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.

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.

Estimating crossover closure velocities is a recurring question for dyno tuners.

Dyno tuners have developed a technique to estimate crossover gap closure velocities by installing a stiff backing plate behind the crossover. When the face shims hit the backer plate the damping force kicks up giving a measure of the shock shaft velocity required to close the crossover gap.

Valving Logic provides an example of the technique on Thumper Talk. However, for this example the dyno velocity limit of 60 in/sec was not able to reach the shaft velocities required to close the crossover gap.

Evaluating crossover gap closure velocities is easier with Shim ReStackor. The shim stack deflection graphic gives a visual indication of crossover gap closure. The shim stack flow area curve shows when the face shims contact the stiffer high speed stack, or in this case the ridged backing plate. Shim ReStackor calculations are also capable of evaluating shock absorber configurations at conditions well beyond the capability of conventional dyno testing. In this case, the crossover gap closes at 73 in/sec – just beyond the dyno test limit at 60 in/sec.

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.