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Flex Capacitor

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10 Best Lightweight Backpacks of 2018 - CleverHiker

Best Ultralight Backpacks of 2018 - GearJunkie

One of The Best Summer Packs of 2017 - Outside Online

Top Gear of the Year 2016 - GearJunkie

The Flex Capacitor features a unique patent pending gusset system that allows the pack to quickly and easily expand from 40-60 liters with the adjustment of a few straps. Instead of expanding up like other packs, the circumference of flex expands to provide a more stable and comfortable load carry.


Key Features

Packbag Key Features

  • Expandable 40-60L volume (volume varies between sizes)
  • Ultralight weight design
  • 2 torso sizes (non-adjustable)
  • 4 hip belt sizes
  • U-shaped top access zipper
  • Zippered stash pocket on lid
  • Removable reservoir sleeve
  • Shoulder strap water bottle pocket
  • Stretch mesh side water bottle pockets
  • Two hip belt pockets
  • Compression straps
  • Ice axe/trekking pole loop

Suspension Key Features

  • Lightweight "Y-FLEX" suspension
  • EVA foam hip belt and shoulder straps
  • Raised lumbar and scapula pads (EVA foam)
  • Load lifter straps
  • Hip belt stabilizer straps
  • Adjustable hip belt


  • Body Fabric: 100D Nylon-Poly Ripstop
  • Secondary Fabric: 420D Nylon Oxford
  • Stay Material: DAC Pressfit Aluminum
  • Number of Stays: 1 "Y-FLEX" stay

Tech Specs and Sizing

TORSO SIZE 16" - 19" 18" - 21"
VOLUME (Liters) 39 - 56L 42 - 60L
VOLUME (Cubic Inches) 2400 - 3400 2550 - 3650
WEIGHT 2 lbs. 9.2 oz. (With S/M hip belt) 2 lbs. 11.2 oz. (With M/L hip belt)
DIMENSIONS (LxWxD) 28.5" x 13 "x 10" 30" x 13" x 10"
MAX COMFORT WEIGHT 35-50 lbs. 35-50 lbs.

WEIGHT (Ounces) 10.2 11 11.5 12.2
WEIGHT (Grams) 290 310 330 350
HIP BELT FIT RANGE* (Inches) <29 29 - 32 32 - 35 >35
HIP BELT FIT RANGE* (Centimeters) 74 74 - 81 81 - 89 89

*This is a hip measurement, not a waist measurement.  Please watch pack fit video to learn how to properly measure.

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    • Sierra Designs Flex Capacitor 40-60 Pack
    • How to pack a backpack: Organization & load distribution
    • How to get measured and pick the perfect Sierra Designs Flex Capacitor backpack, with Andrew Skurka.

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    Product Reviews


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    1. Mixed Bag

      I have used the pack several times, on backpacks and day hikes. It is my intention to use the pack for the John Muir Trail this summer and want to make sure it will work well. I have used the REI Flash pack for the JMT and that was not a good pack at all.

      First off the positives. Well constructed, light weight, and carrys weight well. Large hip pockets are a big plus with good, snug side pockets for bottles, poles, chair, or whatever. You can access the side pockets while wearing the pack. Love the shoulder pocket and I use it to hold my iPhone for quick access. Hip belt stays snug (unlike the REI Flash BTW).

      Now the negatives. I have a SD lightning 2 tent which has built in stays on each end that are about 14 inches give or take. This means I cannot carry the Sierra Designs tent effectively in the Sierra Designs Bag. It cannot be placed horizontally. I need it to be horizontal to be able to fit my bear can. So this is presenting a problem, especially since (my second issue), there is no bottom straps to carry a pad or the tent. I REALLY could use the straps on the bottom for a rolled up pad or a tent. I now have to find someone to add the straps. Third, it a very little thing, but the chest strap has no built in whistle like every other pack.
      on Jun 14th 2017

    2. I have sold all my other packs

      Literally the most comfortable pack I have owned. I am long and skinny and this pack with the ability to change out the hipbelt has been amazing. I have taken it on day hikes and overnights in the CO mountains lugging a ton of camera gear and it has fit wonderfully. The large pockets on the hipbelt are great and the other features are simple yet highly useful. My one negative that I like on other packs that I have is a dedicate insertion point (sleeve) for my ice axe. I currently slip it between the seam when in 40L mode and just am concerned I will pierce the pack at some point, but overall happy as can be! One pack to rule them all!!!!! on May 18th 2017

