Penske Racing Shocks 8900 Series - Manual (Page 5)

COMPRESSION ADJUSTMENT
In the state of low shaft velocities (i.e. corner entry, exit, and power down), oil is
displaced within the damper in direct proportion to the volume of the shaft
entering the body. The displaced fluid passes through the compression adjuster
where it is metered through a fixed, low speed bleed orifice. Due to the small
diameter of this orifice and the viscosity of the damper fluid, a pressure loss
occurs across the orifice. This loss of pressure is a loss of energy in the fluid
due to friction and the subsequent opposing damping force is generated.
As the shaft velocities increase, the same amount of fluid must pass through the
low speed bleed orifice, but at a much higher rate. The viscosity of the fluid
causes a greater resistance to flow at the orifice entrance which in turn
produces a large internal force on the CD housing. The other major internal
components, namely the piston and shim cage, are designed to handle this
extra force by allowing the shims to “blow off” proportionally to the extra force
generated, much like a coil spring compresses proportionally to the axial load
applied. With this arrangement, the low speed bleed orifice still meters fluid
during high speed shaft movements, but the extra forces generated are handled
with the shims which have less resistance to flow at higher velocities. They are
designed to virtually bypass the low speed orifice and form a new fluid circuit.
The force at which this occurs can be varied by turning the compression
adjuster in or out, which preloads the shims. Therefore, as the preload on the
shims increases, the static force required for them to activate is increased as
well. The name designation for the parts also clue one in to their purpose, with
the low speed bleed orifice handling low velocity bleed flows and the piston/
shim arrangement handling high velocity flows. This principle originated in the
main shaft piston/shim arrangement and follows similar behavior.
NOTE: When making adjustments, use the full soft setting (adjuster wound all
the way in against the reservoir body) as a starting point when counting the
number of “clicks” to the desired setting. The full soft setting should correspond
to a clicker number designation of 0. This starting datum has been proven to be
most reliable and repeatable when making compression adjustments. There are
22 +/- clicks of adjustment.
8983 Series Compression Adjuster
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