Rigid or Flexible Multibody Simulation — and How to Tell
At the start of every multibody simulation stands a decision that governs how meaningful the whole model will be: rigid or flexible bodies? Get it wrong and you get forces that are either needlessly expensive or simply wrong. Yet the question can often be pinned to a single criterion.
The Standard Case: Rigid Bodies
In many multibody simulations it is enough to assume the components are rigid. You are interested in motions, velocities, accelerations and the resulting forces and moments in the joints — not in the deformation of the components themselves. The rigid calculation is fast, robust and usually sufficient for load determination.
When Rigid Is No Longer Enough
It becomes critical when the system is statically overdetermined. With an overdetermined support, the force distribution depends on how stiff the components are — and that is precisely the information missing from a rigid model. A rigid calculation then distributes the forces according to the model assumption, not physical reality.
In such cases the relevant bodies must be modelled as flexible. Only when the stiffness of the components enters the model does the force distribution settle as it occurs in the real system.
The Rule of Thumb
Statically determinate support and only interested in motion and joint forces? Then rigid is enough. Statically overdetermined, or the component deformation affects the result? Then there is no way around flexible bodies.
Conclusion
The choice between rigid and flexible is not a question of effort but of physics. Pinning it to the question of static determinacy usually gets it right — and gives you forces that fit reality.
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