    3. Excellent Pack

      This pack is great. The weight transfer to the hips is great. And it doesn't feel like it there's constant tension on your shoulders like a lot of packs. And for the weight it's actually pretty strong material. Very well designed. I actually prefer it over my ULA Circuit. on May 12th 2017

    4. Great pack strong and looks good.

      I used this pack on my trip to Havasu Falls in the Grand Canyon. It was a 10 mile hike down into the canyon on the Havasupai Reservation. I wanted to get a new pack this year and after doing a little research I came across this one. I like the ability to expand from a summer load of 40 liters to a winter load of 60 liters. Im somewhat new to light weight backpacking and wanted to upgrade my equipment and lighten my load. This pack is perfect for people like me that are not fully ready to go ultralight but still want to save weight. I like the single bucket design. Keep it simple. I didn't find attached my sandals and a solar panel to the outside of the pack to be much of a problem. I got mine before they change the sizing. on May 11th 2017

    5. Good pack with a serious issue.

      This is a good pack that can carry heavier loads. The overall weight on the pack is pretty low for it's capability. The ability to compress the pack smaller is very useful.

      The issue I have with this pack is that when the pack is completely full and the compression straps are completely loosened, there is no option to strap things to the outside of the pack. I bought this mostly for winter backpacking and there is no way to strap snowshoes unless I rig something up. There is also no way to strap a foam sleeping pad to the pack. For a pack that has a frame capable of carrying more than what will fit inside of a 60 liter bag, it should really have the ability to strap some extra gear to the outside.
      on Apr 28th 2017

    6. The search is over

      I'm so happy to have come across such an innovative design as the Flex Capacitor. It really does what it intends to do, which is to serve as a do-it-all backpack. I've created a video review here to capture more details: on Jan 12th 2017

    7. Really well designed backpack

      Totally impressed by this backpack. The FC is a great design for being in between lightweight and full backpacks. I have been using an Osprey Atoms 50 AG. The FC carried the weight just as good and I actually found I like the breathability of the FC better than the mesh that covers your back on the AG.

      With no bottom zipper I recommend watching how to pack video. This makes you think more about how to back and I not miss having the lower access. The big open top was great and liked the easy wide access. Plenty of areas to strap on gear on the outside.
      on Nov 27th 2016

    8. Great pack

      I just got back from a 4 night backpack in big bend with this pack. Pack worked great for me and my wife. Fit was great no hot spots from fitment. Loaded the pack sit well. The simplicity of the compartments is really nice. The rear storage compartments on the sides are great for bottles you can still reach them easily with pack on. Storage on the hip belt is great. Bag on the front strap is great place for my GPS. I was really surprised how breathable the back area was . Great pack overall. on Nov 26th 2016

    9. Lighten your load

      I purchased the Flex Capicitor in an effort to reduce my pack weight. I just finished a 10 day, 113 mile section hike on the AT and couldn't be happier with how this pack carried my gear. I loaded the pack with my tent (High Route), sleeping bag, 15 lbs of food, and a few other minimal items. The pack feels like it molds to your back. It has a padded lumbar pad (baby bump) that sits perfectly above your waist when you tighten the waist belt up. This keeps all the weight off your shoulders and makes the pack very comfortable to wear. The belt pockets are large enough to hold a small digital camera, iPhone 6 or a Ruger .380 pistol. The side pockets are plenty large enough to hold a 1 liter Smartwater bottle and a few snacks. As you eat your food and reduce needed pack volume just tighten up the compression straps and hike on. The shoulder strap has a nice water bottle holder that is very accessible. The top compartment is great for maps and smaller items.
      Overall, I was very satisfied with the weight, design, and simplicity of this pack. Can't wait for item 3 from Andrew Skurka.
      on Nov 25th 2016

    10. Great Start Out of the Gate!

      I purchased the Flex Capacitor during the pre-sale period and received it last week. Right out of the box I knew we had something good. As Andrew Skurka and others had mentioned in their blogs and responses, I found the pack to be exactly as described. Simple, capable, potentially tough. To be fair, I have not put it through "the paces" yet, but have hiked 8 miles with a 30-lb load on relatively flat woodland trail. My intent was to gauge the fit under the load and make adjustments in a benign environment. It appears to be well constructed and reinforced in the right places. I will write a follow-up review in a few weeks after further activity. Great job! ps. I am 6'3" 220lbs and was just inside the size large torso measurement for this pack. Make sure you measure before ordering! on Oct 11th 2016

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