Seven post rigs are designed to carry out "circuit simulations",
in which the force-controlled actuators attached to the sprung mass are
used to apply loads computed from track measurements and the position-controlled
wheel actuators are driven by signals computed to minimise the differences
between selected track measurements and similar measurements recorded on
the test vehicle. Clearly, track and rig measurements must correspond closely,
and the track and test vehicles must be matched in order for the process
to stand any chance of success. Arguably, unless all vehicle States can
be measured or deduced in both cases, then more than one solution exists
to the track simulation problem.
In essence, circuit simulations are used as an alternative to track testing.
They have the disadvantage that track conditions cannot be simulated completely
(no side loads acting on the tyres, for example), but have the advantage
that contact patch loads and road input accelerations can be measured directly.
Circuit simulations can apply large loads to the vehicle, and can generate
correspondingly large response accelerations. A consequence of this fact
is that measurement sensitivities are generally lower than would be the
case for a four post rig. In addition the force-controlled actuators usually
generate added "noise" and sometimes constrain vehicle response.
Thus, although a seven post rig can be used for vehicle characterisation,
it would appear to be the case that the results are of lower quality than
would be expected from a dedicated four post rig.
Finally, it is necessary to attach the force-controlled actuators securely
to the sprung mass of the vehicle under test in order to achieve stable
operation of the rig.. This implies that installing a new vehicle on a seven
post rig is an expensive and lengthy process, and one that can result in
damage to the vehicle.
For all of these reasons, seven post rigs tend to be dedicated to a particular
vehicle and to a particular race team.