Instead of a backpack, what about a lumbar pack? They sit low, around your lower back. No shoulder straps. I've just gotten back into cycling after a long hiatus and I used a lumbar pack previously and liked it. What I dislike about a backpack is how my back gets sweaty.

But you're probably not likely to be able to cram everything into a lumbar pack that you did into a decent sized backpack.

In any event, I just ordered this and am looking forward to not having a sweaty back any more.

http://www.amazon.com/Mountainsmith-...1416514&sr=8-1

(Also note the customer reviews; all good except for one.)