Ultralight Pedestrian

Human powered minimalistic approach to life, love and the outdoors by foot, bike and paddle


In the Glen Nevis on the TGOC in Scotland


Ever since I got interested in lightweight and ultralight hiking the name Andrew Skurka has been one of the most respected long-distance hiking professionals in the world. His adventures and exploits are well documented by himself and others on blogs and in books. So, when he announced he was putting his name on a pack from a little-known company called Sierra Designs. I will admit to being very skeptical in the beginning, suspecting the usual ‘I’ll say anything for an endorsement fee’ type of gear, allthough from what I thought, I knew of him, it did seem a bit out of character for him to do something like that if it wasn’t the real deal. Chance would make it so, I was in the USA when the pack was released for sale. I had done my dues and researched it as much as possible and I was intrigued enough to pick one up, while I was there and bring it back home with me to try out.


In the Pyrenees circumnavigating Andorra

Time and use

I bought the pack at full price for my own money in the fall of 2016. I have been using it for backcountry skiing in Norway and on hikes in Denmark, Sweden, the Pyrenees and on the TGOC in Scotland in all season conditions. In total it has probably seen around 50-60 days of use, so I feel confident in giving this long-term review of the pack. The review is my own subjective opinion of the pack and it’s use for hiking and backcountry skiing. The data is supplied by the manufacturer Sierra Designs.

2017-03-01 11.08.43

Backcountry skiing in Norway with snow shovel on the side

Bottom line

This pack has many very positive things going for it in my view and only few minor gripes (see list at the bottom). When I bought it, I had hoped to have a lightweight flexible pack capable of handling most of my hiking trips year-round. Every pack has always been some sort of a compromise, but The Flex Capacitor is truly innovative, as it comes very close to balancing light weight with durability and capacity in a pack that carries weight comfortably on your back. This is an outstanding pack for the price, and when they fix my list of small gripes, I will for sure rebuy it for a nearly perfect pack for me.

Where to buy in Europe

The price in Europe is 192 Euro at Backpackinglight.se, which I think is a very competetive price for a fully featured and awesome pack like this.


The Flex Capacitor a great pack!

The Facts

The Sierra Designs Flex Capacitor is an internally framed lightweight backpack that is expandable from 40 liters to 60 liters using a unique system of straps to adjust the circumference of the pack. I am 187 cm tall and weigh 85 kilos. My original version of the large size fits me perfectly. I say the original version, because I have seen a newer model called medium/large and it seemed to look a little shorter ( a couple of centimeters) in the back length.

The pack features a top lid, with a zipper opening the three sides for easy access to the main compartment. The zipper has a rain guard flap. The top lid has a small pocket that expands downward into the packs main compartment. On both sides are two mesh pockets capable of holding a one liter Nalgene bottle. There is a shoulder strap pocket with stretch mesh capable of holding a 0.75 liter ‘cycle’ type of bottle. The hip belt has two good sized pockets. The front has two loops for ice axe or hiking poles. The inside has a removable hydration pouch but no hole for the hose anywhere.

The pack is made of 100 Denier nylon reinforced with Dyneema (not Dyneema Composite Fabric/Cuben). On the bottom is 420 D nylon. The mesh pockets on the side is robust and stretchy.

The internal frame is a removable Y-Flex DAC aluminum stay. It slots into the hip belt at the bottom holding both in place and help with weight transfer to the belt. There is a rigid type of foam in the hip belt pads, the lumbar pad and further up the back at the shoulder pads and shoulder straps. The pack comes with load lifter straps, waist belt straps and adjustable sternum strap.

The main innovation in the pack is the compression strap system allowing you to expand the pack from 40 to 60 liters. When compressed the pack is quite narrow and is suitable for 3-7 days hikes with light gear. If you need to carry more gear for a winter hike, or just need more volume, you release the straps on the front, and the pack expands to 60 liters. Instead of expanding vertically the pack expands outwards keeping the center of gravity low without compromising the stability of the pack on your back. When everything is properly adjusted the Flex Capacitor carries weight remarkably well.

With the hydration pouch removed and in an original size large my version weighs just under 1150 grams, which is considered light but not LIGHT. You can remove the stay and belt to further bring the weight down, but then I would say that there are better suited packs on the market for that purpose.

Pack Features (from Sierra Designs website)

·        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


Drying shirt on the outside

My opinion


One of the best and most innovative packs I have used in recent years.

Good size range as the 40 liters will see me make a seven day lightweight hike in summer conditions, and the 60 liters is perfect for a prolonged 7+ day hike with or without resupply or a winter expedition.

Control of load and carrying system is very good and comfortable even when expanded and full.

