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).

A recurring theme in dyno testing is the idea of developing a dyno test library archiving performance of low and high speed shim stack. With the library, components of the shim stack could be mixed and matched to create any damping force profile needed.

A thread on Thumper Talk outlines the process and proposed three shim stack configurations spanning the range of tuning:

• g02: Baseline shim stack with balanced low and high speed damping
• g01: Softer low speed stack to improve ground compliance and an additonal shim in the high speed stack to give the bottoming resistance needed for rough tracks
• g03: Baseline low speed stack to keep chassis control and softer high speed for smooth operation on groomed tracks

The proposed configurations make no difference in high or low speed damping. All three shim stacks follow the same damping force curve and are simply stiffer or softer. MXScandinavia knew that, but dyno tested the shim stacks anyway.

Changes to the high or low speed stack make little difference in high or low speed damping. Dyno tests have demonstrated that over and over again. But the myth still persists.

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).

MXScandinavia on Thumper Talk demonstrated operation of a trapped crossover (linky sample apps) and compared performance to a simple single shim crossover. The two shim stacks are identical with the single difference of replacing the crossover with a 34.1/28.1 split shim pair.

By the thickness cubed rule (shim fac), the replacement shim pair should be 40% softer than the single 28.15 crossover shim. In addition, the split shim pair forms a 0.2 mm crossover gap making the shim stack softer than the single shim 0.15 mm crossover gap.

However, dyno testing shows the opposite effect with the split shim pair producing 3% more damping force. The reason for the difference is the larger 34.1 crossover shim becomes trapped in the closing crossover gap reducing the “effective” crossover gap to 0.1 mm instead of the expected 0.2 mm crossover gap.

Shim ReStackor closely follows the dyno test data and confirms the occurrence of the trapped crossover shim increasing the damping force of the theoretically softer stack.