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Shim Stack
Number 1 is the main damping piston that cycles
through the damper fluid, the piston is surrounded by a glide ring
that contributes to the stability of its rapid cycle, housed in the piston
are oil ports, their size determined by the compression or rebound
circuit, and control is in the form of thin damping shims, stacked
pyramid style with the larger shims covering the ports (
2 ) The force of stacked shims contribute to
resistance as oil flow and shock shaft speed increases or decreases,
these shims are stacked on either side of the piston and size and number
of shims determine compression or rebound
oil flow.
The flat red lines in figure 1 indicates the
oil travelling through the piston ports and ( high speed ) oil flow is
regulated by the shim stack, the oils initial transfer into the piston
is slowed by a deliberately choked entrance and then met with
considerable resistance before exit, the oil flow determined by shim
configuration......altering the shims, alters the damping
characteristics.
The piston is attached to the hollow shock
shaft ( 3 ) which also houses the rebound needle, oil flow through the
hollow shaft is metered by the needle valve, this a separate port
from the piston and the oil flow is restricted as the needle is
adjusted in, ( by means of an external adjustment control ) or the
opposite effect when adjusted out, the thin red line indicates
oil flow ( low speed ) through the hollow shock shaft.
The direction of flow in figure 1 is the
compression stroke which makes it obvious that rebound and compression are
affected by the same adjustment by the needle, although increased oil
flow ( high speed ) is directly effected by the shim configuration on
either side of the damper piston. The over all design of the
needle orafice, piston ports and
stacked shims all contribute to damper
operation at various shock shaft speeds, and high and low speed
compression and rebound forces are effected by this
configuration.
The diagram on the left shows the shim stack , damper
piston and their location in the shock internals.
Shims and stationary pistons ( valves ) can often be
located elsewhere in the damper, a blow off valve normally mounted between
the main damper piston and internal floating piston consists of a valve
and shims, some designs use the stationary valve and shims in separate oil
routes and directly effected by external adjusters for high and low speed
damping, or in some cases a single shim is used in an area to divert oil
flow elsewhere but allowing a low resistants on the oils
return.
Shims can wear out over time, the constant opening and
closing causes fatigue and in some situations the largest shim covering
oil ports on a piston will split an area the same shape as the oil port it
is covering, often the rebound shims suffer first, and once a shim fails
the damping becomes non existent, the small bits of debris will damage
other internal parts if left with out
repair. | 