The pads on the back makes it ride a little away from the back making it breathable and possible to get a little air on a sweaty back.

Easy to use and comfortable hip belt with usable large pockets.

The flexibility of the pack makes it able to replace other packs in my inventory saving me space and money.

It is a good-looking pack with the grey and red color system.

Easy access to main compartment.

Bottle pockets is stretchy and very usable up to 1-liter bottles.

In the beginning I missed an outside stretchy stuff pocket, but with more and more use, I have begun to use the top lid pocket, the hip belt pockets and the side pockets instead. For drying gear like towel, socks and shirt on the outside I just use the compression straps now.

Minor gripes 

A little on the heavy side (1150 grams) compared to other lightweight packs, this is offset by the usability of the pack, but still…

Stiff backside and the lumbar pad takes some getting used to.

Top lid pocket is on the small side to be really usable and it expands downward. Make it wide all the way out to the edge of the lid and expand upwards, so you can use it to stuff your windjacket and other assorted things into. The zip opening is similar to the packs main opening, sometimes making you grab the wrong zipper, it is a minor complaint and could be solved by using a different color zipper string.

The rain guard and cover to the zipper system can get tangled and snag in the zipper, which is super annoying. Maybe a rainproof zipper og a roll top closure instead (which would mean no lid pocket)?

Where is the whistle on the sternum strap, come on, I thought that was standard by now?


9 thoughts on “Sierra Designs Flex Capacitor backpack long term review

  1. David says:

    Tak for anmeldelsen Niels, virkelig interessant taske.
    Konceptet ligner meget Exped Lightning 60, har du nogle umiddelbare tanker ift. de to modeller over for hinanden? (http://www.exped.com/international/en/product-category/backpacks/lightning-60-terracotta#prettyPhoto).
    Mvh. David


    1. Niels Blok says:

      Jeg har kun benyttet Expeds Lightning på korte ture. Det er i mine øjne en fin rygsæk, der sagtens kan bruges til kort og lange ture. De to kan godt minde om hinanden i opbygning med lænde støtte og et hovedrum. Expeds har ikke muligheden for at justere volumen, og i mine øjne bærer den heller ikke vægt lige så godt som Flex Capacitor. I sidste ende er det jo ens behov, der afgør det, og skulle jeg købe en ny rygsæk og valgte var mellem de to ville jeg vælge Flex’eren.

      Liked by 1 person

  2. Arvy says:

    Hi, this nice review is two years old. Do you still use Flex Capacitor or did fnd better pack?


    1. Niels Blok says:

      Thanks for your comment. Yes I still use the pack and will keep doing. It was with me in Norway this summer on a Glacier hike.



  3. Angie says:

    I have this pack as well, though I haven’t put as many miles on it yet as you have, so I haven’t really developed any strong preferences for the way I load my pack. I have the “newer” model with the strap pocket everyone who reviewed the old model thought they needed. It makes me crazy to carry water there b/c it’s right up in my face, so I use it for snacks. I agree with all the positives; my thoughts were in favor of having one pack that worked for various lengths of hikes. The only thing that truly bothers me is the difficulty I have zipping the belt pockets closed while wearing the pack. It just can’t be done with one hand. You need both, one providing tension or pushing the sides of the zipper together. Maybe once they get a little more broken in? A small consideration overall, and certainly not a dealbreaker.


    1. Niels Blok says:

      Hello Angie.

      Thanks for your comments. I have not had the issue with the zippers on the belt pockets. You could try to give them som candle wax so they will glide smoother? Just a thought.

      Cheers Niels


  4. Lad says:

    hello, I got a new version of this pack.. while I see great things abt it the stiching on shoulder straps at all angles is just appalling!
    my 20l lowe alpine day pack got straps stitched more properly.
    I just see SD shoulder straps to start falling apart from all directions very soon. not sure if I wanna try it though.
    have u had any issues with durability?
    Thank you.


    1. Niels Blok says:

      Hi, sorry to hear it. I have had zero issues with mine. Return it for a warranty repair?


      1. Lad says:

        hm.. nothing has happened yet, but it wasn’t really tested in real life.
        I think it s just fundamentaly bad design on some spots that supposed to bear weight.
        I seen some ppl report the same but it s very few.
        one Russian adventurer crossed whole country over 3000km, he used this pack.
        and it failed on first 1000 km if I m. not wrong, so. he had to repair it and reinforce. but despite this he said he would still buy it again just before use he would do custom reinforcements for shoulder straps.. I think it s a bummer.
        so you can confirm after all use straps are not falling apart? it s just one line of stiching sewn directly onto thin material.. very impractical,unless we carry 10kg only, but then why need a bag that expands to 75l?it could have been really good pack.


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