Shim stacks are extremely sensitive to trace contaminants. A single particle from 800 grit sandpaper (0.025 mm diameter) trapped in the shim stack changes the damping force by 120 lbf.

Particles trapped inside of the shim stack clamp produce a crossover gap and reduce the damping force. Particles trapped outside of the clamp preload the shim stack driving the damping force up. A single flex from 800 grit sandpaper creates a +/- 20% damping force uncertainty depending on location of the particle inside or outside the stack clamp diameter.

For fielded shocks, any dirt, grit or lint trapped in the shim stack is ground up and spit out after a couple of hours of operation. Free of debris the shock returns to the nominal “clean stack” damping force.

Dyno tests, on the other hand, are only run for a couple of minutes. Trace contaminates, in the range of 800 grit sand paper, significantly alter the damping force measured. That creates problems comparing back-to-back tests with subtle differences in shim stack tuning and +/- 20% damping force noise.

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.

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

Valving Logic dyno testing of an rmz250 used a split shim crossover. Deflection of the high speed stack by the interactive crossover delayed closure of the crossover to 64 in/sec. However, the dyno was not capable of testing beyond 50 in/sec. The data provides no information on the expected damping force increase after the crossover gap closes.

High speed closure of crossover gaps and valve port flow restrictions can significantly increase high speed damping making extrapolation of dyno data beyond the test range risky.

MXScandinavia dyno tested shim factor equivalent stacks on Thumper Talk to evaluate the accuracy of shim factors for scaling shock absorber damping force. The test replaced all of the 0.20 mm shims in the shim stack taper with a pair of 0.15 mm shims. By shim factor theory a pair of 0.15 mm thick shims should be 16% softer than a single 0.20 mm shim.

Dyno tests of the shim stack configurations shows the theoretically softer stack actually produces 5% more damping force as shown by the MXScandinavia dyno data points. Shim ReStackor calculations are shown by the lines and verify the dyno test data and the 5% damping force increase for the theoretically softer shim stack.

The difference in damping force is caused by shim friction. Replacing the stack taper shims with a softer pair of 0.15 mm shims doubles the shim interface area and the resulting friction. The friction increase results in the theoretically softer shim stack producing more stiffness and damping force than the baseline configuration.

Read more: Stack taper shim factors