Shim stack deflection
A recurring question in crossover tuning is determining the actual edge lift of the shim stack which sets the shock shaft velocity where crossover gaps closes.
Dyno tuners have developed a method to estimate shim stack deflection by installing a stiff backing plate behind the shim stack. When the face shims hit the backer the damping force kicks up giving a measure of the shaft velocity where the gap closes.
MXScandinavia shows an example of the dyno test technique on Thumper Talk.
The dyno data shows a shaft speed of 35 in/sec closes the 0.20 mm gap (data points). MXScandinavia also tested a 0.30 mm gap however his dyno could not produce the shaft speed needed to close the larger gap. Shim ReStackor calculations shown by the lines indicate the larger 0.30 mm gap closes at a shaft velocity around 65 in/sec with a damping force of 1200 lbf. The capability to evaluate shock absorber configurations at conditons well beyond the capability of conventional dynos is a unique capability of Shim ReStackor.
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.
Ultra-high speed data
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.
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.