ReStackor User Manual

Finally computer software to tune a shim stack

ReStackor Calculation Output

When you click on the "Run Once" button the ReStackor inputs in the spreadsheet are written to a file and execution of the 11,000 lines of ReStackor code is started. Outputs from the calculations are loaded into the spreadsheet by clicking the "Load_Output" button. Results of the ReStackor stack deflection calculations are loaded into the "stack" tab of the worksheet and the stack stiffness results are loaded into the "ReStackor" tab. All of the plots in the spreadsheet are automatically updated with the current calculation results whenever the "Load_Output" button is clicked.

The output parameters computed by ReStackor calculations are defined in the frames below as well as some discussion of the plots produced by ReStackor.

Stack Deflection Table

The "stack" tab of the worksheet contains the data needed to produce the stack deflection plot on the main "Plots" tab of the ReStackor excel worksheet. The stack deflection plot is computed for both Shim ReStackor and ReStackor pro calculations. Data on the "stack" tab is simply a table containing the deflection of each shim at all of the major force balance node points used in the ReStackor shim stack force balance. The table consists of:

d.n [=] The diameter of each node point for shim n. For the stack computed here ReStackor used 28 node points across the face shim and the diameter locations go out to column AB, well off the right hand side of the screen shot below.

y.n [=] The deflection height, in inches, of the shim n at the node point d.n.

The stack force balance used by ReStackor solves up to 5,000 simultaneous equations to balance all of the radial and axial forces within the stack. The force balance determines the overall stiffness of the stack as a function of edge lift height and the forces necessary to close any crossover gaps in the stack. Details of the calculations used in the ReStackor force balance are given here.

The data in the table itself is relatively uninteresting. The table data is far more interesting when examined using the plot on the main page of the ReStackor spreadsheet. Multiple runs of ReStackor at progressively higher stack force levels produces the following sequence of plots. These plots allow you to determine the stack force required to close the crossover gap and the effects created by the stack taper on the stiffness of the stack after the crossover gap closes.

For drawing the shims in excel each shim is surrounded by imaginary white space so that lines from adjacent shims do not overlap. For crossover gaps the white space results in a small gap between shims even though the crossover gap is closed. If face shims are traversing several shims to contact a backing plate the sum of the white space between shims can leave a noticeable gap.

Stack Stiffness

Data tables containing the stack stiffness calculation results for both Shim ReStackor and ReStackor pro calculations are on the "ReStackor" tab of the spreadsheet. The Shim ReStackor table contains the stack deflection as a function of force applied to the stack and the ReStackor pro table contains the stack deflection as a function of suspension velocity and the resulting fluid dynamic forces applied to the stack.

Shim ReStackor Calculation Results:

The results of Shim ReStackor calculations are in columns A through E. Definition of each of these parameters are given below.

Force [=] The force applied to the face shim of the stack in lbf.
Yport [=] The edge lift height of the stack face shim in inches.
Aclsd [=] The stack face flow area between the valve face and stack face shim in square inches. This measurement is taken with the clickers in the closed position. In addition to the valve port edge area defined by W.port and N.port, the stack face flow area also depends on the radial length along the edge of the valve port defined by D.port. These parameters are defined in the input file.
Aclk [=] The stack face flow area as defined above plus the flow area available through the bleed clickers at the requested clicker position. From the ReStackor input file shown at the top of this thread the requested clicker position is 7 clicks.
Awo [=] The stack face flow area as defined above plus the flow area through the bleed clickers in the wide open position. From the ReStackor input file shown at the top the wide open position is 20 clicks and the bleed port diameter is 3 mm.

The maximum applied force to the stack for this calculation was 3 lbf as defined by the F.max parameter in the input file. The Shim ReStackor table above computes the stack deflection at progressively higher applied force levels up to the maximum value requested. The calculations increment the applied force using a logarithmic curve focusing more points at low force levels to accurately describe the opening process of the valve shim stack.

ReStackor pro Calculation Results:

ReStackor pro calculation results are shown in columns G through M. ReStackor pro includes all of the shim stack deflection calculations of Shim ReStackor and adds the capability to compute fluid dynamic flow losses through the suspension fluid circuits and determine the fluid dynamic forces applied to the shim stack face. This allows ReStackor pro to determine shock absorber damping forces as a function of suspension velocity and gives you the capability to design the shim stack to provide specific damping forces at specific wheel velocities. The added capability of ReStackor pro gives you the capability to design shim stack crossover points to close at a specific bump height and scale suspension systems from one bike to another adjusting for effects of suspension valve geometry differences.

