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My full size super c on the expedition rack is very stable. It starts to wobble when I've been to the shops and bought to many tins of beans/beer but even then it holds well. I have the qr too, but have never used without so can't comment on extra stability. Also can't comment on stability with the rack vs without - I've only ever used this setup.
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Depending on how heavily loaded the bag is, it can get fairly wobbly when out of the saddle in my experience (but others may offer differing opinions as you can read). I like the QR system as I can just take the whole bag on and off really easily eg when commuting, or leaving the bike on a ferry or train. Can't say whether it changes anything stability-wise.
Considering buying a bagman sport rack for my super c audax, how much movement is there when bag is sat on the rack? And does the qr bracket version make it more stable, or just easier to pinch?