Upcoming changes to Flex Capacitor sizing & hipbelt

Sierra Designs will receive a new shipment of its Flex Capacitor 40-60 Pack at the end of February. The pack has been well received by customers and the media, and we’re optimistic about its future.

A few changes have been made to this next round of production. I’ve mentioned these changes in several places, but figured I would dedicate an entire post to it.

What’s not changing

The changes are best described as “tweaks.” We are happy with the product as-is, and we have not even begun to discuss a second-generation. (A much higher priority is a smaller version.)

So nearly everything will stay the same: colors, fabrics, body shape, pockets, stay shape and material, and of course the unique expanding gusset.

Torso sizing

The most noteworthy change is that we have consolidated sizes, to two from three. The new sizes will still fit most backpackers, but by eliminating the SKU count by one-third the production becomes more economical and we can keep its retail price at just $200.

Currently, the Flex Capacitor is available in Small, Medium, and Large. Each size fits a 3-inch range: 16-18 inches, 18-20 inches, and 20-22 inches, respectively.

The new sizes will be Small/Medium and Medium/Large. The range is still 3 inches, but they will now fit 17-19 inches and 19-21 inches, respectively.

My apologies to those with much shorter-than-average or longer-than-average torsos. If the Flex continues to be a success, maybe we will add back a third size, or maybe even a fourth. In the meantime, Sierra Designs still has some inventory of the S, M, and L sizes.

By consolidating sizes to S/M and M/L, we can still fit most backpackers while still hitting a $200 retail price.

By consolidating sizes to S/M and M/L, we can still fit most backpackers while still hitting a $200 retail price.

Hipbelt sizing

Included with the shipment of new packs will be extra hipbelts, so that we can properly fit customers with atypical proportions, e.g. someone with a very long torso but a skinny waist.

To this point, we have not been able to offer this customization. Now, additional hip belt sizes will be available on our website. We will have four:

  • XS/Small: 25-37 inches
  • Small/Medium: 28-40 inches
  • Medium/Large: 31-43 inches
  • Large/XL: 34-46 inches

Hipbelt design

The hipbelt will now consist of several bonded EVA foams (hard and soft), to better balance load-carrying performance and body comfort. The main piece has also been cored, so that there is a deeper “pocket” for the hip. The current hipbelt is made of single layer of foam. The overall thickness is about the same.

A schematic of the new hipbelt. It contains multiple layers of bonded foam, to better balance load-carrying and body comfort.

A schematic of the new hipbelt. It contains multiple layers of bonded foam, to better balance load-carrying and body comfort.

A strip of 420d Oxford was added to the bottom of the hipbelt for improved abrasion-resistance.

A more abrasion-resistant strip of 420d Oxford nylon was added to the buttom of the new hipbelt (top).

A more abrasion-resistant strip of 420d Oxford nylon was added to the buttom of the new hipbelt (top).

Finally, the hipbelt buckle has a two-point anchor, which should allow the user to get a better fit and better distribute pressure on their hips.

The new 2-point hipbelt (top), which will contour better to individual body shapes.

The new 2-point hipbelt (top), which will contour better to individual body shapes.

Want one of these new hipbelts?

If you’re happy with the current hipbelt, you should stick with it. The new hipbelt will not necessarily be an “improvement” for you.

But if the current hipbelt is missing the mark, check the SD website in March once the additional size are offered.

Have an additional question? Leave a comment.

Posted in on January 17, 2017


  1. Brad on January 17, 2017 at 9:26 am

    Thanks for the post Andrew. Now I’m anxious to hear about the smaller version you mentioned. Are you talking for ultralight backpacking or daypack?

    • Andrew Skurka on January 17, 2017 at 10:28 am

      It would be a backpacking pack, although it would be small enough when collapsed to be used for longer and more involved day-hikes (e.g. several layers of clothing, a full day of snacks, small med kit, etc.)

