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

Dyno tuners have developed a test technique to estimate crossover gap closure velocities by installing a stiff backing plate behind the crossover. When the face shims hit the backer, 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 a ridged backing plate. The calculations also allow computing damping force without limit to determine the shaft velocity required to close the crossover gap. In this case, the crossover gap closes at 69 in/sec – just beyond the dyno test velocity limit.

Dyno tests use a stiff backing plate to estimate face shim deflection and shaft velocities required to close crossover gaps