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Flex Capacitor

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2017, 2018, and 2019 Budget Pick -

10 Best Lightweight Backpacks of 2018 - CleverHiker

Best Ultralight Backpacks of 2018 - GearJunkie

One of The Best Summer Packs of 2017 - Outside Online

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.

config_file_url: /content/webrotate360/360_assets/flexCapacitor/flexCapacitor.xml
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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. An excellent pack, but the bottom material is not hardy enough.

      I used the pack, for the first time, on a eight day cross-country backpacking trip in the Sierra. I have nothing praise for the overall design. My one dislike is the insufficient material at the bottom of the pack. The granite cut numerous holes in the material. I don't think the cloth is strong enough for cross-country hiking. I understand there is a trade off between weight and durability, but the lower section of the pack is durable enough. on Sep 18th 2017

    2. Great pack

      I took this pack out about a month ago for a three day, forty-ish mile trip in Desolation Wilderness. Before I bought this pack, I would have used my Gregory Z55, but I wanted to try something lighter, and so I ended up with this. I have no regrets. I really like having one big internal compartment rather than several-- makes it easier to stay organized (I always forgot which compartment contained what), and it allows you to weatherproof your load from inside the pack (as the guy recommends in the video). The external pockets were all the right size and easy to access, the load carry was super stable, and back ventilation left nothing to be desired. I really like this pack, and I plan to use it for longer trips. on Sep 8th 2017

    3. Great pack, carried on the GR20 in Corsica

      I just returned from a 13 days trekking on what's called Europe's most difficult long distance trail. I've been so happy with this pack in the mountains. I did not fill it complete, so average weight carried was up to 12kg in the morning with 2L waterbladder filled. What is surprising is that with that load I do not feel the weight on my hips or shoulders. Thanks to the lumbar pad doing a lot of the carrying work ! but it is also not felt. My experience is that the load is spread over 3 zones instead of 2 as other packs do . (I was in Spanish Pyrenees last year with a high quality Lowe Alpine backpack and my hips hurt like hell and felt numb for weeks after the trip.) One of the 2 red straps at the bottom i used for holding the bottom of the trekking poles. Other people's remarks about missing straps to attach stuff was not an issue to me : I used the "expansion" straps for that and for sock and clothes drying :)
      on Aug 15th 2017

    4. Awesome pack

      Just bought one of these great packs to start backpacking again. It has been 33 years since I last backpacked, too busy with many other outdoor activities (day hiking, trail running, mountain biking, backcountry skiing, snowshoeing, kayaking) and work has kept me from backpacking adventures. We just had out first trip and I could not be more pleased with this pack. Carried very well, super easy to pack and organize and was the perfect size. Thanks to SD, I am again super excited about backpacking again. I am now excited to buy one of the SD backcountry quilts for a sleep system. on Aug 12th 2017

    5. 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

    6. 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

    7. 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

    8. 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

    9. 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

    10. 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