Tatra introduced an air-cooled flat-four engine in the 1926 Tatra 30, followed by the T52 in 1930, T54 in 1931, T57 in 1931, and T75 in 1933, all with air-cooled flat-fours of varying displacements. The 1936 T97 model pioneered the rear-engined, air-cooled flat-four, backbone chassis layout, later copied in the Volkswagen KdF-Wagen.