A common misconception in shim stack tuning is the face shims control low speed damping and the stack taper controls high speed. Dyno tests and Shim ReStackor show shim stacks do not work that way.

The dyno example below from the MXScandinavia thread on Thumper Talk replaces the 0.2 mm shims in the stack taper section with stiffer 0.25 mm thick shims.

The dyno test results show the shim stack is stiffer everywhere across the speed range, not just at high speed. Shim ReStackor calculations (lines) show the same thing.

High speed damping can be stiffened by tuning the crossover or softened using a preloading ring-shim (linky, fundamentals).

MXSCandinavia went on in the thread to demonstrate the shim stack modifications needed to gain separate control of high and low speed damping:

• Preloading a soft shim stack increases damping force at low speed and softens damping force at high speed
• A crossover does the opposite with softer low speed and stiffer high speed
• A soft tapered shim stack matches the crossover stack at low speed and drops off at high speed

MXSCandinavia went on in the thread to demonstrate the shim stack modifications needed to gain separate control of high and low speed damping.

Tuning to get the desired damping force curve shape is simply done by hacking: adding or removing shim stack preload and crossovers to get the desired damping force curve shape and hacking around on stiffness to get the desired damping force value.

The numerical test bed of Shim ReStackor makes that simple, easy and intuitive.

A faux crossover gap never closes. Faux gaps are created by large crossover shim diameters, stiff low speed stacks or soft high speed stacks that do not produce enough force to close the crossover gap. MXScandinavia provides dyno test examples of faux crossovers.

In dyno testing, faux crossovers behave like a interactive crossover. Changes to the low or high speed stack changes the damping force leading many dyno tuners to believe the crossover gap is active.

However, the crossover gap height never changes as the shim stack deflects. The crossover shims could be moved further up in the stack forming a simple tapered shim stack with the same damping force.

In dyno testing, there is no way to know the crossover gap is faux until the shock is pushed to high enough speed to observe the crossover closing. Soft closures of interactive crossovers make those events difficult to spot in damping force data.

Split crossovers (linky sample apps) use two shims to form the crossover gap. Split shims smooth the bend radius at the crossover shoulder and help prevent kinking of the face shims on the sharp shoulder of a single crossover shim.

Valving Logic dyno testing of an rmz450 used a spit shim crossover with a crossover gap of 0.3mm. The large gap coupled with the stiff low speed stack produces a crossover closure velocity of 120 in/sec, roughly twice the dyno test speed capability.

The data illustrates a frequent dyno test frustration. At the limit of the dyno the crossover gap has not closed. Damping force will increase after the crossover gap closes, but there is no way to estimate the high speed performance from the data obtained.

The capability of Shim ReStackor to compute high speed performance helps to relieve those uncertainties and determine the effect of high speed crossover closures and valve port flow restrictions that kick in at high speed.