rmz450 trapped crossover
Valving Logic on Thumper Talk tested trapped crossover shim stack configurations. Differences in the number of face shims, crossover configuration and high speed stack stiffness makes it difficult to guess the expected damping force differences between the two shim stack configurations.
Computing the damping force of complex shim stack configurations with multiple differences in the crossover configuration, high speed stack and clamp is the central purpose of Shim ReStackor. The computed results (lines) closely follow the dyno test data (symbols).
Faux crossover gap
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.
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.
Interactive crossover dyno testing
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.
