Penske Racing Shocks 8900 Series - Manual (Page 11)

11
Compression Adjuster
8986 Series Compression Adjuster
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 compres-
sion 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.
Fixed Low Speed Bleed Circuit High Speed Flow Circuit