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.

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.

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.

Valving Logic dyno tests of an rmz450 shock on Thumper Talk demonstrate the operation of a faux crossover.

Shim ReStackor analysis of the configuration shows the crossover gap eventually closes at a shaft velocity of 180 in/sec. However, the soft high speed stack used in the configuration produces virtually no change in damping force at the crossover closure. In that sense, the configuration demonstrates “faux” over the range tested.