Big Trail Hiking with Sierra Designs' Larger Flex Capacitor

The Flex Capacitor's... light weight and flexibility make it easy to stay agile over mixed terrain, remain secure when navigating obstacles or hopping across water, and not feel heavy on extra-long trail days, or when slogging up high passes.

Last year, between pandemic-induced forest closures, and wildfire-burning forest closures, we squeezed in a few multiday trips with Sierra Designs’ then-new Flex Capacitor 60–75L backpack. This larger version of the original, 2016 Flex Capacitor 40–60L retains the same basic design and features, but has a slightly modified construction that keeps the weight comparable to the original. And while the 60–75L doesn’t add any spectacular new features, we feel it’s a significantly superior pack to its predecessor.

Out of the Bag

The first thing we noticed on unpacking the Flex 60–75L was its remarkable light weight for a big-trip backpack. We had to check the specs of the original and found that the newer, larger version ticked in at nearly the same weight of the original (both size M/L). This was achieved through a few smart design revisions, as it’s still constructed of similar 100D ripstop nylon. The Y-shaped, internal DAC frame—also similar to the original, and one of the features for its cinematic name tribute (Back to the Future’s “flux capacitor”)—keeps the entire back panel rigid and upright, yet allows a remarkable amount of lateral torque (more on that below).

Looking the new Flux over, it retains essentially the same semi-minimalist style and feature set as the original Flux. The big main compartment is accessed only by way of the top, reverse-opening, zipper lid. Inside, the only thing is a removable, dual-purpose mesh bladder sleeve. (More on this below.) A small clip inserts into the lid of the sleeve to hang the bladder (sold separately); the hose port is in the top center of the back panel. On the outside, there are seven pockets for organizing essentials. This is one more than the original. These consist of a zippered lid pocket, two stretchy side pockets, two zippered waistbelt pockets, and two mesh shoulder strap pockets.

The main feature of the pack, its variable capacity, is obvious by the expandable center panel. This is what gives the Flex Capacitor its “flex” capacity. This panel runs from the bottom of the pack, all the way up through the lid, so when you’re adjusting the pack size for whatever trip you’re taking—short and light, long and heavy, etc.—the whole pack expands or contracts accordingly. This is accomplished by means of a series easily adjustable clips, buckles and straps across the back of the pack. Near the bottom of the pack are two gear loops for trekking poles or ice tools. Same with the original, a rain cover is not included.

Loading Up

Loading up the bigger new Flex is pretty straightforward—just start dropping stuff in. Because of its size, and its single top opening, it’s a good idea to consider how much you’re carrying, and load with purpose. That way things like rain gear and food are accessible near the top, while camp items can go down toward the bottom. Then you won’t need to have a trailside garage sale every time you need something. We also found that by setting the compression straps at about the halfway point, it’s easier to load up then cinch them in or out as needed. The main compartment can easily accommodate a BearVault or similar food canister if necessary.  

The external pockets are mostly good for small essentials. The side pockets are big enough to hold 1-liter bottles or a compact, ultralight camp chair. (Must have easy access to both!) The waist belt pockets are good for snacks, smartphone, hand sanitizer, etc. And we used the shoulder strap pockets for our SPOT and sunglasses. These can also be used to hold some of the smaller, flexible water bottles, if not using a hydration system. The large lid pocket is a good place for the first aid kit, toilet kit, water filter and personal effects.

There are no big exterior pockets for larger items, but you can attach the mesh bladder sleeve to the exterior of the pack via toggles at the top and bottom. This makes an excellent external pocket for rain gear, camp shoes, etc. Alternatively—depending on how full the Flex is loaded—it is also possible to cinch up a sleeping pad or tent into the compression straps on the back. These straps also provide plenty of convenient places to clip things to the outside. The great thing about the adjustable capacity is that you can fit most everything inside, and keep it clean and clutter-free on the outside.     

Keep the outside of the Flex Capacitor sleek and clutter free, or attach the mesh sleeve for even more carrying capacity.

