Model Physics

  Stack Stiffness

    Shim Stiffness

    Force Balance

    Stack flow area

  Fluid Dynamics

  Cavitation

  Spring mass damper

Shim Stack Stiffness

The effects of shim stack stiffness on suspension damping requires calculation of three basic components:

  • Calculation of the stiffness of each individual shim. 

  • Calculation of the stiffness of the stack structure accounting for effects of crossover gaps, stack taper and the stack clamp shim diameter.

  • Calculation of the fluid flow area at the valve face as the stack progressively deflects at higher applied force levels.

ReStackor models each of these components using physics based relationships developed through years of technology development by the spring manufactures. Coupling shim stiffness calculations with a thorough FEA stack force balance allows complex shim stack configurations to be reliably computed accounting for effects of multiple crossover gaps, stack taper, shim thickness variations in the stack and the effect of backer shims to limit the stack travel.

The physics based analysis used in development of each of these models are discussed in the threads below.

ReStackor uses physics based relationships to accurately compute shim stiffness

ReStackor integrates Hooke's law and Young's modulus of elasticity through the curvilinear coordinates of shim to evaluate the internal stresses created when the shim is bent. These same physics based stress integrations are used in the design of leaf springs and coil springs. The relationships are well known and well anchored through years of years of development in industrial spring manufacturing technology. Within the limits of material property definitions these spring design equations are exact (more).

Detailed force balance used to accurately compute stack stiffness and interactions of design features within the stack structure

A central feature of ReStackor is the capability to compute a detailed stack force balance, accurately account for stack edge lift and the complex stack structures used in practical shim stack configurations. The ReStackor force balance includes effects of shim thickness, stack taper, clamp diameter, crossover shims, backing shims and the interactions of these features as the stack structure changes at different edge lifts. A stack of 50 shims requires ReStackor to solve 4950 simultaneous equations to determine the stack structure and edge lift values (more) .

ReStackor shim stack force balance solves 5,000 simultaneous equations to determine the stack stiffness.

Flow Area At The Shim Stack Face

Simple calculation of stack edge lift is inadequate to determine the fluid flow area at the valve face. The flow area is a function of the valve port geometry and the bend profile of the stack face shim. ReStackor integrates the fluid flow area along the outside edge and side of the valve seat to determine the flow area and accurately account for the bend profile of the shim stack face shim. This allows ReStackor to interpret the subtle effects of stack taper on the valve port flow resistance due to changes in the bend profile of the face shim (more) .

ReStackor accounts for effects of face shim bend profiles in evaluation of fluid flow area available at the valve face.

Shim Stack Stiffness Tuning

The baseline Shim ReStackor code allows you to compute the stiffness of your current shim stack and compare potential stack modifications in terms of you many "clicks" stiffer or softer the modified stack will be. In the example below the stack clamp diameter was increased. The larger clamp produces high speed damping rates equivalent to riding your current stack with the clickers fully closed. If this is beyond the change you intended you then know you need to work on the stack taper, not the clamp diameter. 

Shim ReStackor references shim stack changes in terms of clicker settings allowing you to easily interpret the effect of each change to the shim stack.

The capability of Shim ReStackor to reference stack stiffness to the clicker settings can save you a huge amount of time tuning shim stacks. Referencing the stack face flow area to the clicker flow area allows you to use your real world riding experience to evaluate shim stack changes and search for stack configurations that provide two clicks stiffer high speed damping or insert a crossover shim to provide three click softer low speed damping while maintaining the original high speed damping rate. Using software to evaluate these changes allows you to confidently change shims in the stack or the stack clamp diameter to get the stiffness you want without getting wrapped up in the age old question: Is this stack stiffer or not?. Tuning in software also allows you to test hundreds of stack configurations to find the best combination of face shims and stack taper to get the stiffness you want. Referencing those changes to the clicker flow area in ReStackor helps to eliminate stacks that are "way outside the box".