      Don’t hold your breath for it. Spring 2018 at the earliest. I’ll hopefully be testing a prototype this summer.

  2. Susan S on January 17, 2017 at 6:15 pm

    As someone with a 15.5″ torso length, my lack of options makes me sad. For the record, I bought the small iteration while it exists.

  3. John on January 23, 2017 at 9:03 pm

    Have been following this since day one and was very interested in the new spring offering but you just lost me and my two sons as we would all likely need the 21-22″ size with about 32″ waist.


    • Andrew Skurka on January 23, 2017 at 9:14 pm

      Bean poles.

      If they are 21″ torso the 32″ waist is no problem since the huobelts can be swapped out. But if they are 21+ inches on the torso then, yes, afraid we have lost you for now.

  4. Illimani94 on January 23, 2017 at 10:08 pm

    My torso length is gonna fall right on the dividing line between S/M and M/L. Got advice for someone on the dividing line? Go small or go large?

    • Andrew Skurka on January 24, 2017 at 5:13 pm

      How small or big is the rest of you? If you are bigger than average, the hipbelt and shoulder straps of the Large size will fit you better. And vice versa.

      Packs are a little bit like shoes. If you’re unfamiliar with the brand or model, it’s best to get into a store and try on multiple pairs. I don’t expect the distribution of the Flex to be massively widespread, so you’ll probably be limited to online shopping. In that case, order the size that you think will fit you best, and exchange it if you’re not satisfied. Or, order both and return one. Not convenient, but I prefer it over having to get in my car and drive to the store while a nice sales person hovers over me as I try on a half-dozen pairs of running shorts.

  5. Ray on February 28, 2017 at 11:52 pm

    I received my FC-Medium today. My torso is 18″. I am new to backpacking and curious if perhaps it should sit a little higher on my hips. Tired a small Baltoro in the store and was not in love with the fit. Tired my friends FC-Medium and it was much better fit. I am 5’11”, have a low body fat % fit build with broad shoulders and 34″ waist. Can you explain the upcoming 2-point hip belt? What body types will it help with?

    • Andrew Skurka on March 1, 2017 at 9:09 am

      Without seeing in-person, or at least a photo of it, it is difficult for me to say if it’s riding too low. Send a photo via email or post a link here. Make sure that the pack is weighted with about 20 pounds so that it will actually sit properly.

      In general, the hipbelt should center over the illiac crest, which is the pointiest part of your hip, aka “love handles.” Situate the hipbelt there, and then tighten the shoulder straps until it feels secured.

      The 2-point hipbelt will help more with curvy body types, because you can nuance the tension on the top and bottom of the hipbelt. For a more ruler-shaped body like yours, it won’t make much of a difference.

  6. Keaton D on March 1, 2017 at 10:20 pm

    Hey Andrew, I have been trying to find information about whether or not the new sizes have arrived for the Flex. I have been waiting for them because I am planning on picking one up, but I am not sure what to look for at the moment. Any information will be appreciated.

    • Andrew Skurka on March 2, 2017 at 9:06 am

      They will be in stock shortly.

      When they become available, you will notice that the sizing on the website has changed from the current Small, Medium, Large, to Small/Medium and Medium/Large.

  7. Chris S on March 2, 2017 at 9:18 am

    I’d love for Sierra Designs to ship to Canada.

    Please, please, please drop the hint next time you are in SD HQ!

    • Stephen B on March 4, 2017 at 8:48 pm

      We do ship to Canada!
      Many of our products are available through MEC, and they can certainly special order anything that they do not regularly carry in their inventory.
      Thanks for being a fan and mega thanks to Andrew for all his does for Sierra Designs!
      Keep smiling,
      Chief Bottle Washer @ SD!!

      • Grayson on March 17, 2017 at 4:54 pm

        Hi Stephen B., I live in British Columbia and I spoke with a MEC rep yesterday who stated that there is no way they could special order a flex capacitor pack from SD. Do you of any other way I could get one if these packs (other than driving to the states, buying one, and bringing it back)?


