rmz450 trapped crossover
Valving Logic on Thumper Talk tested trapped crossover shim stack configurations. Differences in the number of face shims, crossover configuration and high speed stack stiffness makes it difficult to guess the expected damping force differences between the two shim stack configurations.
Computing the damping force of complex shim stack configurations with multiple differences in the crossover configuration, high speed stack and clamp is the central purpose of Shim ReStackor. The computed results (lines) closely follow the dyno test data (symbols).
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.
MXScandinavia interactive crossover
Interactive crossovers (linky sample apps) use a shim diameter that is larger than the shim stack clamp. The larger crossover diameter transfers force from the face shims directly into the high speed stack forcing the high speed stack to deflect before the crossover closes. Interaction with the high speed stack softens the crossover closure event.
MXScandinavia dyno tests on Thumper Talk evaluated the performance of an interactive crossover configuration.
Shim ReStackor analysis of the configuration shows the crossover closes at a shaft velocity of 26 in/sec. Due to the shim stack configuration there is virtually no change in stack stiffness or damping force at the crossover closure confirming the dyno test results.
Bent clamp washer
Shim stack clamp washers are thick, heavy washers purposely overdesigned to never bend. However, hard hits can bend the clamp washer. When bent, the clamp washer significantly reduces the damping performance of a shock absorber.
A “bent” washer does not mean the clamp washer is bent around into a cup shape. Bent means the clamp washer is very slightly displaced from perfectly flat.
The Shim ReStackor example below evaluates a 35 mm clamp washer bent 1/10,000 of an inch at the outside edge. Assuming a smooth bend, the displacement at the 25 mm clamp shim stack is 0.4/10,000 of an inch.
A bent clamp washer does not support the outside edge of the clamp shim producing an “effective” clamp diameter of zero. As the shim stack deflects, the clamp shim eventually contacts the bent washer, but that requires significant deflection of the face shims.
The end result is a 1/10,000 of an inch bend at the clamp washer edge produces a 260 lbf drop in shock absorber damping force.
To obtain reliable performance clamp washers need to be perfectly optically flat.