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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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9 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.

  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.

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