Ultra-high speed data
In a unique dyno test series MXScandinavia on Thumper Talk 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 the shim factor linear stiffness theory performs poorly in scaling shim stacks.
Shim ReStackor analysis of the tested shim configurations closely follows the nonlinear finger press stiffness data and the damping force data up to the dyno velocity limit of 120 in/sec. At 120 in/sec the shim stack edge lift was 0.02 inches, approximately one third of the finger press test range.
The finger press measured stack deflections well beyond the dyno test limit up to deflections of 0.06 inches equivalent to hitting a four inch bump at 200 mph.
The finger press data verifies Shim ReStackor shim stack stiffness calculations, verifies the nonlinear stiffness behavior of shim stacks at high deflections and gives confidence applying Shim ReStackor calculations at extreme conditions well beyond the limits of conventional dyno testing.
Shim stack deflection rmz250
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.
Shim stack deflection
A recurring question in crossover tuning is determining the actual edge lift of the shim stack which sets the shock shaft velocity where crossover gaps closes.
Dyno tuners have developed a method to estimate shim stack deflection by installing a stiff backing plate behind the shim stack. When the face shims hit the backer the damping force kicks up giving a measure of the shaft velocity where the gap closes.
MXScandinavia shows an example of the dyno test technique on Thumper Talk.
The dyno data shows a shaft speed of 35 in/sec closes the 0.20 mm gap (data points). MXScandinavia also tested a 0.30 mm gap however his dyno could not produce the shaft speed needed to close the larger gap. Shim ReStackor calculations shown by the lines indicate the larger 0.30 mm gap closes at a shaft velocity around 65 in/sec with a damping force of 1200 lbf. The capability to evaluate shock absorber configurations at conditons well beyond the capability of conventional dynos is a unique capability of Shim ReStackor.
Split crossover
Split crossovers (linky sample apps) use two shims to form the crossover gap. Split shims smooth the bend radius at the crossover shoulder and help prevent kinking of the face shims on the sharp shoulder of a single crossover shim.
Valving Logic dyno testing of an rmz450 used a spit shim crossover with a crossover gap of 0.3mm. The large gap coupled with the stiff low speed stack produces a crossover closure velocity of 120 in/sec, roughly twice the dyno test speed capability.
The data illustrates a frequent dyno test frustration. At the limit of the dyno the crossover gap has not closed. Damping force will increase after the crossover gap closes, but there is no way to estimate the high speed performance from the data obtained.
The capability of Shim ReStackor to compute high speed performance helps to relieve those uncertainties and determine the effect of high speed crossover closures and valve port flow restrictions that kick in at high speed.
