The united body is calculated correctly for the mass of the overlapped part between bodies. However, in the case of merged body, the mass properties are just summed with each body’s mass property.
Figure 1 Overlapped part between Body A and Body B
Additionally, when the user merged a sphere or a cylinder created by RecurDyn, it is possible to apply the primitive contact. However, in the case of the united body, the only general contact should be used.