  8. Cory on September 3, 2017 at 9:22 am

    I just received a M/L FC and so far I like every thing about it, except for one. The shoulder straps are not adjustable, and I have a small gap between the top of my should blade and the shoulder strap (20 inch torso). Are there any plans to make the shoulder straps adjustable (i.e. move them up or down on the frame)? Or is the small gap not an issue?


    • Andrew Skurka on September 6, 2017 at 2:01 pm

      There are no plans to make a model with adjustable torso length at this time. That feature alone adds a half-pound and $50.

      The gap is not ideal, but I’d be more worried if you said the pack is not comfortable. Load up the pack and take it for a hike. How does it feel? If it’s comfortable, the gap is not a problem. Also, the gap might go away if you load some weight into the pack (so it sits more on your body) and if you adjust the shoulder straps and load-lifter straps.

  9. Noel on September 21, 2017 at 1:28 pm

    Is there any plan to add bottom straps for carrying a foam pad etc? Also, I’d second the request to be able to get them in Canada, through MEC or otherwise. Thanks

    • Andrew Skurka on September 21, 2017 at 10:35 pm

      We will look at that. Seems like an easy (and inexpensive) add-on, and I agree that it would add some utility options.

  10. Randy on March 14, 2018 at 1:30 pm

    I’m curious if X-Pac materials were ever considered for this pack, or for future revisions. From some feedback I’ve heard with other packs it seems like it’s tough stuff and holds up long-term, unlike many other ultralight materials.

    • Andrew Skurka on March 14, 2018 at 1:35 pm

      We looked at it, but the $200 price point would not have been viable if we’d used it.

      • Randy on March 14, 2018 at 3:04 pm

        Ah, I see. Any idea if a new revision of the Flex is on the horizon? That’s currently my 1st choice for a new pack but I won’t be pulling the trigger until next fall/winter.

        • Andrew Skurka on March 14, 2018 at 4:27 pm

          There’s discussion of updates for Spring 2019. I haven’t seen any drawings or prototypes yet.

          • Randy on March 14, 2018 at 6:25 pm

            Thanks. And I really appreciate your contribution to the backpacking world.

          • Randy on April 5, 2018 at 3:35 pm

            And, by the way, gray looks great but coyote tan would be just swell.

  11. Joanne on May 11, 2018 at 4:54 pm

    Almost all the online reviews for the FC have male names attached. Any comments on how well the FC carries weight and fits on the average female frame, wider hips/smaller waist, with torso length of 16″ – 18″?

    • Andrew Skurka on May 11, 2018 at 8:36 pm

      I haven’t heard anything specific. Maybe customer service has.

      In general, I think the closer you are to typical male proportions (i.e. some curve, but not Kim Kardashian level), the better it should fit. Women packs assume greater difference between hip and waist, narrower shoulders, and sometimes a better accommodation of breasts.

  12. Ryan on September 21, 2018 at 7:37 pm


    I’m a big guy backpacker (6′-2″ / 280lbs) and I’m working on losing weight. I’m really interested in this pack as an upgrade to my Osprey Volt 60. Always looking to lighten the load and the Flex Capacitor would certainly shave a couple of pounds easy. I take several weekend trips per year and would like to get out there more…going lighter has certainly made trips way more enjoyable.

    Without a local retailer to see how these fit…I’m left to order one to see how it fits…but I’m curious to hear if anyone (during product testing / someone on here / or you yourself) have seen a person of my girth using this pack?

    Enjoyed reading through your honest take on a product you associated your name with…it seems like an innovative and well thought out pack!

    • Andrew Skurka on September 23, 2018 at 6:45 pm

      Uncertain how the Flex will fit a guy of your size, sorry. Maybe SD customer service would have some more info.

Leave a Comment