ReStackor Model Physics

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Finally computer software to tune a shim stack

In the past, experience and intuition were the only guides for tuning suspensions. ReStackor introduces an entirely new era of suspension tuning using fundamental physics and computer software to generate a suspension tuning tool capable of describing the performance of your suspension. Numerical modeling of a shock absorber allows you to test hundreds of stack configurations, in software, across the entire range of wheel speeds and fine tune the stack to produce the damping characteristics you want. The shock absorber numerical analysis of ReStackor gives you the capability to confidently modify suspension shim stacks, stiffen high speed damping or loosen the low speed damping with predictable results.

The Problem With Seat-of-the-Pants Tuning

Suspension tuners rely on the empirical data of seat-of-the-pants test rides to develop suspensions. The problem with development through test is it requires a huge experience base to interpret the suspension behavior and develop the capability to guess the modifications needed to fix a shock. Dyno tests are an example. Dyno data will give you the exact performance of a shock. But, from a single test the data provides no information on how to adjsut the shock to obtain the damping rates you want. Tuning requires several more runs using different combinations of shim configurations to produce enough data to allow you to guess the shim stack configuration needed to produce the damping profile you want. With enough testing the shock can be tuned. But the data from that shock provides no information to tune another shock which may have a slightly different valve or body diameter. Changing something as simple as the oil viscosity requires the entire dyno tuning process to be repeated. The fundamental problem of empirical test data is it is not scalable from one configuration to another. Scaling requires a model. Development of a model using fundamental physical principals makes it scalable over a broad range of operating conditions and shock absorber valve geometries. ReStackor attempts to develop that model.

ReStackor gives you the capability to numerically test the shock, test hundreds of stack configurations, develop your own experience base, and modify the the shock to produce specific results

ReStackor uses fundamental physics to determine the performance of shock absorbers. The physics based models of ReStackor use the fundamentals of fluid dynamics to determine damping rates and basic principals of FEA analysis to evaluate the structural stiffness of shim stacks. Examples are integration of Young’s modulus of elasticity over the tangential stress and strain developed in bending a flat washer to arrive at the Belleville washer equations. Or use of Newton ’s F=ma equation to evaluate fluid momentum changes in the valve flow circuits to determine the valve pressure drop and shock absorber damping rate. These basic physics models are scalable and capable of evaluating the performance of your suspension and accurately scaling those effects to determine effects of modifications. This allows you to modify the valve flow circuits and the valve shim stack to retune the shock to provide the desired performance and accurately evaluate that performance over the entire range of wheel speeds. The capability of ReStackor to do this in software allows you to test hundreds of stack configurations, in software, and save a huge amount of time over the old method of random shim shuffling and intuitive guesses. With ReStackor modifications to your suspension system can now be designed to produce predictable results.

The simple graphical plots produced by ReStackor identify the influence of each shim in the stack. Damping force calculations are referenced to the clicker settings allowing you to instantly recognize the effect of stack changes in terms of how many clicks stiffer or softer each modification will be. This simple clicker reference keeps you from wasting your time installing and testing shim stacks that are outside the range of suspension stiffness you want.

The physics based models used in ReStackor are explored below:

Shim stack stiffness: Shim ReStackor and ReStackor pro compute a detailed shim stack force balance to determine stack stiffness, edge lift and valve face flow area. This allows you to tune all features of the shim stack including shim diameter, shim thickness, stack taper, stack float, crossover shims and the stack clamp diameter. Results of calculations are plotted with the bleed circuit "clicker" area allowing you to instantly relate changes in stack stiffness in terms of how many clicks stiffer or softer each modification will be.
Fluid dynamic flow losses: ReStackor pro carries the calculations a step further to include fluid dynamic flow losses and flow resistances through the valve circuits. This allows calculation of damping rates as a function of suspension speed and bump height. Calculation of the fluid dynamic flow resistance gives you the capability to retune the valve flow circuits and vale port diameters as well as evaluate the effect of oil viscosity changes on the suspension performance.
Flow Cavitation: At high suspension velocities stiff mid-valve shim stacks can result in cavitation of the rebound chamber. ReStackor gives you the capability to compute the combined damping force of base valves and mid-valves, the suspension velocities where cavitation initiates and the damping force produced when the suspension is driven beyond the cavitation limit.
Spring-Mass-Damper system: The fundamental theories describing the motion of a spring-mass-damper system provide equations that relate suspension response to damping rate. These equations also provide the relationships needed to set damper requirements as a function of spring rate, bike weight and rider weight changes. The fundamentals of spring-mass-damper theory can be used to scale the damping rates of a stock suspension, and the rider weight it was designed for, to the spring constants and damping rates needed for a heavier or lighter rider. These same relationships can also be used to scale the suspension characteristics of your current bike to a new bike that has a different weight, spring rate and suspension valve geometry and reproduce the ride characteristics of your old bike.