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If you want to increase capacity with that set-up, consider a cape roll from Carradice as well. It lashes on top of a large saddlebag. I don't need mine too often with the Zimbale's 18 liter capacity (which IIRC is comparable to a Nelson), but it's nice to have when I need it. It's a great way to carry a coat or sweater or something like that, saving the internal space of the bag for other things.