Analysis Help: Free Modal Analysing with Ansys 13

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In free-free modal analysis using Ansys 13, the expected outcome includes six rigid body motions with zero frequencies, but the user observes only three linear displacements at zero frequency while the rotational motions have non-zero frequencies. This discrepancy raises questions about the analysis setup. For a box-like structure with wheels, the analysis should ideally be performed unconstrained, as the wheels are not fixed to the ground. Clarification on the correct modeling approach and potential reasons for the observed frequency behavior is needed. Understanding these factors is crucial for accurate modal analysis results.
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I am a new user in this forum. In free- free modal analysing, there is 6 rigid body motion (3 displacements and 3 rotations) with frequencie's of 0, but in my analysis there is 3 rigid body motions with frequencie's of 0 (linear displaements) and the frequencie's of others (rotational rigid body motons) are not zero. I do not know this reason. Please help me. I am analysing the model using Ansys 13.
 
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Hi,
If I wanted to get the natural frequencies of a box-like structure with four wheels (at each corner of the bottom surface) by means of FE-analysis, should it be performed as constrained or unconstrained ? Indeed, the wheels are not bounded to the ground, so please, could you help me?
Thank you very much
 

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