The agricultural flexible chassis (FC) is a four wheel hub motor independent drive, four wheel omni-directional independent steering chassis. Its structure is very simple, even the steering motors used in the ordinary steer-by-wire system are cancelled. It steers through an offset steering shaft mechanism. It is very flexible and can realize special movements such as four-wheel steering, in-situ turning and lateral driving. When the vehicle body is stationary and the offset arm is not locked，the rotating electric wheel will steer around the offset steering shaft to switch the vehicle form for in-situ rotation or lateral driving. The study on tire mechanics performance of static steering based on offset steering shaft (SSBOSS) is the key and foundation for in-situ rotation and lateral driving control of FC. The tire patch kinematics was analyzed to study the longitudinal and lateral slip. Then, the existing tire longitudinal slip LuGre model was extended to the longitudinal slip-lateral slip combined SSBOSS LuGre model. The SSBOSS tire mechanical characteristics test device was designed. A two-factor test was used to test the effect of offset distance and load on the lateral and longitudinal friction of the tire. The parameters of LuGre tire model were identified based on the measured results, and the tire friction during the static form switching of the FC was simulated based on the identified results. The results showed that the measured and simulated results of the lateral and longitudinal frictional forces had a good consistency. The tire was subjected to a longitudinal frictional force that hindered rolling and a lateral frictional force that pointed to the outside during FC static form switching. And the magnitude of the longitudinal frictional force was proportional to the 1.82th power of the load and inversely proportional to the 1.61th power of the offset distance. And as the offset distance was increased，the lateral frictional force was firstly increased and then decreased，but the change was relatively flat. The research result can provide a basis for the estimation of the steering drive torque and the optimization of the device parameters for the FC.