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Sierra Designs Flex Capacitor 40–60 review: a clever sleeping bag with mammoth capacity

The Sierra Designs Flex Capacitor is an innovative yet lightweight pack with an impressive 20 litres of expandable capacity and plenty of practical pockets

Sierra Designs Flex Capacitor 40–60
(Image: © Sierra Designs)

Our Verdict

For such a lightweight pack, the Flex Capacitor carries well thanks to a clever Y-shaped internal stay and a well-padded harness and hipbelt. It has a roomy, useable main compartment and plenty of practical, quick access pockets for convenience on the trail. But the pack’s best feature is its versatile expanding capacity and efficient compression system, which means you can transform it from a ‘fast and light’ 40-litre sack to a gear-swallowing, 60-litre load lugger.


  • Stable and comfortable
  • Lightweight
  • Hydration-compatible
  • Innovative side-expanding capacity


  • No raincover
  • Awkward top-loading bucket zip
  • Compression straps tend to flap around

First impressions

Designed with considerable input from US backpacking legend Andrew Skurka, the Sierra Designs Flex Capacitor series are genuinely innovative packs with unique horizontal compression systems that enable the packs to expand outwards. In this case, it transforms the capacity from 40 litres to a generous 60 litres.

And, unlike most expanding packs with floating lids and extending collars, which tend to feel top-heavy and unwieldy when overstuffed, this sideways expansion system ensures that the pack remains stable and comfortable even when fully loaded. The secret is a central front gusset combined with a series of compression webbing straps that can be cinched as required.

Impressively, the Flex Capacitor is still a lightweight pack. It has a Y-shaped internal aluminium frame to provide rigidity without adding too much weight, plus strategically placed back panelling that provides adequate padding as well as some airflow without employing the heavily engineered suspended mesh systems of some other rivals.

The main compartment is accessed via a bucket-style zipped flap, and inside there is a minimalist removable mesh insert for a hydration reservoir. On the exterior of the main body here are twin large mesh stretch side pockets and trekking pole loops. You also get two useful zipped hip belt pockets, and unusually, stretch pockets on the shoulder harness.

For more on the features described here, check out our guides on the parts of a backpack and how to choose a backpack.


RRP: $130 (US) / £165 (UK)
Volume: 40–60L / 2,563–3,661 cu in
Weight: 1168g / 2lb 9.2oz
Dimensions (HxWxD): 70 x 28 x 25.4cm / 27.5 x 11 x 10in
Sizes: S/M and M/L torso lengths plus S/M and M/L hip belt sizes
Fabric: 100D Nylon-Poly Ripstop/420D Nylon Oxford

In the field

There’s no denying that the Flex Capacitor is a very versatile beast. When fully cinched in, the 40-litre capacity is a good size for quick overnighters or shorter ‘fast and light’ trips (see: What size backpack do I need?). But the fact that it will expand outwards to 60 litres makes this a viable option for extended adventures too. It also allows you to overstuff the pack if required – e.g., if you’re traversing remote and isolated terrain with no place to resupply and therefore need to carry additional food or water supplies for a few days.

The pack’s minimalist internal Y-frame uses high-quality, large-diameter aluminium alloy DAC poles – the same as those found in the best tents for backpacking. This ensures that the Flex Capacitor is a surprisingly capable gear hauler. There’s plenty of shoulder, back and hip padding to soak up pressure points from a fully laden pack, and the hip belt in particular is extremely well-designed, taking plenty of weight.

The single main compartment is easy to pack, being a practical barrel shape, with no awkward corners or rigid, curved back panels intruding on the interior space. This makes it a very ‘usable’ rucksack. The design of the pack means there is no front stretch pocket, but the roomy side pockets largely make up for this shortcoming. They offer plenty of useful additional storage, though you need to make use of them to avoid them being a bit baggy. Similarly, on the trail, we appreciated the zipped hip belt pockets and stretch mesh pockets on the shoulder straps, all of which ensure quick and easy access to everything from Jelly Babies to smartphones. It’s a pack that is clearly and cleverly designed for those who like to keep moving.

Our only niggles were pretty minor and almost all related to the top-access bucket zip. It’s a U-shape design that allows the whole top flap of the pack to open out – easy for loading, but in our experience difficult to zip and unzip quickly. That’s partly because it tends to snag on the fabric storm flap. This seems a design shortcoming that we suspect was probably added because the zipper itself is not especially water-resistant, but attempting to solve one problem seems to have caused the other.

This is, after all, a pack from a US brand designed in part by a US thru-hiker. As such, it’s perhaps a little less well suited to the wet maritime climate of other countries, like the UK, where you need to worry about water ingress through that top zip. The pack has no rain cover either. It’s an issue that would be solved by good packing, with the use of dry bags for your kit (see: best dry bag) or a large roll-top rucksack liner though.

Final observations: the compression system of the Flex Capacitor requires a lot of straps that can be untidy and flap around in windy conditions, while also occasionally getting caught and snagged along the trail. Our test sample also came in ‘birch’, a very light-coloured white-grey that is possibly the most impractical colour it is possible for a backpacking rucksack to be. It meant that after just a few hours in the hills of Snowdonia, the pack was already very grubby. Not a dealbreaker, and certainly not egregious enough to keep it off our list of the best hiking backpacks, but we’d advise playing safe and going for the darker ‘peat’ colourway.

Those very minor quibbles aside, we were impressed with this pack’s innovative design approach and practicality in use. It’s also a pretty comfortable carry even when fully laden, despite weighing in at a very respectable 1.2kg.

An outdoors writer and editor, Matt Jones has been testing kit in the field for nearly a decade. Having worked for both the Ramblers and the Scouts, he knows one or two things about walking and camping, and loves all things adventure, particularly long-distance backpacking, wild camping and climbing mountains – especially in Wales. He’s based in Snowdonia and last year thru-hiked the Cambrian Way, which runs for 298 miles from Cardiff to Conwy, with a total ascent of 73,700 feet – that’s nearly 2½ times the height of Everest. Follow Matt on Instagram and Twitter.