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.
Shim ReStackor FEA calculations compute the forces acting on the top and bottom surface of each shim in the shim stack allowing the calculations to accurately determine the forces acting on each shim interface and the resulting friction and its effect on stack stiffness.
The MXScandinavia dyno test demonstrates shim friction can make a theoretically softer shim stack dyno test 5% stiffer due to shim friction. Shim friction effects on increasing the bending stiffness of shim stacks is not included in shim factors.
