SCHWALBE INNER TUBE 16 x 1 3/8 - 18x 1.35 SCHRADER VALVE AV4 Schwalbe tubes are made with a special butyl rubber compound which makes them extremely airtight and extremely reliable. Less pressure drop means safer cycling, fewer punctures and reduced rolling resistance, plus you don't have to inflate them so often. Airtightness. Apart from the quality of the ingredients, the purity of the rubber mixture is crucial to the quality of the tube. The base material is therefore sieved at enormous pressure through seven filters before being extruded. All tubes are inserted into a mould and inflated for vulcanization to ensure constant wall thickness and high air pressure retention. Reliability. All tubes are inflated and stored for 24 hours in order to test for air pressure .