See also: Soft Baggage Comparison
I said this already: soft bags may be ancient, pack animal-era technology but they have very much caught on in advworld. Some are little different to the things I was throwing over my bike 40 years ago; one or two feature significant innovations in mounting, fabrics, lockability and more.
On advrider (as well as in my own review) questions got asked about the volume claims of the GL Siskiyou pannier: 34L said GL, while me and another guy measured l x h x w as near as we could and came up with 24L.
‘Aha!’ the bloke from GL replied – we establish volume by filling out our bags with beans until they bulge out and that way get 35 litres so that’s what we rate them at. It sounded plausible and got me thinking: what is the maximum volume of a shaped, non-elastic but flexible rectilinear container like a motorcycle side pannier? Logic suggests as the box form flexes out sideways under the weight on contents, the shorter side will pull in and the volume will remain constant.
But intuition (or maybe logic too) suggests capacity ought to increase: the classic Envelope Test performed by an obscure Cartesian monk, Antoine de Connerie in front of a disbelieving King Philippe V in 1444:
An envelope is a flat container with a volume of next to nothing; open it a little and volume increases, open it a lot and volume increases some more up to a point when opening it out too much will reduce volume to near zero again as it folds back in on itself.
Al Jesse [Luggage] and I discussed this: he reckoned volume of a rectilinear vessel is fixed, but I was not convinced and now I have the answer: If the flexible container is a cube (l x h x w all the same) volume when filled (with beans, water, anything non-compacting) will not be altered much. There may be some fabric bulge.
But a rectilinear flexible box (‘suitcase’) seeks to attain the geometric nirvana of cubic equilibrium and does deform and expand substantially. L x w x h on my Magadans rolled up and clipped came in at 24L (left). It doesn’t sound so much and would be identical to a 24L metal box.
But, fill the Mags with water and you’ll easily get 40 litres in each side, as the pictures right and below show. Seems hard to believe but there are no less than two fills of that 20L white bucket inside the Mag bag, rolled up, clipped down and ready to roll were it not for the fact that it would give me a hernia trying to lift 40kg (88lbs).
Does this all really matter? Yes, it does because for a start, the l x w x h method doesn’t truly represent the maximum potential volume (MPV) of a flexible, non-cuboid container, even if the maxed-out 40L capacity demonstrated on the left is unlikely to be achieved in the real world of packing your panniers with normal travel stuff.
It matters all the more when trying to compare stated fabric pannier volumes with rigid metal or plastic boxes as a guide to buying one or the other. My comparisons in the table at the bottom uses the l x w x h method but that only compares like against like. In all cases you can get more in your bags.
Even then, I think the dimension ratios of a flexible container may also have something to do with it. I recall the guy from Enduristan saying something like the reason their original Monsoons (left, reviewed here) are wide (closer to a cube form) is that they have/can make more volume (by presumably having less far to go to reach ‘cubic optimisation’).
But on a motorcycle I still believe slimness is a desirable attribute and is something that for example, Jesse Luggage strive to maintain in their mounting systems and cases. Al likes to boast that some of his rack and box setups are narrower than competitors’ racks alone.
So, in summary, think carefully when comparing stated rigid box volumes against fabric panniers. A rigid box’s capacity is immutable but a soft bag may be more than you think.
The Magadan was used because it was the pannier I used at the time, but this test would work and give similar results with any similar product.