  This paper discusses the utility of using simple stiffness and vibrations
models, based on the Jacobian matrix of a manipulator and only the rigidity of
the actuators, whenever its geometry is optimised. In many works, these
simplified models are used to propose optimal design of robots. However, the
elasticity of the drive system is often negligible in comparison with the
elasticity of the elements, especially in applications where high dynamic
performances are needed. Therefore, the use of such a simplified model may lead
to the creation of robots with long legs, which will be submitted to large
bending and twisting deformations. This paper presents an example of
manipulator for which it is preferable to use a complete stiffness or vibration
model to obtain the most suitable design and shows that the use of simplified
models can lead to mechanisms with poorer rigidity.
