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Sierra Designs Flex Capacitor: Drive By

by , January 25, 2018

Sierra Designs teamed up with Andrew Skurka, National Geographic’s 2007 Adventurer of the Year and professional outdoorsman, to design a range of products for the more ambitious adventure crowd. For Sierra Designs’ first foray into technical backpacks, the Flex Capacitor is nothing short of awesome.

A quick pitch at basecamp and I’m ready for dinner. The U-Zip top access on the Flex Capacitor makes rummaging for my stove and fuel an easy task.

The Flex is unique in more than one way. First, it utilizes a patent pending compression system, and secondly, Sierra Designs reached out to the outdoor industry’s favorite tent pole manufacturer, DAC, to create a super rigid frame stay. The result of all this is a feature-rich, sub 3-pound, 40-60L pack that can handle most of your adventures.

I’ll be honest. I didn’t think I would like this bag, I didn’t think much of it when it came out, and I never considered Sierra Designs as a bag company (because they weren’t). While I have always looked up to Andrew Skurka and heeded his sage advice, the bag felt a little gimmicky. It had a wacky name and a never-before-seen compression system. Seemed like a long shot. But after using the bag this summer/fall, it looks as though I’m having crow for dinner because I’m convinced this is one of the best values in the market.

I do miss a traditional, symmetrical compression system on the Flex. You can pack in additional supplies, but it ain’t pretty.

Who It Suits

Beginners, enthusiasts, experienced backcountry travelers—the Flex Capacitor is a great all-rounder for everything from overnights to five-day trips in fair weather.

Who It Doesn’t

Heavy load haulers, I can think of more appropriate bags to do your grunt work.


Weight: 43.2 oz / 1.25 Kg (large frame with medium belt)
Material: 420D bottom panel, 100D body
Dimensions: (L x W x D) 30 x 13 x 10 inches / 76.2 x 33.02 x 25.4 cm
Pockets: 2 side, 2 zippered hip belt, 1 zippered lid, 1 shoulder

My first outing with the bag was a breeze. Looking for waterfalls here day hiking in Shenandoah National Park.

The Good

One of the marks of a good bag is convenience, and the Flex Capacitor delivers in this category twofold. First, the stretch woven shoulder pocket is clutch. I feel like this pocket was either a military thing back in the day or started by one of the ultra-light brands offering easier access to water and essentials—either way, I love to see features like this bleed into the mainstream market. I kept my sunglasses in there most frequently, but a soft water bottle and a can of bear spray found their way in a time or two. The two large hip belt pockets also made trail life easier.

“This is a feature-rich, sub 3-pound, 40-60L pack that can handle most of your adventures.”

This stretch woven shoulder pocket is a thing of beauty.

Secondly, let’s talk about zippers. I’m a rolltop guy, but the U-Zip is fast and offers good enough access to your stuff, especially if you’re more experienced and pack intelligently. Bonus! It’s a 10mm YKK coil.

The large U-Zip gives good enough access to your belongings and is faster and easier than a cinch top with a lid or a rolltop. Note the Y-FLEX frame stay courtesy of DAC. A simple hydration sleeve attaches to the Hypalon slots at the top. 

The Flex Capacitor checks a lot of boxes outside of convenience, too. The suspension is about as good as it gets in this weight class. The articulating shoulder straps have ample padding and contour; they’re comfortable and should fit most body types. The hip belt is generously sized, has two large zippered pockets, and transfers weight well. It’s simple, no frills, and rides great. I carried out 54 pounds of food and water on a resupply drop without issue. It isn’t my favorite bag to haul that much weight, but that’s no small feat for a 43 oz bag.

The articulated shoulder straps add a little comfort to the suspension by accommodating a wider range of body shapes.

Sorry for the poor quality photo–This was taken as I arrived at my party’s camp. I had the Flex Capacitor loaded up with 10 liters of water and about 30 pounds of food.

The Not So Good

My biggest gripe with the pack is the DAC Pole frame stay will occasionally make contact with my lower back. It happened while bending over or taking big steps—might be a me-issue, though. Fortunately, the stays are rounded and the discomfort only lasts for a split second—think of it more as a reminder to maintain good hiking posture. Don’t let this scare you; I really like the suspension.

Note the main stay in the lumbar area. During big dynamic movements I could occasionally feel the stay making contact with my lower back.


The top lid pocket could be larger. Also note that it bellows into the main compartment. I could use a little more space for my knick-knacks, ya know? 

Note the pleats sewn into the top lid pocket. This pocket expands down into the main pack bag when full.

The top lid pocket is rather small. That’s not a deal breaker, but it doesn’t offer much in the way of organization. Shown here loaded with a pair of winter gloves and a thin wool cap.

Wouldn’t be a review without a controversial statement, right? This would be a better bag with a traditional compression system. I appreciate the innovation and risks associated with bringing new design elements and features to market, but I think the compression system is a miss. Let me explain:

All cinched down for a day hike.

The Flex Capacitor is, in my opinion, a solution to a problem that doesn’t really exist. What I mean is that, fundamentally, we should NOT think of bags as expandable—rather, we should think of them as compressible. As much as Sierra Designs achieved building a very good 40L  –> 60L expandable pack, I’m arguing that it would be a great 60L –> 40L compressible one. And it is—just not primarily.

The real innovation here is the sewn gusset which runs down the center of the pack face where excess pack material gathers under compression rather than on the sides near the frame of a bag with a traditional compression system.

“I appreciate the innovation and risks associated with bringing new design elements and features to market, but I think the compression system is a miss.”

From Sierra Designs’ website, “Instead of expanding up like other packs, the circumference of flex expands to provide a more stable and comfortable load carry.” That’s essentially the same principle with the panel geometry present in packs with traditional compression systems. Lengthen the compression straps: Increase the circumference of the pack.

A happy medium for me would be to keep the dimensions of the main pack bag, drop the gusset altogether, and incorporate a symmetrical compression system. With the current asymmetrical webbing channels, we’ve lost the utility that a standard symmetrical compression system brings, which I sorely missed trying to neatly carry a closed cell foam pad and sandals through Shenandoah. You can do it, but it’s a little awkward.

Still—it’s an awesome pack and one I recommend taking a close look at.

Three days in, no hiccups.

Alternatives to Consider

60L backpacking bags near the $200 mark… The only other choice out there for me would be the Granite Gear Crown2, which fills the same role but isn’t really comparable outside of price and capacity. I’ve heard people mention the Osprey Exos 58 within the same sentence as the Flex, and I can understand that—though the Exos is a little more specialized where I feel the Flex serves a wider range of users.

“The suspension is about as good as it gets in this weight class.”


If you’re on the fence, go for it. The Flex is a must-consider for beginners and should be at the top of the list for enthusiasts on a budget looking to upgrade their older packs. Skurka’s years of experience are evident in just how livable this bag is—my final takeaway from the pack was that it just made sense.

Putting in the miles with Pops. He can still motor when he wants to.

The Breakdown

1 2 3 4 5 6 7 8 9 10

Geek (Performance)

Space & Access

Style (Design)

Look & Feel
Build, Materials & Hardware

Stoke (Experience)

Warranty & Support
Brand experience
X Factor

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