Ultra-durable and 100% waterproof with a special oval shaped bottom. These use a much thicker material than the silnylon stuff sacks most people are used to. Pair it with a 3-bolt mount like the Many Things Cage or strap it to your handlebars with the Handlebar Harness. Bikes with minimal handlebar-to-tire clearance will need the 5L versions. The opaque/see-through ones have a PurgeAir™ valve.