Valving Logic demonstrated the effect of adding a crossover to a simple tapered shim stack. Adding the crossover makes the damping force softer everywhere, not just at low speed.

Tuning crossovers to produce the single effect of softer low speed damping requires multiple changes to the shim stack:

1. Adjust the crossover position
2. Adjust the crossover diameter to produce the desired low speed damping
3. Adjust crossover gap to produce the desired closure velocity
4. Adjust the high speed stack to produce the desired high speed damping

There is no algebraic equation to “design” a crossover. Crossovers are tuned by hacking around on each of the above four parameters to hit the desired ow damping target while maintain the same high speed damping. That is a tedious process on a dyno, but rapid Shim ReStackor calculations make the process easy.

Tuning crossovers to hit a damping target requires multiple simultaneous changes to obtain the single result of softer low speed damping. Multiple simultaneous changes frustrates many tuners committed to the “one thing at a time” approach to tuning.

A general misconception in shim stack tuning is the high speed stack controls high speed damping. That creates the perception removing shims from the high speed stack “loosens-up” high speed damping.

Ride tests of the “jake norm” stack determined the setup needed softer high speed damping to get better ground compliance. Jake wanted to replace the stack taper with a straight stack of six shims with the thinking the softer high speed stack would “loosen-up” high speed damping.

MXScandinavia ran the dyno test and the two configurations (data points) show no difference in damping. Shim ReStackor calculations (shown by the lines) show the same thing.

To get softer high speed damping while keeping low speed the same requires using a ring-shim to preload the stack (linky, fundamentals).

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.

Read more: Shim stack deflection