A common mistaken concept in shim stack tuning is the face shims control low speed damping and the stack taper controls high speed.

Dyno tests and Shim ReStackor calculations show shim stack 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 tuning the crossover or softened using a preloading ring-shim (more).

Dyno tests show stiffening the high speed stack stiffens damping force at both low and high speed

A recurring theme in dyno testing is the idea of developing a dyno test library archiving performance of low and high speed shim stack. With the library, components of the shim stack could be mixed and matched to create any damping force profile needed.

A thread on Thumper Talk outlines the process and proposed three shim stack configurations spanning the range of tuning:

• g02: Baseline setup with balanced low and high speed damping
• g01: Rough track setup. Softer low speed stack to improve ground compliance and an additional shim in the high speed stack to give the required bottomin resistance needed for rough tracks
• g03: Groomed track setup. Baseline low speed stack to preserve chassis control and softer high speed for groomed track operation

Proposed shim stack configurations to handle wide range of MX track conditions

Dyno library test results

MXScandinavia dyno tested the three configurations....

A faux crossover gap never closes. Faux gaps are created by large crossover shim diameters, stiff low speed stacks or soft high speed stacks that do not produce enough force to close the crossover gap. MXScandinavia provides dyno test examples of faux crossovers.

In dyno testing, faux crossovers behave like a interactive crossover. Changes to the low or high speed stack changes the damping force leading many dyno tuners to believe the crossover gap is active.

However, the crossover gap height never changes as the shim stack deflects. The inactive faux crossover shim could be moved further up in the stack forming a simple tapered shim stack giving the same damping force and curve shape.

In dyno testing, there is no way to know the crossover gap is faux until the shock is pushed to high enough speed to observe the crossover closing. Soft closures of interactive crossovers make those events difficult to spot in damping force data.

Faux crossover gap never closes creating confusion in operation of the shim stack

A recurring question in crossover tuning is determining the actual edge lift of the shim stack and the shock shaft velocity where crossover gaps close.

Dyno tuners have developed a method to estimate deflection of the stack face shim 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 needed to deflect the shim stack.

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). Retesting with a larger 0.30 mm gap requires nearly double the shaft velocity (lines computed by Shim ReStackor). MXScandinavia’s dyno could not produce the shaft speed and 1200 lbs of force needed to close the larger crossover gap.

Separate dyno tests run with a backing plate estimate shim stack deflection and closure of crossover gaps