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 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.
High speed stack (mx3)
A common misconception in shim stack tuning is the face shims control low speed damping and the stack taper controls high speed. Dyno tests and Shim ReStackor show shim stacks do not work that way.
The dyno example below from the MXScandinavia thread on Thumper Talk replaces the 0.2 mm shims in the stack taper section with stiffer 0.25 mm thick shims.
The dyno test results show the shim stack is stiffer everywhere across the speed range, not just at high speed. Shim ReStackor calculations (lines) show the same thing.
High speed damping can be stiffened by tuning the crossover or softened using a preloading ring-shim (linky, fundamentals).
Damping curve shape tuning
MXSCandinavia on Thumper Talk demonstrated the shim stack modifications needed to separately control low and high speed damping:
- Preloading a soft shim stack increases damping force at low speed and softens damping force at high speed
- A crossover does the opposite producing softer low speed and stiffer high speed damping
- A soft tapered shim stack matches the crossover stack at low speed and drops off at high speed
Tuning shim stacks to get the desired damping force curve is simply done by hacking: adding or removing shim stack preload and crossovers to get the desired damping force curve shape and hacking the shim stack stiffness to get the desired damping force value. Obviously, multiple iterations are needed to simultaneously achieve the desired damping force value and curve shape.
The numerical test bed of Shim ReStackor makes that simple, easy and intuitive.
