This attack pack executes its mission perfectly: efficiently deliver fluids to mountain bikers, skiers and trail runners in a compact unit. The internal Osprey Hydraform reservoir holds two liters of water to help you stay hydrated on longer rides and runs. The water hose emerges from the pack atop the shoulder and wraps around the front of the chest. Strong magnets keep the mouthpiece securely fastened to the chest strap so that you’re not constantly searching for the hose. Viper 4 is long and narrow with a low profile so that it doesn’t impede your movements or bounce around even while you’re bombing down rough trails. Storage is minimal with a small heat-embossed stash pocket and shoulder-strap pocket for essential effects like multi-tools and nutrition. Viper 4 includes an outer bungee lacing system to stow extra layers of clothing and the Osprey LidLock attachment to carry your helmet for transport.