Damping curve shape tuning
MXSCandinavia went on in the thread to demonstrate the shim stack modifications needed to gain separate control of high and low 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 with softer low speed and stiffer high speed
- A soft tapered shim stack matches the crossover stack at low speed and drops off at high speed
MXSCandinavia went on in the thread to demonstrate the shim stack modifications needed to gain separate control of high and low speed damping.
Tuning to get the desired damping force curve shape is simply done by hacking: adding or removing shim stack preload and crossovers to get the desired damping force curve shape and hacking around on stiffness to get the desired damping force value.
The numerical test bed of Shim ReStackor makes that simple, easy and intuitive.
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).
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.
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.
