Shims or
Valves
A damper piston attached to a shock shaft has
specifically designed oil ports machined through the piston, the
entrance to each port is positioned on the outer edges of the piston which
exit closer to the centre on the opposite side, depending on
compression or rebound circuit the size of the port is relevant to flow,
to control the amount of flow, shims of varying size are stacked pyramid style, with the largest
shim covering the exit of particular oil ports, the remainder of the shims
are stacked according to size, finishing with the smallest. This
configuration of shims is a variable gate and only effects oil going one
way, on a standard shimmed damper the shim stack is on both sides and
surrounded by the strategically placed oil port entrances. i.e. rebound
shims and compression port entrances on the same side of a damper piston,
etc.
External Control
A standard shimmed damper piston controls oil flow in both
directions via stacked shims covering oil ports, if an external rebound
control is introduced it requires a specific internal oil port, this is
achieved by an orifice in a hollow shock shaft where oil can bypass the
shimmed piston and flow is metered by a needle that seats into
the tapered base of the damper piston bolt, the bolt is hollow and
this contributes to an oil transfer port that allows minimal oil flow
to both sides of the damping piston. When rebound flow of oil is
adjusted via the needle orifice, compression flow is effected also. This
also contributes to equalized pressure on both sides of the damping
piston.
Low Speed Compression 1 ( Fox DHX
)
With some platform dampers the minimal compression
flow through the fixed shaft orifice is closed
( indefinitely ) by a one way valve ( situated on the hollow
piston bolt ) that allows rebound oil flow only, and the returning
oil ( rebound ) is now controlled by slight resistance of the one
way valve. Compression flow is now forced through the compression
shim stack ( high resistance at low speed ) on the damping
piston and this modification alone inhibits rider induced
movement.
Adjustable Propedal ( Fox DHX
)
The more complicated ( adjustable ) propedal still retains
a standard damping piston with stacked shims either side, complete with
rebound needle orifice and tapered hollow bolt low speed oil transfer is
still possible. To attain a separate compression
circuit a position sensitive valve hovers at the base of a reservoir, the
position of the valve is dictated by pressure behind the floating piston
and opposing forces of shock shaft and damper piston as it cycles through
compression and rebound. An external propedal knob adjusts the
threshold of low speed oil flow and although the propedal adjustment
is integrated with the position sensitive valve it effects low speed
compression resistance only.
Low Speed Compression 2 ( Manitou, 5th
Element, Curnutt )
By utilizing the one way valve ( situated on the hollow
piston bolt ) rider induced movement is reduced, and compression flow can
now be diverted to one area of the damping mechanism ( SPV Valve ), the
pressure change that occurs on either side of the damping
piston during compression or rebound can be isolated and unaffected
by the minimal oil flow as in a standard shimmed damper Specific
pressure must be maintained either side of the damping piston if the SPV
is to operate correctly.
Stable Platform Valve ( Manitou, 5th
Element, Curnutt )
Designs like SPV divert oil flow ( compression ) directly
through the damper piston ( no compression shims ) to the face of the
SPV situated directly behind the piston, ( standard rebound shims are
retained ) the SPV is two parts, cupped and sealed with two quad rings,
integrated, the two parts create an internal cavity of atmospheric
pressure ( pneumatic spring ). The face of the valve blocks the exit of
compression oil ports, this due to the higher pressure behind the floating
piston, in this state the pneumatic spring created inside the SPV valve is
compressed, the valve under pressure will only open ( to allow compression
oil flow ) when the change of opposing forces dictate, obviously increased
oil flow from bump force, ( followed by a drop in pressure behind the
damping piston on the compression stroke ) decreasing
the resistance on the SPV and it opens accordingly to allow oil flow.
With a drop in pressure the SPV valve closes and shuts off
compression oil flow, this also prevents the piston travelling
further than necessary into the compression stroke, with the valve shut,
the shock rebounds, and diverts oil flow through the rebound only
shim stack. As the collective actions of plunging shock shaft, shimmed
piston with attached stable platform valve takes place, the
opposing forces regulate oil flow during compression, adjusting pressure
and volume as the particular threshold dictates.
When the pressure behind the floating piston is increased
( via shock pump through the schrader valve ) the threshold of
resistance is raised and a larger bump force is required to unseat the
SPV, when pressure behind the floating piston is decreased, less force is
required to open the SPV, and the suspension becomes more
active.
