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The Ortlieb Seat-Pack bag is great for bikepacking and bicycle touring. It is made of polyurethane laminated rip stop nylon, holds a volume capacity of 8 to 16.5 liters, and features easy to use fasteners. This bag can withstand exposure to harsh elements including rain, snow, and mud. I would highly recommend this bag to all riders.
-Elastic chord allows you to attach additional items with easy access
-Central distribution of weight to the seat post minimizes the lug/heaviness of your load
-Maintains shape due to inner stiffener
-Can be bulky and cause tire rubbing the closer your saddle is to your seat collar. (If you ride with your seat post extended up this will not effect you)