In a unique dyno test series on Thumper Talk, MXScandinavia dyno tested two shim stack configurations and also obtained direct shim stack deflection measurements using a finger press.
A finger press inserts metal rods through the valve ports to directly measure the force required to produce a specific deflection. The MXScandinavia data shows the stiffness of the shim stack is nonlinear and the nonlinear behavior increases with stack lift. Nonlinear stiffness is one reason why shim factors perform poorly in scaling shim stacks. Shim factors assume a linear constant spring stiffness over the deflection range.
Shim ReStackor analysis of the data shows the finger press shim stack stiffness and deflection measurements are consistent with the damping force measured on the dyno up to the dyno test limit of 120 in/sec producing a stack deflection of approximately 0.02 inches.
The finger press data measured stack deflections well beyond that limit up to a deflection of 0.06 inches equivalent to hitting a four inch bump at 200 mph.
The finger press data verifies Shim ReStackor stack stiffness calculations and gives confidence in applying the calculations at extreme conditions well beyond the limit of conventional dyno testing.