Sierra Designs Flex Capacitor 40-60 Backpack | Review - Outdoors Magic

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Sierra Designs Flex Capacitor 40-60 Backpack | Review

A truly versatile gear-hauler with a unique expansion system that gives you up to 20 litres of additional capacity, the Flex Capacitor really is like no other backpack out there

US brand Sierra Designs has been one of the most consistently innovative gear makers of recent years. From their zipperless sleeping bags to the eminently practical 3,000 tent series, we’ve tried and tested a lot of their kit, and invariably come away impressed. This includes the Flex Capacitor backpack, which has ended up being perhaps the most notable success in Sierra Designs’ recent history.

The Flex Capacitor is a lightweight backpacking rucksack with a genuine USP: a clever gusset system that allows the pack to quickly and easily expand in volume by up to 20 litres, simply with the adjustment of a few straps. Rather than increasing storage vertically, like most top-loading designs, it expands outwards, providing useful additional capacity in the main compartment without adversely affecting load-carrying stability or comfort. For any backpacker who’s ever needed to overstuff a pack in order to take on extra food, water or other supplies, it’s a genuine game changer.


The main body of the pack is built from 100D nylon honeycomb fabric, with 420D Oxford fabric in the base. Those fabrics are reasonably tough whilst remaining commendably weight conscious – which makes sense. After all, this is a gear-hauler designed with lightweight backpackers in mind.

The Flex Capacitor accompanying OM editor Will on a trip to the Cairngorms. Photos: Dave Macfarlane

The internal framing is similarly minimalist. Rather than chunky stays or a perimeter frame, the Flex Capacitor instead uses a Y-shaped frame made from lightweight 7075 aluminium alloy. Zoned shoulder, back and hip padding made from foam with mesh overlays offers good cushioning, plus a little airflow, and the excellent hip belt transfers weight efficiently. But again, it’s all fairly weight conscious in terms of design.

The result is a pack that weighs in at a fraction over 1.2kg (or 1.4kg for the M/L size, which is designed for longer torsos). Given its load-lugging ability and generous capacity, that’s a competitive weight – especially when you compare it to most rivals. A lot of packs in this class from manufacturers like Osprey and Gregory tip the scales at well over 2kg.


As we’ve established, the Flex Capacitor really is like no other backpacking rucksack out there, thanks to that impressive 20 litres of adjustable volume. This gives you the option of packing 40L or 60L, which is the difference between an overnight camp and a multi-day trip. That in itself is unusual, but not unique – plenty of packs also offer the ability to overstuff them with extra gear. But only this one expands outwards, not upwards – keeping your load stable and secure rather than teetering and top-heavy, even at full capacity.

In addition to that expanding main compartment, there are plenty of clever pockets too. This includes a stash pocket in the lid, two roomy side pockets, two stretchy chest pockets on the shoulder straps and zippered hip belt pockets to keep your kit organised.

“It’s a capable and comfortable gear-hauler.”

Access to the main body is via a bucket-style opening, covered by a flap with a chunky U-shaped zipper. It’s easy to pack, with no awkward corners or curved back panel to intrude on interior space. Inside, a mesh hydration sleeve provides space to hang your water reservoir. However, it is fixed to the inside of the pack via a simple toggle system. This means it can double as a removable ‘summit sack’, though we tend to use it as a handy shopping bag for resupplies on the trail.

The pack is internally framed with a Y-shaped aluminium alloy stay. Dual adjusting shoulder straps tailors pack fit your torso, while the back panel consists of three cushioned lumbar and shoulder pads. Air channels between these areas offer good ventilation.

The exterior of the pack features two large mesh stretch side pockets and trekking pole loops. There’s no front pocket, because the compression system requires a central gusset and a lot of straps. In the previous generation of the pack, these were exposed, offering various potential lashing points for attaching extra gear. On the other hand, when cinched in they were a little untidy and could snag on trail branches and undergrowth. Maybe that’s why the latest generation of the Flex Capacitor has fabric sleeves to cover the webbing, making for a neater and more streamlined pack.

All in all, it’s a capable and comfortable gear-hauler, yet one that is still a viable option for lightweight backpacking and wild camping trips.

Sierra Designs Flex Capacitor 40-60 Backpack

Selected for the Outdoor 100 Winter 23/24
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