Initially, the first aim of the present study is to model 3-axle rigid truck by developing a code in Matlab. Secondly, each equation should be drived by acquisition of Lagrange approach. After deriving equations successfully, validation is performed by some result which was mentioned in Bohao Li's master thesis. Although the present approach is totally different from the approach that Boahao Li used in his analysis but the deviation of the present results from that thesis on average is 7 percent which shows an acceptable accuracy in our current numerical approach. In the present study 19 degree of freedoms is used for determining natural frequency and dominant motions. By having dominant motion, it is possible to clarify which region is in endangering of resonance. This paper presents a portion of the results that have been gathered and for watching new result in this field, it is recommended to read our future papers. Solidworks CAD software is also used to calculate mass properties of several components that used in our present study. پرونده مقاله