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The Valve Stack
I will describe how the valve stack works within a
hydraulic damper – i.e. - fork, or shock. In this discussion I will
only be dealing with the speed or pressure sensitive type system, not the
position sensitive type system.
As I explained in my last column, the damper unit is a
closed system which utilizes a chamber or air space through which the oil is
circulated. A piston is mounted on a shaft or rod that travels in a
cylinder – i.e - shock body or cartridge tube. The piston is actually
a disc with holes machined into the top continuing through the bottom, in a
special pattern such that some are machined more deeply than others,
allowing two actual levels of holes on each surface – one set acting as exit
holes and one set acting as entrance holes. The entrance holes are the
recessed holes of each surface while the corresponding exit holes on the
opposite surface are flush. To control the restriction of oil through these
holes, thin spring steel shim washers, when stacked on each surface of the
piston, cover only the exit holes on each side. The recessed entrance holes
allow oil to pass through the piston, compressing the oil against the end of
the damper unit and the shim stack on the exit surface of the piston. This
creates pressure on the shim stack. The shim stack on the top of the
piston controls the compression dampening while the shim stack on the bottom
of the piston controls the rebound dampening. As the piston moves in the
damper, the pressure increases and will become great enough to flex the
outer edges of the shim stack, allowing oil to begin to flow into the
opposite chamber in the damper unit. The amount of flexing of these
shims controls the resistance of the oil flow, thereby slowing or increasing
the damper’s movement. As the speed of the piston increases, the
volume of the oil being pumped is increased and therefore the pressure
exerted against the shim stacks is also increased, causing the shims to flex
more. (Smaller bumps create less piston speed, therefore creating less
resistance, while larger bumps create greater piston speed, therefore
creating more resistance.)
As I stated earlier, shims are thin spring steel
washers. They will be stacked up in pyramid fashion, with the largest
diameter shim against the piston surface, and decreasing in diameter as the
stack height increases. More shims, thicker shims, smaller diameter
shims create more resistance, while less shims, thinner shims, larger
diameter shims create less resistance. A GOOD suspension
tuner can adjust or control the amount of flexing of the shim stack by
varying the thickness, the diameter, and the amount of the shims used in the
stack. Usually the REAR SHOCK will contain shims varying in
size from 22mm in diameter to 40mm in diameter, and the thickness will vary
from .1mm to .3mm. FORK shims are much smaller,
usually varying from 10mm to 24mm in diameter and from .1mm to .15mm
in thickness.
The ADJUSTMENT SYSTEM usually consists of a hole
drilled through the damper unit SHAFT entering the shaft below the
piston and exiting the shaft through cross drilled holes located above the
piston, serving as a bypass of the shim stacks. If this bypass passage was
not controlled in some way, the shim stack would not offer as much
restriction of the oil flow because some of the oil would be allowed to flow
around the piston and shim stack. Therefore, a tapered adjustment
screw is added to control the amount of oil bypassing the shim stack. When
this tapered screw is turned clockwise, the bypass passage is closed down,
allowing less oil to bypass the shim stack and
therefore more oil to be forced through the shim
stack, thereby creating more restriction - resulting in harder suspension.
When the screw is turned counterclockwise, the bypass passage is opened up,
allowing more oil to bypass the shim stack and
therefore less oil to be forced through the shim
stack, thereby creating less restriction - resulting in softer suspension.
Most FORKS actually have a separate rebound
piston and shim stack and a separate compression piston and shim stack.
The compression piston in the fork does not move. It is usually
referred to as a base valve because it is mounted in the base or bottom of
the fork. The rebound piston does all the work. It moves up and
down in the cartridge tube, forcing the oil through the compression (base
valve) piston and valve stack and then sucking the oil back into the
cartridge tube and rebound stack for rebound restriction i.e. - dampening.
Each of these pistons has a spring-loaded one-way shim on the opposite
piston surface from the actual shim stack. This one-way shim will lift
so as not to restrict the oil flow in one direction and will close allowing
restriction or dampening in the opposite direction. This feature
allows the two pistons, compression and rebound, to work independently of
each other, and keeps the cartridge full of oil and ready for the next
cycle. |
