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.
Trapped crossover
MXScandinavia on Thumper Talk demonstrated operation of a trapped crossover (linky sample apps) and compared performance to a simple single shim crossover. The two shim stacks are identical with the single difference of replacing the crossover with a 34.1/28.1 split shim pair.
By the thickness cubed rule (shim fac), the replacement shim pair should be 40% softer than the single 28.15 crossover shim. In addition, the split shim pair forms a 0.2 mm crossover gap making the shim stack softer than the single shim 0.15 mm crossover gap.
However, dyno testing shows the opposite effect with the split shim pair producing 3% more damping force. The reason for the difference is the larger 34.1 crossover shim becomes trapped in the closing crossover gap reducing the “effective” crossover gap to 0.1 mm instead of the expected 0.2 mm crossover gap.
Shim ReStackor closely follows the dyno test data and confirms the occurrence of the trapped crossover shim increasing the damping force of the theoretically softer stack.
Crossover tuning
Valving Logic demonstrated the effect of adding a crossover to a simple tapered shim stack. Adding the crossover makes the damping force softer everywhere, not just at low speed.
Tuning crossovers to produce the single effect of softer low speed damping requires multiple changes to the shim stack:
- Adjust the crossover position
- Adjust the crossover diameter to produce the desired low speed damping
- Adjust crossover gap to produce the desired closure velocity
- Adjust the high speed stack to produce the desired high speed damping
There is no algebraic equation to “design” a crossover. Crossovers are tuned by hacking around on each of the above four parameters to hit the desired ow damping target while maintain the same high speed damping. That is a tedious process on a dyno, but rapid Shim ReStackor calculations make the process easy.
Tuning crossovers to hit a damping target requires multiple simultaneous changes to obtain the single result of softer low speed damping. Multiple simultaneous changes frustrates many tuners committed to the “one thing at a time” approach to tuning.
High speed damping (mx9c)
Softening or stiffening the high speed stack changes the damping force everywhere, not just at high speed.
The example below from the MXScandinavia thread on Thumper Talk replaces the 0.2 mm shims in the stack taper with stiffer 0.25 mm shims.
The dyno test results (data points) show the shim stack is stiffer everywhere across the speed range, not just at high speed. Shim ReStackor calculations (shown by the lines) shows the same thing. High speed damping can be stiffened by tuning the crossover or softened using a preloading ring-shim (linky, fundamentals).
