kxf450 split shim crossover
Split crossovers (sample apps) use two shims to form the crossover gap. Split shims smooth the bend radius at the crossover shoulder to prevent kinking of the face shims on the sharp shoulder of a single crossover shim.
Valving Logic dyno testing of kxf450 shim stack on Thumper Talk used a split shim crossover. Shim ReStackor analysis of the configuration shows the crossover closes at a velocity is 64 in/sec, just beyond the dyno test range of 50 in/sec. Damping force is expected to sharply kick up after the crossover closes due to the stiff high speed stack used in the configuration.
The data illustrates a typical dyno test frustration. At the velocity 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 stiffness increase from the data obtained.
The high speed capability of Shim ReStackor helps elevate those uncertainties and evaluate high speed crossover gap closers, and valve port flow restrictions that kick in at speeds beyond the limit of conventional dyno testing.
Stack taper shim factors
MXScandinavia dyno tested shim factor equivalent stacks on Thumper Talk to determine the accuracy of shim factors in scaling suspension setups. The test replaced all of the 0.20 mm shims in the shim stack taper with a pair of 0.15 mm thick 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 testing 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 shown by the lines in the figure below verify the dyno test results and the 5% damping force increase for the theoretically softer stack expected by shim factors.
The difference in damping force is caused by shim friction. Replacing the stack taper shims with a softer pair of shims doubles the shim interface area and the resulting friction. The friction increase results in the theoretically softer shim stack producing a higher stiffness and damping force.
High speed damping rmz250
A general shim stack tuning misconception is high speed damping is controlled by the high speed stack. That creates the perception removing a couple of shims from the high speed stack will “loosen-up” high speed damping while having little effect on low speed.
Dyno testing and Shim ReStackor calculations show that approach does not work.
Valving Logic provides a dyno test example on Thumper Talk. The rmz250-248 shim stack runs a full stack taper. The rmz250-152 configuration completely removes the high speed stack creating the expectation of “loosening-up” high speed damping.
Dyno tests of the two configurations shows no difference in high speed damping (data points). Shim ReStackor calculations (shown as lines) show the same thing.
To get softer high speed damping while keeping low speed the same requires using a preload ring-shim in the shim stack (linky, fundamentals).
Shim factor scaling
MXScandinavia dyno tested shim factor equivalent shim stack configurations on Thumper Talk to determine the accuracy of shim factors in scaling suspension setups. By shim factor theory (linky, physics), a stack of 4x40.3 face shims should be 3.8% softer than a stack of 14x40.2 shims.
Dyno test results shows the actual difference was approximately double that at 7.7% shown by the data points in the figure below. Shim ReStackor analysis (lines in the figure below) show the same 7.7% difference in damping force.
