At the time when the first vague ideas about the S-tank started to take form (late 50’s), weapons technology was far more advanced than armor technology. It seemed prohibitively hard to design a tank that had sufficient protection against modern HEAT and APDS shells while also maintaining decent mobility and staying within reasonable weight limits. When the French and German governments cooperated on the Europa-Panzer project (what would become the AMX-30 and the Leopard 1), one of the stated design goals was to sacrifice armor strength in favor of mobility and weight savings. The British Chieftain was considerably better protected but especially in its early versions suffered from high weight and poor cross-country mobility.