The big one a new manufacturing process makes this the stiffest44 most weather resistant mtx bags topeak have produced perfect rear rack mount storage bag with side panniers for light mountain touring44 road touring44 commuting and errands large capacity expandable main compartment with two large side panniers44 and a separate top compartment foam walls provide padding and insulationfor use as a cooler or camera bag 420 denier nylon for superb weather resistance elastic top bunge