This recommendation applies to operator seats which may be fitted to agricultural wheeled tractors per SAE J1150 (December 1976) and/or earthmoving machines per SAE J1057 JUN81. Agricultural wheeled tractors and earthmoving machines having similar vibration spectra are grouped into defined classes with corresponding test vibration input spectra defined for each class in SAE J1385 and SAE XJ1386. ¹ Performance for all applications within a defined class requires tests with subjects of specified weights. The specified power spectral density at the seat mounting base defines the response required from the vibration test stand during a given laboratory seat test.