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.

MXSCandinavia on Thumper Talk demonstrated the shim stack modifications needed to separately control low and high 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 producing softer low speed and stiffer high speed damping
• A soft tapered shim stack matches the crossover stack at low speed and drops off at high speed

Tuning shim stacks to get the desired damping force curve is simply done by hacking: adding or removing shim stack preload and crossovers to get the desired damping force curve shape and hacking the shim stack stiffness to get the desired damping force value. Obviously, multiple iterations are needed to simultaneously achieve the desired damping force value and curve shape.

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

Interactive crossovers (sample apps) use a shim diameter that is larger than the 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.

Shim ReStackor analysis of an interactive crossover tested by MXScandinavia on Thumper Talk produces a crossover closure velocity of 60 in/sec. However, the MXScandinavia dyno could only test to shaft speeds of 40 in/sec.

The test illustrates a typical dyno frustration. At the velocity limit of the test no unusual behavior is observed. However, the crossover gap has not closed creating uncertainties the shock absorbers high speed performance.

Shim ReStackor helps relive those uncertainties with the capability to compute high speed shock absorber performance and evaluate crossover closure velocities and the effect of valve port flow restrictions that kick in beyond the limits of conventional dyno testing.

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.