Modern motocross bike suspension uses very complicated hydraulics.
The hydraulic systems have variable, position and speed sensitive oil pressure relief capabilities.
Valving shims are used to regulate and control the amount of oil flow through your suspension. The thicker and/or larger diameter of shim, the less likely the shim will allow oil to bypass it, causing what we refer to as "damping".
These work similar to a reed valve in a 2 stroke, as far as reed tention,
thickness, and tip surface area are concerned.
These shims are located on something called a piston.
The suspension's oil is pushed through the ported orfices within the piston, and the valving on either side of the piston, depending on the direction of oil flow. The valving shims on one side of the piston flex when oil pressure/oil flow is applied in the appropriate direction. The amount of shim flex is determined by the shim's size and tention, much like a normal reed valve.
When a series of shims are assembled together, they are considered a "shim stack". This is a variety of shims assembled starting with a large diameter shim, progressively getting smaller in diameter and or thickness as the shims are positioned together.
Most properly set-up shocks have at least 5 of these shim stacks. The "low speed compression stack", which is usually the largest/stiffest stack in the shock, is positioned closest to the piston.
The next shim stack to follow is the "mid speed compression stack". This is positioned right behind the low speed stack. It's largest shim is positioned behind the smallest diameter shim on the low speed stack. The small diameter shim at the end of a shim stack is usually reffered to as the "transition shim", if there is another stack postitioned behind it.
The next shim stack is called the "high speed compression stack". This is positioned behind the mid-speed compression stack, in the same manner.
All of these compression shim stacks are on the same side of the piston.
These shim stacks work basically the same as a "Boyesen 3 stage reed block", where there is 3 different sized/tentioned reeds stacked on top of each other. You probably have seen pictures of these in magazines. The operating principals are very similar.
At very high amounts of shock travel, the total low speed damping is done exclusively with the low speed compression stack. The total amount of mid speed damping is done with the low speed AND the mid speed stack combined. The total amount of high speed damping is done with the low speed, mid speed, AND the high speed stacks combined.
At very low amounts of shock travel, the low speed stack will do the duties of low, mid, and high speed compression damping.
At moderate amounts of shock travel, the low and mid speed stacks will do the duties of low, mid, and high speed compression damping.
The high speed stack only works fully at very high amounts of high speed compression.
This is like a main jet in a carburetor. The only time the main jet works to it's full extent is if the engine RPM is very high, and it has been there for a moment or two.
Just like a carburetor, the total high RPM jetting is determined by the
low, mid and high speed circuits COMBINED.
On the opposite side of the piston, we have a low speed rebound stack,
and a high speed rebound stack.
These work identically as the compression stacks, they just work in the opposite direction.
The thickness of a valving stack's transition shim determins the rate and time of which that particular stack will start to open the next stack behind it.
Forks use the same basic principals. Most forks have seperate compression and rebound pistons. The low and high speed compression valving is on the compression piston, and the mid speed compression valving is done on the BACK of the REBOUND piston.
The rebound valving is on the rebound piston.
On a shock, the compression/rebound piston moves through the shock oil.
On a set of forks, the rebound piston moves through the fork oil,
and the compression piston is stationary.