The Sierra Designs Flex Capacitor is a lightweight, full-suspension backpack that is expandable from 40 liters to 60 liters using a unique, patent-pending gusset system. I’m 5’9” and 155lbs. The medium size fits me perfectly.

Screen Shot 2017-01-24 at 1.50.13 PM.pngThe backpack features a top-lid zipper for easy access into the main compartment, a top lid pocket, two mesh side pockets, a shoulder strap pocket, two hip belt pockets, and two ice axe/trekking pole loops. It also comes with a removable hydration pouch.

screen-shot-2017-01-24-at-1-50-02-pmThe Flex Capacitor is made of 100D nylon-poly ripstop (which is nylon fabric reinforced with Dyneema) and a 420D nylon oxford fabric on the bottom. The mesh pockets are made using the most robust stretch mesh that Sierra Designs could find.

Screen Shot 2017-01-24 at 1.51.01 PM.pngThe suspension consists of: a removable “Y-flex” DAC aluminum stay (similar to those found in tent poles), and EVA foam in the hip belt pads, lumbar pad, scapula pads, and shoulder straps. It also has load lifter straps and an adjustable sternum strap and waist belt. When everything is properly adjusted, the Flex Capacitor carries weight incredibly well.

Screen Shot 2017-01-24 at 1.51.51 PM.pngWhen compressed to 40L, the pack is quite narrow and short, making it great for those who wish to travel light and fast. If you carry a bear canister or just need a bit more volume, you can expand the gusset and the pack will hold up to 60L of gear and supplies. Rather than expanding vertically, the pack expands horizontally outwards to keep your center of gravity low. And as you consume your food on the trail, you can compress the pack down.

With all its bells and whistles, the Flex Capacitor weighs in at 2 pounds 10 ounces. It can be further lightened by removing the 1.5oz hydration pouch and the 2.5oz aluminum stay. You could even remove the hip belt if you carry ultralight loads, though there are packs on the market better suited for that purpose. Currently, the pack has an MSRP of $200, which is in line with packs from big names like Osprey, Gregory, and Granite Gear.

Here is what I particularly like about the backpack:

  1. Volume – For 3-season use, 40L is just the right amount of volume for my 11lb loadout (pack included). Initially I was concerned that my items would bounce around inside the pack if I didn’t fill up all the space but you can use the compression straps to reduce the volume even further if needed. And boy, is this a nice looking pack. During the winter or when I backpack in parks that require a bear canister, the 60L can hold all my gear without the need to lash anything to the outside.
  2. Sturdy suspension – The aluminum stay design really helps spread the load to the hip belts, which are nice and tall to reduce pressure on the hip area. And the lumbar pad fits nicely into the small of my back despite how prominent it felt when I put on the pack for the very first time.
  3. Zippered top lid – Sierra Designs uses a nice, beefy #10 YKK zipper. This was a major selling point for me since there is nothing easier to operate than a zipper. The rainguard provides some protection but any experienced backpacker knows that they should waterproof the interior in some way, like with dry sacks or trash bags.
  4. Shoulder strap pocket – This heavy-duty mesh pocket can hold a 20oz water bottle or bear spray. It’s one of those features, like the zippered top lid, where you will never be able to go back to a pack that doesn’t have it. Am I too greedy in wishing for two of these shoulder pockets?
  5. Side pockets – The side pockets are just the right height for me to grab a water bottle on the go without taking the pack off. They are made of a thicker and more tightly woven mesh than what you would find on an Osprey pack.
  6. Color – I love the muted color scheme with the red accents.
  7. Sizing – Backpack fit can be very personal. My torso is 19” long and my waist is 31”, which puts me in the middle of the range for the size medium. The sizing seems accurate. Sierra Designs plans to eliminate the medium size and to only have two backpack sizes, small-medium and medium-large, starting in the Spring of 2017.

Here are some things that could be improved:

  1. Weight – There are many packs that are lighter than the 2.5lb Flex Capacitor. There are certainly some design compromises that could be made to lighten the pack further. However, I think the weight is perfectly acceptable given that it’s the only pack I need to cover the whole range of trips that I do — from weekend trips to long summer hikes in the Sierras to winter backpacking in Canada.
  2. Non-adjustable shoulder straps – The shoulder straps are attached to the pack and are not adjustable. Depending on your body shape, this means that you may not be able to dial-in the perfect fit.
  3. No back mesh pocket – I think if Sierra Designs could think of a way to include a large mesh shove-it pocket without impinging on the gusset functionality, they would include it. It’s what a lot of users seem to want. However, I’ve found that it’s really not necessary given how easy the top lid opens. For example, normally I would put my sit pad into a shove-it pocket but with the Flex Capacitor, I just put it towards the top of the pack. Any wet clothing can be hung on the compression straps to dry. The lack of this pocket may take getting used to but it’s not a hard feature to give up.
  4. Top lid zipper can snag – It’s a hate it and love it situation with zippers. One solution is to keep the rainguard flipped up in good conditions.
  5. No whistle – I’m used to packs that have a handy whistle built into the sternum strap buckle. I rarely use it but it’s a nice to have on group trips, especially with virtually no weight penalty. You can attach an external whistle to the sternum strap on the Flex Capacitor to achieve the same function.

With this purchase, I now have one light-but-capable backpack that can handle all of my trips. Whether you’re already a lightweight backpacker or someone looking to shed weight from your pack, the Flex Capacitor will help prevent you from packing unnecessary items to fill that empty pack space. You can rest easy knowing that even as you upgrade your gear piece by piece, you won’t need to replace your old pack with a smaller one.

See the full details in the video review below.

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