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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