How much do you need to carry in the backpack?

They do make cycling specific backpacks designed to keep airflow going over your back as much as possible, which is generally the biggest problem with a backpack on a bike. There's others, but I'm lazy and don't feel like writing about it. :-)

Here's one with a decent carrying capacity - the Ergon GC3:
http://www.ergon-bike.com/us/backpac...c0it728244sn4#




Here's another one that wouldn't carry a lot but it also doesn't obstruct your view backwards and it can take a water thingy (like a camelback):
http://detours.us/product_info.php?c...roducts_id=110





However, NO backpack is going to be as comfortable as putting a rack on your bike and using panniers or a trunk bag.