Stack taper shim factors
MXScandinavia dyno tested shim factor equivalent stacks on Thumper Talk to evaluate the accuracy of shim factors for scaling shock absorber damping force. The test replaced all of the 0.20 mm shims in the shim stack taper with a pair of 0.15 mm shims. By shim factor theory a pair of 0.15 mm thick shims should be 16% softer than a single 0.20 mm shim.
Dyno tests of the shim stack configurations shows the theoretically softer stack actually produces 5% more damping force as shown by the MXScandinavia dyno data points. Shim ReStackor calculations are shown by the lines and verify the dyno test data and the 5% damping force increase for the theoretically softer shim stack.
The difference in damping force is caused by shim friction. Replacing the stack taper shims with a softer pair of 0.15 mm shims doubles the shim interface area and the resulting friction. The friction increase results in the theoretically softer shim stack producing more stiffness and damping force than the baseline configuration.
Dyno library
A recurring theme in dyno testing is the idea of developing a dyno test library archiving performance of low and high speed shim stack. With the library, components of the shim stack could be mixed and matched to create any damping force profile needed.
A thread on Thumper Talk outlines the process and proposed three shim stack configurations spanning the range of tuning:
- g02: Baseline shim stack with balanced low and high speed damping
- g01: Softer low speed stack to improve ground compliance and an additonal shim in the high speed stack to give the bottoming resistance needed for rough tracks
- g03: Baseline low speed stack to keep chassis control and softer high speed for smooth operation on groomed tracks
The proposed configurations make no difference in high or low speed damping. All three shim stacks follow the same damping force curve and are simply stiffer or softer. MXScandinavia knew that, but dyno tested the shim stacks anyway.
Changes to the high or low speed stack make little difference in high or low speed damping. Dyno tests have demonstrated that over and over again. But the myth still persists.
Interactive crossover rm85
Interactive crossovers use a shim diameter that is larger than the shim 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 (linky sample apps).
Valving Logic on Thumper Talk demonstrated the performance of an interactive crossover. The damping force data shows no evidence of the crossover closing. However, Shim ReStackor calculations of the shim stack deflection and stack flow area make it easy to spot the crossover closure velocity at 20 in/sec.
Soft closures of interactive crossover with a soft high speed stack often do not produce any specific “event” in the damping force curve. The data at 20 in/sec where the crossover closes shows that.
Two stage crossovers
Two stage crossovers (linky sample apps) have a similarity to split crossovers in spreading out the bend radius of the crossover shim shoulder. Valving Logic dyno testing of an rmz250 shock demonstrates performance of a two stage crossover.
Shim ReStackor analysis of the configuration shows the upper crossover gap closes first followed by closure of the lower crossover gap at shaft velocities near 37 in/sec.
The soft high speed stack used in the configuration produces little increase in damping force when the crossover gap closes.
