Osprey viper 13 hydration pack the largest volume in the viper series the viper 13 has a front stretch mesh pocket interior dividersin shove it pocket that creates three seperate compartments and a lower compression strap to pull weight close to the body with these extra features the viper 13 is the pack best suited for 3 hours plus of riding with the added capacity for extra food layers and tools this hydration pack is also ideal for longer trail runs and day hikes features ergopull hip