Friction of suspension linkages and and any sliding elements, including
the dampers, will add to the overall damping forces generated by the suspension.
Friction forces can, with care, be estimated during a rig test. In any case,
a rig-derived suspension set-up will take friction forces into account automatically.
It is generally considered to be advisable to minimise suspension friction,
because it (and hence overall damping of the vehicle) can vary with time
(wear), load (particularly true of sliding friction), temperature, and assembly
Most vehicles exhibit a finite suspension "installation"
compliance. This makes effective suspension motion ratios (defined as
wheel velocity per unit damper velocity) depend upon the loads reacted by
the suspension. Since damper loads increase with increasing frequency, the
effective motion ratios will also increase with increasing frequency. Installation
compliance can be estimated from the results of a rig test, provided damper
positions are measured.
Is installation compliance important? It can be. One vehicle tested on
the Thetford rig had been raced with an effective spring stiffness that
was only 50 percent of that computed using kinematic motion ratios at the
heave mode frequency. At higher frequencies the reduction in effective damping
meant that the hub modes were poorly controlled, so that tyre grip was lower
than it should have been.