Ergon BX2 Backpack
Ergon's mid-sized mountain bike pack has an innovative 'Adaptive Carrier System' that holds the load close to the body and has rock solid stability. 4 position back adjustment allows every rider to find the perfect fit and comfort. The pack has a capacity of 10 liters (expandable to 11.5 liters using the zippered expansion facility) and features a main compartment, bladder compartment, rain cover, hip belt pockets, and organization compartments. Also features self-correcting shoulder straps.
I used to rock a Camelback that didn't have a waist strap and oh boy is it awesome to ride with a pack that does. This pack is plenty big for day trips and is INCREDIBLY comfortable. The pack really hugs your body and there is even an aluminum bar in the lower back that you can bend to the shape of your body. The small vs. large only affects shoulder strap size but I don't know more than that so I just ordered a large(6ft 155lbs). The pack stays in one place on rides and fits my Camelback 48oz baldder great! The pack also includes bungee straps for a helmet etc... and a rain cover.