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.

Valving Logic demonstrated the effect of adding a crossover to a simple tapered shim stack. Adding the crossover makes the damping force softer everywhere, not just at low speed.

Tuning crossovers to produce the single effect of softer low speed damping requires multiple changes to the shim stack:

1. Adjust the crossover position
2. Adjust the crossover diameter to produce the desired low speed damping
3. Adjust crossover gap to produce the desired closure velocity
4. Adjust the high speed stack to produce the desired high speed damping

There is no algebraic equation to “design” a crossover. Crossovers are tuned by hacking around on each of the above four parameters to hit the desired ow damping target while maintain the same high speed damping. That is a tedious process on a dyno, but rapid Shim ReStackor calculations make the process easy.

Tuning crossovers to hit a damping target requires multiple simultaneous changes to obtain the single result of softer low speed damping. Multiple simultaneous changes frustrates many tuners committed to the “one thing at a time” approach to tuning.

In a unique dyno test series on Thumper Talk, MXScandinavia dyno tested two shim stack configurations and also obtained direct shim stack deflection measurements using a finger press.

A finger press inserts metal rods through the valve ports to directly measure the force required to produce a specific deflection. The MXScandinavia data shows the stiffness of the shim stack is nonlinear and the nonlinear behavior increases with stack lift. Nonlinear stiffness is one reason why shim factors perform poorly in scaling shim stacks. Shim factors assume a linear constant spring stiffness over the deflection range.

Shim ReStackor analysis of the data shows the finger press shim stack stiffness and deflection measurements are consistent with the damping force measured on the dyno up to the dyno test limit of 120 in/sec producing a stack deflection of approximately 0.02 inches.

The finger press data measured stack deflections well beyond that limit up to a deflection of 0.06 inches equivalent to hitting a four inch bump at 200 mph.

The finger press data verifies Shim ReStackor stack stiffness calculations and gives confidence in applying the calculations at extreme conditions well beyond the limit of conventional dyno testing.

A faux crossover gap never closes. Faux gaps are created by large crossover shim diameters, stiff low speed stacks or soft high speed stacks that do not produce enough force to close the crossover gap. MXScandinavia provides dyno test examples of faux crossovers.

In dyno testing, faux crossovers behave like a interactive crossover. Changes to the low or high speed stack changes the damping force leading many dyno tuners to believe the crossover gap is active.

However, the crossover gap height never changes as the shim stack deflects. The crossover shims could be moved further up in the stack forming a simple tapered shim stack with the same damping force.

In dyno testing, there is no way to know the crossover gap is faux until the shock is pushed to high enough speed to observe the crossover closing. Soft closures of interactive crossovers make those events difficult to spot in damping force data.