On the Trail

Loaded up and tromping down the trail, we found the new Flex to be an exceptionally comfortable pack, even more so than its predecessor. The internal Y-flex DAC frame keeps the back panel rigid to prevent bunching and slouching, and the chonky scapula pads and lumbar support provide plenty of cushion. The padding is also spaced out enough to allow a good amount of airflow between your back, and the back of the pack. This helps keep things cool, and not get too sweaty. Cinched up properly, the Flex also pivots with your body exceptionally well, holding to you as you twist and turn down the trail and hop over obstacles. The 100D ripstop body is durable enough to take a mild beating, and withstand the occasional brush, poke or snag with trees and rocks. 

The Flex’s waistbelt and shoulder straps are lined with breathable mesh fabric to improve airflow and keep things from getting too swampy, and both are padded enough to keep it comfortable and prevent rubbing or chafing, even on long hauls. Load lifters on the shoulder straps and cinchers on the waistbelt help in tailoring the fit to keep it in close, and prevent it from sloshing around. Fetching things from the outer pockets is no problem. The waistbelt pockets are positioned well for easy item access. The outer stretch pockets require a little more effort to reach, but it’s possible to retrieve items, and put them back, without wrenching an arm or needing to take the pack off.

In camp, unloading requires the same method as loading: everything comes right back out the top, either by methodically unpacking, or just turning it over and dumping it out. Emptied out and flattened down, the Flex easily fits under most average size tent vestibules to keep it out of any potential inclement weather. (Ideal, since it doesn’t come with a rain fly.) It’s also not so cumbersome that if you need to stash it in your tent, it doesn’t take up too much room. It also has a lift/grab strap for hanging, if that’s an option wherever you’re camped.

The Rundown

Overall, we found the new Flex Capacitor 60–75L to be an exceptionally comfortable and versatile pack. It’s light weight and flexibility make it easy to stay agile over mixed terrain, remain secure when navigating obstacles or hopping across water, and not feel heavy on extra-long trail days, or when slogging up high passes. Its adjustable capacity is especially nice for fine-tuning its size for specific trip needs, without needing to own both a long-trip pack and a short-trip pack. The easily accessible waist and side pockets are great for keeping essentials within reach.

Similar to how we reviewed the original Flex, we still wish it had a removable lid to stash essentials in the tent, or use as a day tripping bag—but this is easily remedied by just packing along an extra stuff sack. Its water repellency is decent for a while in drizzle and light rain, but it definitely requires packing along a rain shell (sold separately) if the forecast looks ominous. Barring these two very minor critiques, the newer, larger Flex Capacitor is pretty much everything you need for going the distance comfortably in a semi-minimalistic pack, no matter if you’re out for a few days, a few weeks, or a few months.

Sierra Designs Flex Capacitor 60–75L

S/M WEIGHT: 2.6 lb
S/M WAIST: 29–32 in
M/L WEIGHT: 2.7 lb
M/L WAIST: 32–35 in
TORSO: 18–21 in

MATERIAL: 100D Ripstop Poly
PRICE: $200

Eli "Lounger" Boschetto

Eli "Lounger" Boschetto

Eli is the founder of PCT: Oregon, and the author of three Mountaineers Books guides: Hiking the Pacific Crest Trail: Oregon, Day Hiking: Mount Hood, and Urban Trails: Portland. He is also a brand ambassador for SPOT and National Geographic Maps, and is on the advisory council for the Oregon Trails Coalition. Eli lives in Portland, Oregon.

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These reviews are based on the field results of PCT: Oregon’s gear testing team. Reviews are subjective, and are based on a variety of factors, including, but not limited to, temperature, weather, elevation, and trail/camp conditions, as well as personal size, comfort and body function. Individual results may vary. PCT: Oregon and its gear review staff are not associated with featured gear brand(s), and are not paid for reviews. 

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Additional gear shown, not included with backpack: SPOT X Satellite Messenger, NEMO Tensor Sleeping Pad, NEMO Chipper Foam Seat, Hydro Flask Trail Series Insulated Bottle.

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