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).

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.

Interactive crossovers (linky sample apps) use a shim diameter that is larger than the shim stack clamp. The larger crossover diameter transfers force from the face shims directly into the high speed stack forcing the high speed stack to deflect before the crossover closes. Interaction with the high speed stack softens the crossover closure event.

MXScandinavia dyno tests on Thumper Talk evaluated the performance of an interactive crossover configuration.

Shim ReStackor analysis of the configuration shows the crossover closes at a shaft velocity of 26 in/sec. Due to the shim stack configuration there is virtually no change in stack stiffness or damping force at the crossover closure confirming the dyno test results.

Interactive crossovers use a shim diameter that is larger than the shim stack clamp. The larger crossover diameter transfers force from the face shims directly into the high speed stack forcing the high speed stack to deflect before the crossover closes. Interaction with the high speed stack softens the crossover closure event (linky sample apps).

Valving Logic on Thumper Talk demonstrated the performance of an interactive crossover. The damping force data shows no evidence of the crossover closing. However, Shim ReStackor calculations of the shim stack deflection and stack flow area make it easy to spot the crossover closure velocity at 20 in/sec.