The frame consists of a lightweight spring steel and aluminium frame attached to a high-density polyethylene (HDPE) sheet.
This is what prompted me to splash out on an MH “Crag Wagon”- it appeared to be the only bag with climbing-friendly features (I don’t share Moose’s enthusiasm for the multi-pocketed trekking style) with a proper back system.
The big advantage of the Crag Wagon is that the zip is a kind of U-shape on the
front of the bag though- these Lowe and DMM ones seem to have the zip going round the sides so doing a lot of the structural work of holding it all together, particularly at the point when you zip it all up at the end of the day.