A bicycle tube is predominantly made of Butyl rubber. Butyl is a very elastic and airtight synthetic rubber. But as with the tire, other fillers are necessary to make up the rubber compound. The quality of a tube can vary significantly depending on the rubber compound. For instance Schwalbe tubes have extremely high air retention and elasticity. This high elasticity allows a wide range of different tire sizes to be covered.\n\nThere is a difference between heat molded and autoclave tubes. Vulcanizing in a mold improves the uniformity of wall thickness, thus reducing weight and improving air retention. For this reason the autoclave tubes which can be produced easier have become less common over the past years. Schwalbe tubes have always been made using the heat molding process.\n\nEvery tube also needs a valve, which is bonded to the tube during the vulcanizing process.