"Four Post" Rigs
comprise four road input electro-hydraulic actuators, one supporting each
wheel. The position of each actuator is controlled independently to follow
a prescribed displacement time history. Four post rigs are usually equipped
with two or more passive devices to simulate non-varying aerodynamic forces
acting on the vehicle sprung mass.
"Seven Post" (or
even eight post) Rigs comprise four road input electro-hydraulic actuators,
one supporting each wheel, and a further three (or four) actuators attached
directly to the vehicle sprung mass. The forces applied by the additional
actuators are controlled independently to allow pitching and rolling moments
acting on the sprung mass to be simulated, as well as varying aerodynamic
forces acting on the sprung mass.