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.