The Flex Capacitor has been a favourite backpacking pack since I first used the original version six years ago. The latest version has the same back system as before but a redesigned, and in my opinion improved, packbag. I’ve used the original on many long walks including a month in the Colorado Rockies and found it hard-wearing. The materials and construction haven’t changed so the latest model should prove just as durable.

Chris Townsend’s Best In Test

The latest version of the Sierra Designs Flex Capacitor has the same back system as before but a redesigned, and in my opinion improved, packbag.
  • Expandable Volume
  • Pockets
  • Load carrying ability
  • Lightweight
  • Cost
  • Lid can’t be raised or compressed for different sized loads
Quick specs
Price: £180         
Weight: 1.265kg (S/M)     
Capacity: 40-60 litres        
Materials: 100D Nylon Honeycomb/420D Oxford Nylon
Closure: zipped lid           
Back: Y-shaped tubular frame, padded scapular and lumbar pads         
Hipbelt: padded, shaped         
Pockets: 2 stretch mesh front, 2 stretch mesh side, 2 stretch mesh shoulder strap, 2 zipped, hipbelt, 1 zipped lid, inner hydration sleeve
Features: Expandable gusset, side/front straps, ice axe loops    
Sizes: S/M. M/L     
Women’s/Men’s Version: No

The Flex Capacitor has a rigid curved Y-shaped frame that’s longitudinally stiff but which twists from side to side so the pack can move with you. Thick mesh-covered foam pads hold the frame and the contents away from your back. There’s an air gap across the lower back where the frame curves for ventilation. The hipbelt is wide and well-padded. This back system is comfortable with 20kg loads.

The packbag has a central gusset that can be opened and closed with four straps that run across it, hence the variable capacity. This is much more efficient than increasing the volume by extending the pack upwards as it doesn’t affect stability. Two of the straps just run across the gusset, two of them right round the pack. The latter run in channels on each side of the pack. They can be removed from the channels if you want to use them to attach items. I don’t see any advantage to the channels but there’s no disadvantage either. The long straps can be used to attach a foam pad to the back of the pack, an improvement on the original where they were too short.

New and welcome additions to the pack are the big stretch pockets each side of the gusset. This is the one feature I really missed on the original. There are also smaller mesh pockets at the bottom of each side that can be used for water bottles, zipped hipbelt pockets, stretch shoulder strap pockets, and a zipped lid pocket so there’s ample room for everything you might need during the day without needing to open the pack.

The pack opens with a U-shaped zip-round lid. There are no drawcords or straps so this gives quick access to the contents. The disadvantage is that you can’t raise the lid to carry anything under it but outside the main bag. You can’t tighten the lid when the pack isn’t full either.

The Flex Capacitor is lightweight, comfortable, and tough, and comes at an excellent price.