MXScandinavia dyno tested shim factor equivalent stacks on Thumper Talk to evaluate the accuracy of shim factor scaling. By shim factor theory, a stack of 4x40.3 face shims is 3.8% softer than a stack of 14x40.2 shims.

Dyno test results shows the actual difference was approximately double that at 7.7% (data points). Shim ReStackor analysis of the two shim stack configurations shows the same thing (lines).

MXScandinavia dyno testing of shim factor equivalent shim stacks shows 7.7% less damping force

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 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.

Valving logic tests of an rmz450 shock on Thumper Talk demonstrate the operation of a faux crossover.

Shim ReStackor analysis of the configuration shows the crossover gap eventually closes at a shaft velocity of 180 in/sec. However, the soft high speed stack used in the configuration produces virtually no change in damping force at the crossover closure. In that sense, the configuration demonstrates “faux” over the range tested.

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

MXScandinavia on Thumper Talk demonstrated operation of a trapped crossover and compared performance to a simple single shim crossover. The two shim stacks are identical with the single difference of replacing the crossover with a 34.1/28.1 split shim pair.

By the thickness cubed rule, the replacement shim pair should be 40% softer than the single 28.15 crossover shim. In addition, the split shim pair forms a 0.2 mm crossover gap making the shim stack softer than the single shim 0.15 mm crossover gap.

However, dyno testing shows the opposite effect with the split shim pair producing 3% more damping force. The reason for the difference is the larger 34.1 crossover shim becomes trapped in the closing crossover gap reducing the “effective” crossover gap to 0.1 mm instead of the expected 0.2 mm crossover gap.

Shim ReStackor closely follows the dyno test data and confirms the occurrence of the trapped crossover shim increasing the damping force of the theoretically softer stack.

Shim trapped in closing crossover gap forces gap to close earlier increasing damping force