Originally Posted by
Jim from Boston
I use my carbon fiber endurance bike (Specialized S-Works) for commuting, and my best solution has been to use a seat mounted (not seatpost) Arkel 15 Liter Bikepack. I have sufficient room to carry just about everything I need. I might add a lightweight backpack for bulky, but light items. 
That's a nice bag, I might look into this. Thanks for posting.