Schwalbe 18" Inner Tubesretain air pressure considerably longer than others. Each Schwalbe tube is individually inflated and inserted into a mold. This ensures the even wall thickness and excellent air retention. Schwalbe 18" Inner Tube Features The quality and the material purity of the rubber mixture directly effects the quality of the tube. Consistent quality control brings with it the security of use Each tube is inflated at the factory and stored under pressure for 24 hours in order to test the air retention Group Tube System. Available Size Options 18 x 1.50/1.75" Schrader Valve 18 x 1.75-2.35" Presta Valve