I've a Osprey Atmos 50ltr sack, it's a decent sack but I wouldn't even entertain carrying it on the bike, that's for sure. I'd agree with Peter Clinch above, panniers would be more comfortable.
You could check out second hand ones if money's an issue. Otherwise a mountain marathon sack like the KIMM (now OMM) 35ltr sack or MacPac Amp 20ltr is probably the best bet.
Thanks for your interest in our Atmos packs. To answer your questions, the most recent model of the Atmos 50 has the colour coded water-resistant zips on the outside/front pockets (as can currently be seen on our website. The earlier model had black ones). Also, the most recent model does not have access to the main compartment through the front pocket(s). It's designed as a top-loading pack, also benefitting from a floating lid. Hope this clears things up a bit for you!
I really think that if you are going to cycle with a rucksack on you need one that is held close to your back (no fiddly air frame system) and doesnt have internal stays or frame.
Just an opinion but having done lots of cycle camping, I find a small frameless rucksack of the OMM ilk and panniers or just pannier frame (to attach tent and other bulky items) is a really good system. After all, if you have a bike and want to set up camp and THEN walk around, why use a 50litre sack when you can get away with a lightweight 25litre one?
