High speed stack (mx3)
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).
Shim factor scaling
MXScandinavia dyno tested shim factor equivalent shim stack configurations on Thumper Talk to determine the accuracy of shim factors in scaling suspension setups. By shim factor theory (linky, physics), a stack of 4x40.3 face shims should be 3.8% softer than a stack of 14x40.2 shims.
Dyno test results shows the actual difference was approximately double that at 7.7% shown by the data points in the figure below. Shim ReStackor analysis (lines in the figure below) show the same 7.7% difference in damping force.
rmz450 trapped crossover
Valving Logic on Thumper Talk tested trapped crossover shim stack configurations. Differences in the number of face shims, crossover configuration and high speed stack stiffness makes it difficult to guess the expected damping force differences between the two shim stack configurations.
Computing the damping force of complex shim stack configurations with multiple differences in the crossover configuration, high speed stack and clamp is the central purpose of Shim ReStackor. The computed results (lines) closely follow the dyno test data (symbols).
Shim stack deflection rmz250
Estimating crossover closure velocities is a recurring question for dyno tuners.
Dyno tuners have developed a technique to estimate crossover gap closure velocities by installing a stiff backing plate behind the crossover. When the face shims hit the backer plate the damping force kicks up giving a measure of the shock shaft velocity required to close the crossover gap.
Valving Logic provides an example of the technique on Thumper Talk. However, for this example the dyno velocity limit of 60 in/sec was not able to reach the shaft velocities required to close the crossover gap.
Evaluating crossover gap closure velocities is easier with Shim ReStackor. The shim stack deflection graphic gives a visual indication of crossover gap closure. The shim stack flow area curve shows when the face shims contact the stiffer high speed stack, or in this case the ridged backing plate. Shim ReStackor calculations are also capable of evaluating shock absorber configurations at conditions well beyond the capability of conventional dyno testing. In this case, the crossover gap closes at 73 in/sec – just beyond the dyno test limit at 60 in/sec.
