Review: Sierra Designs Flex Capacitor 40-60 Backpack

Sierra Designs Flex Capacitor 40-60
$200, 2 lbs. 9 oz. (men’s S/M pack with S/M hipbelt)
Sizes: men’s S/M (fits torsos 16-19 inches) and M/L (fits torsos 18-21 inches), plus four hipbelt sizes (XS/S to L/XL)

Many avid backpackers eventually find themselves facing an expensive quandary: the need for a second or even third pack to better handle the range of trips they take. Sierra Designs confronts that challenge with the Flex Capacitor, which changes size to cover a range of trips from weekends to a week or even a thru-hike. Curious about how it performs, I took it on a trip where a pack with that capacity range would come in handy: on a nine-day hike of the 105-mile Tour du Mont Blanc, where on some days I’d be carrying two people’s stuff, and on other days only my own (when that second person didn’t hike).

Sierra Designs Flex Capacitor 40-60 front.
Sierra Designs Flex Capacitor 40-60 front.

The pack’s unique feature is a gusset system that expands and contracts the capacity in a range from 40 to 60 liters—or more precisely, 2,400 to 3,400 c.i. in the S/M size and 2,550 to 3,650 c.i. in the M/L. Most packs that have a design which allows exceeding their normal capacity do so through a so-called “floating,” or extendable lid, which essentially lets you overload the pack by expanding it upward. But that makes the load top-heavy and significantly less comfortable to carry. By expanding its girth, the Flex Capacitor remains comfortable even at maximum capacity. Plus, its capacity expands by more than 40 percent, far more than packs with a floating lid can usually expand upward.

Horizontal compression straps reach around the pack body, helping to both shrink the pack down and attach an ice axe or trekking poles (the latter using either side pocket, too). With a lightweight, Y-shaped internal stay, a moderate amount of EVA foam padding in the shoulder straps and hipbelt (which comes in four sizes), as well as some rigidity in the pre-curved hipbelt, the Flex Capacitor was comfortable carrying about 35 pounds of climbing gear for three to four miles a day, and over 30 pounds (mostly clothing and personal items for two people) on the Tour du Mont Blanc.

I think many backpackers would find it comfortable with 35 to 40 pounds. That’s good for a pack barely north of two-and-a-half pounds, making it a legitimate ultralight pack. Still, while I found the hipbelt comfortable with the loads I described, it does not quite achieve the comfort of some of the best backpacking packs I’ve used (most of which are also heavier).

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Sierra Designs Flex Capacitor 40-60 suspension.
Sierra Designs Flex Capacitor 40-60 suspension.

A thick and firm lumbar pad and two upper-back pads help soften the load while allowing good air circulation across my back. With a fixed (non-adjustable) suspension, the pack comes in two sizes; the S/M fit my 18-inch torso well, as did the S/M hipbelt on my 30-inch waist.

In lieu of a traditional lid and a main compartment with a drawcord closure, this top-loader’s U-shaped top zipper gives one-step access to the main compartment, with a wide mouth that facilitates easy loading and unloading and swallows a bear canister no problem. The zipper on the main compartment tends to snag on the rain flap, but it’s easy to unsnag.

One compromise with this pack: It offers limited organizational convenience. It lacks a feature I like, a side zipper to provide additional access to the middle of the main compartment. There are no external pockets that would normally hold items you’d want to access during the day or keep outside the main compartment, such as water treatment or a wet jacket or rainfly. The lid pocket has almost no bellows to it, so it’s impractical for storing more than thin items like a map and phone.

But the two zippered hipbelt pockets are spacious enough for three to four energy bars each—though filling them completely can cause them to stick out far enough for forearms to brush against them while hiking—and the mesh side pockets each hold a liter bottle. A stretch-mesh pocket on the right shoulder strap holds a small water bottle or a phone. The pack body consists of 100-denier fabric, and the bottom of more durable, 420-denier nylon; this is a relatively tough pack compared to others in its weight class.

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The Verdict

For ultralighters or any backpacker seeking a sack with capacity versatility and a low weight, at a good price, the Sierra Designs Flex Capacitor 40-60 deserves a close look.

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NOTE: I tested gear for Backpacker Magazine for 20 years. At The Big Outside, I review only what I consider the best outdoor gear and apparel. See categorized menus of all of my gear reviews at The Big Outside.

—Michael Lanza


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13 thoughts on “Review: Sierra Designs Flex Capacitor 40-60 Backpack”

  1. Have you had a chance to try the updated model? Thinking about pulling the trigger on deals of the old one, but the revised looks quite nice with extra loops for trekking poles or to hang a stuff bag. Granted one could probably do that on the older model too, but if it’s there on the new one already…, plus I think the gussets might be stronger now. Any thoughts would be appreciated.

    • Hi Jan,

      I have not used the updated model, but it doesn’t appear to have changed all that much; cosmetically more than anything. With all the compression on this bag, attaching trekking poles to the exterior is easy. I’d go for a good deal on one.

        • No, those straps really are true compression straps, not intended to hold anything with bulk. When the pack is under-filled, there is more slack in those straps for fitting items underneath them, but I don’t think they would hold a tent very securely, especially with the pack’s volume changing. I suggested the straps could fit trekking poles because poles are slim and would likely fit under the straps even when the pack is at max capacity.

          I’m generally not a fan of having a lot of stuff attached to the outside of a pack; it can get damaged and it definitely affects the balance and carrying comfort of the pack. That applies especially to a tent because it’s one of your heaviest items. Plus, you don’t need it while hiking, only once in camp, so it belongs inside a pack. Check out my video on how to pack a backpack.

          • Thanks for the video link on packing my pack. I will utilize it along with Skurka’s video explicit to this pack. I usually put my tent vertically and stuff around it(to tight horizontally), unless I can strap the tent on the bottom. One issue is not all of us have the lightest compact gear.

          • I purchased the flex Capacitor through REI. Due to my gear not fitting well into the pack(I can make it fit, but kinda defeats the point of the being able to compress) I think I will look towards the 60-75 liter bag they have or perhaps an Exped Thunder 70. I wish I could try these bags first in a store. Oh well.

          • Yes. REI definitely has a good return policy. I will be returning it along with the Atmos AG 65 that I got almost a year ago (load transfer to hips is not on par to the Flex Capacitor). I wish they carried the larger Capacitor, but they don’t plan too.
            I don’t presently use the top lid on the Atmos so that’s another thing to consider that the Exped Thunder 70 has that’s not removable. Although, on the Exped it appears to be an integral part of amount of gear that the pack can hold.

          • Yup. Now just the struggle between the 2. Oh well, nothing is perfect, but if you’re not comfortable we’ll that never good on ones body and health lol.

          • Jan, my standard practice with a tent is to take the rainfly out of its stuff sack and keep only the interior tent in the sack, because it’s more vulnerable to tears (in the mesh). That makes the tent stuff sack easier to fit inside a pack. I stuff the rainfly loose around my sleeping bag at the bottom of my packs, filling in every available nook and cranny there. This helps maximize use of your pack’s volume and keeps your tent from being an unwieldy object to pack.