Sierra Designs Flex Capacitor 40-60L Backpack

£152.90 £175.00


The Sierra Designs Flex Capacitor features a unique patent pending gusset system that allows the pack to quickly and easily expand from 40-60 litres with the adjustment of a few straps.

Instead of expanding up like other packs, the circumference of flex expands to provide a more stable and comfortable load carry.  Available in Birch - as shown in photos.

The Sierra Designs Flex Capacitor 40-60L Backpack is an Innovative Designed Backpack. Valley and Peak Stock Two Different Sizes for Different Back Lengths. The Flex Capacitor Features a Unique Gusset System that Allows the Pack to Quickly and Easily Expand from 40-60 Litres with the Adjustment of a Few Straps. 

What We Think at Valley And Peak

Sierra Designs as per usual has come up with another innovative design in the form of an adjustable volume backpack, meaning you can have a backpack for an overnight or week long trips, simply by adjustment of straps tightening a gusset which cinches the Flex Capacitor down, so everything remains stable on your back.

There is a nice airflow through the gap created by the Shoulders and Lumber pads, to keep you cool during hot weather.

Designed by Andrew Skurka, the well known long distance backpacker. Watch the video above from Andrew to get a good fell for all the features and benefits.    The Flex Capacitor Backpack carries very comfortably even with heavier loads where many packs would feel uncomfortable and unstable .

Read Chris Townsend's review in TGO here



  • Expandable 40-60L volume (volume varies between sizes)
  • Ultralight weight design
  • 2 torso sizes (non-adjustable)
  • 2 hip belt sizes
  • U-shaped top access zipper
  • Zippered stash pocket on lid
  • Removable reservoir sleeve
  • 2 Shoulder strap pockets
  • Stretch mesh side water bottle pockets
  • Two hip belt pockets
  • Compression straps
  • Ice axe/trekking pole loop

Suspension Key Features

  • Lightweight "Y-FLEX" suspension
  • EVA foam hip belt and shoulder straps
  • Raised lumbar and scapula pads (EVA foam)
  • Load lifter straps
  • Hip belt stabilizer straps
  • Adjustable hip belt


  • Body Fabric: 100D Nylon-Poly Ripstop
  • Secondary Fabric: 420D Nylon Oxford
  • Stay Material: DAC Pressfit Aluminum
  • Number of Stays: 1 "Y-FLEX" stay
TORSO SIZE 40 - 48 cm 46 - 53 cm
VOLUME (Liters) 39 - 56L 42 - 60L
VOLUME (Cubic Inches) 2400 - 3400 2550 - 3650
WEIGHT 1.16 kg (With S/M hip belt) 1.22 kg  (With M/L hip belt)
DIMENSIONS (LxWxD) 72 x 33 x 25 cm
72 x 33 x 25 cm
MAX COMFORT WEIGHT 16kg - 22.7 kg 16-22.7 kg


Small/Med Med/Large
WEIGHT (Ounces)
11 11.5
WEIGHT (Grams)
310 330
29 - 32 32 - 35
HIP BELT FIT RANGE* (Cm) 74 - 81 81 - 89