Dynamic modelling of too heavy truck

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Hello all,

I have to model an articulated truck dynamically.The truck is very heavy (> 15000 kg). I am doing that using xacro format but still have some problems.
1- How can one choose the right specifications of the wheels to be as possible realistic (mechanically the wheel can be represented by a spring and damper in parallel). actually I am using for that the parameters (mu, kp, kd) but I have no idea how to choose their values.
2- In the picture below, how can one define the spring/damper between the axle and the truck chassis? And how can model the shown "A_frame" ?

Thanks in advance.