not available.

    So, I was thinking of cutting the distal part of the belt where it is sewn to itself in order to create a loop, I think to make it easier to grasp the belt and cinch it tight. Undoing the sewing would extend the belt a few inches which are about what I will need. I don’t think doing this will impact the belt staying in place when cinched. If needed, I could buy webbing and have the belt extended further.

    1 – any concerns I am not seeing about my doing this?
    2 – Is the L/XL belt any different other than the size? for example, would the pockets be further forward on the L/XL for me?

    Aside from this personal issue, I have had many packs and this pack fits my needs and torso like no other pack I have ever owned. Also, reading so much of Andrew’s content causes me to really appreciate the thinking that went into his design.

    Thank you,

    Lowell

    • Lowell Kleinman on September 30, 2020 at 3:44 pm

      Answering my own post…

      I called SD and they offered to send me a belt extender, no charge. Also, they suggested I could also undo the sewing, fold over a very small section on itself, use a lighter to melt it in place, and this would prevent the belt from pulling through.

      LK

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