We want to look at the shim stack as a multi-dimensional spring made up of its various components (the shims).
We change the shims within the stack to affect the stacks overall spring rate, which in turn affects the damping force. Since the shim stack is designed to operate through a specific range of deflections, we have to consider the shim deflections and corresponding forces throughout it's entire range.
When a shock is compressed, oil bends the shims as it passes through the piston ports. The slower the piston moves, the less the shims deflect and the lower the damping force (i.e. low speed). The faster the piston moves, the greater the shim deflection and greater the force (i.e. high speed).
Lower velocities produce lower damping force and higher velocities produce higher damping force. We refer to the relationship between velocity and force as the force-velocity curve.
A force-velocity curve refers to the actual stiffness of the damper as measured on a suspension dyno.
To make valving changes, we alter the shim stack to tune the damper.
When making shim changes, the affect on the entire damping curve must be taken into account. When shims changes are made, they almost always affect the entire damping curve, not just one particular range of the curve.
For example, we could add a shim to the low speed stack to stiffen low speed damping, but it would also have an affect on high speed damping.
Knowing how the shim changes affect the damping curve is the key to revalving. There are two ways to make this determination.
Make a valving change and test ride the bike. The rider notices the difference, and the tuner translates the rider feedback based on his interpretation of the valving change.
This method is not always accurate. The shim change does not always do what the tuner intended.
Make a valving change and dyno test the damper on a suspension dyno. The dyno provides a very precise force-velocity curve for the damper. The tuner can correlate the rider feedback to these force numbers from the dyno. The tuner then adjusts the force-velocity curve (via shim changes) to alter the damping.
Method 2 eliminates guessing what the shim changes did. The shim changes are quantified by the dyno, and the tuner is making a direct comparison of rider feedback to actual damper forces.