For a 30L backpack, takes a close look at the Osprey Momentum 26 pack. It looks perfect to what you're after, and designed with bike commuting in mind. It's 26L standard, and expands to 31L. Available in "bamboo" and "carbide" colors.

http://www.ospreypacks.com/en/produc..._1/momentum_26

Here's a link to the owners manual:
http://www.ospreypacks.com/en/document/download/155

Hint: Osprey packs tend to run a little larger than expected, in comparison to other brands, so don't be afraid to look at the S/M version, instead of the M/L version.

Osprey has the best warrantee in the business too:
http://www.ospreypacks.com/en/web/all_mighty_guarantee