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.