Uclk [=] The suspension damper rod velocity in inches/sec with the clickers set at the requested position. For this calculation the clickers are set at the 7 click position based on the input file shown at the top of the thread. Using this setting ReStackor pro calculations determine the pressure drop through the suspension fluid circuits, the resulting fluid dynamic forces on the stack face, the stack deflection and the pressure drop through the flow area produced by the stack deflection. Summing these force up gives the damping force produced by the shock absorber.
Uwo [=] The suspension damper rod velocity in inches/sec with the clickers in the wide open position. The difference between Uclk and Uwo is the fluid flow rate through the clicker bleed circuit. With the clickers wide open a higher suspension velocity is needed to provide the additional flow through the clicker bleed circuit.
Uclsd [=] The suspension damper rod velocity in inches/sec with the clickers in the closed position. The difference between Uclk and Uclsd is there is no flow through the bleed circuit. The pressure drop of the Uclk calculation can be matched by a suspension velocity that is reduced by the amount of flow through the bleed circuit.
Gv [=] The fluid flow rate through the combined valve and bleed circuits with the clickers set at the requested position, liters/min.
Fstack [=] The fluid dynamic force applied to the shim stack face shim in lbf.
Force [=] The damping force produced by the shock absorber in lbf.
Pressure [=] The pressure drop across the valve face. This is the pressure drop across the valve and does not include any compression pressure created by changes in the oil height. Oil height effects the spring forces of a shock but does not effect the damping force produced by a shock other than the influence of static pressure on cavitation.

Shim ReStackor Plots

Shim ReStackor produces three plots to help you with tuning your suspension. These are the stack deflection plot, the stack edge lift plot and the valve face flow area plot. A central feature of ReStackor is the use of an excel interface, through excel you can manipulate the data and generate additional plots to analyze the calculation results to suit your particular needs.

The stack structure force balance of Shim ReStackor solves up to 5,000 simultaneous equations to determine the stack structural stiffness and edge lift height.

The edge lift plot for this stack produces a linear curve. The shape of the curve can be manipulated by changing the crossover gap height, diameter or position, or the taper of the backing shims behind the crossover. While the shape of the curve can be influenced through changing of these parameters the behavior of the stack edge lift curve will remain essentially linear. This leads many suspension tuners to rely on the basic rules of thumb that the stack stiffness scales with the diameter squared and the thickness cubed. These basic relationships are derived from the Belleville washer equations. While these are solid rules of thumb there are other basic geometry effects at play that also influence the damping forces produced by a shock absorber.

Stack structure plots of Shim ReStackor show you the influence of each shim in the stack and allow you to easily modify the stack structure to control stiffness across the range of operation.

Fluid flow through the shock absorber valve ports exits through the perimeter gap as well as the side of the vale port. The valve port side flow area is controlled by the bend radius of the stack and the bend radius is continuously changing based on the taper of the stack structure. The bend radius also influences the geometric location of the minimum gap area on the perimeter of the valve port. Accounting for the bend radius in computing the available flow area between the valve face and shim stack produces a much different flow area than would be expected based on the linear plot of edge lift height shown above.

Shim ReStackor calculations compute the available flow area through the shim stack to accurately determine effects of stack taper and index the flow area to clicker settings allowing you to instantly recognize effects of stack modifications.

Plotting the flow area computed by Shim ReStackor shows the stack gets stiffer at high lift, an unexpected result based on the linear stack edge lift height plot shown above. This effect is created by the simple geometric effect of the face shim bend profile on the available flow area. The capability of Shim ReStackor to accurately account for the simple geometric effect of the stack face shim bend profile will save you a huge amount of time in tuning a shim stack and help you avoid the unexpected surprises of a stack taper modification suddenly resulting in unexpected changes in stack stiffness. This central feature of Shim ReStackor removes the guess work out of shim stack modifications and allows you to focus on the stack modifications you need without having to contemplate the age old question "Is this stack stiffer or not?".

The above Shim ReStackor plot shows the flow area available through the suspension valve with the clickers set to the requested 7 click position as well as the wide open and closed position. This baseline plot allows you to overlay potential stack modifications onto the stack you are currently riding and determine how many clicks stiffer or softer the stack will be across the range of stack deflections. This simple notion of comparing stacks based on clicker position allows you to instantly recognize how much stiffer or softer each modification to the stack will be based on your riding experience with your current stack. Stack modifications that are stiffer than your current stack with the clickers fully closed can be immediately eliminated allowing you to quickly focus in on the range of face shims, clamp shims and stack taper modifications that are within the range of interest. The capability of Shim ReStackor to guide you into stack configurations that are a single click stiffer or softer is a huge streamlining of the entire tuning process.

For example applications of Shim ReStackor calculations to practical tuning problems of real world suspension systems check out the sample application section of the ReStackor web site.

ReStackor pro Plots

ReStackor pro includes all of the shim stack force balance calculations of Shim ReStackor and all of the plot generation capability of Shim ReStackor. ReStackor pro adds the capability to compute fluid dynamic flow losses and forces through the suspension fluid circuits. This gives you the capability to compute damping forces as a function of wheel speed and design the shim stack to produce specific damping forces at specific wheel speeds.

The capability to compute forces applied to the shim stack as a function of suspension velocity allows you to design crossover gaps to close at a specific wheel velocity. This allows you to tune the crossover shim diameter, thickness and position to be open on a one inch bump and closed on a two inch bump. Stack taper rates can be tuned to remain soft through the whoops but stiff enough to handle jump landings. The oil viscosity effects built into ReStackor pro allow you to test your suspension with different oil viscosities and retune the stack to duplicate those damping rates. This lets you retune the stack for 2.5wt oil at low speed and 10wt oil at high speed.

The capability to compute fluid dynamic phenomena through the shock absorber flow circuits also gives you the capability to determine the suspension velocities where the valve ports restrict the suspension performance. ReStackor pro calculations can be used to retune the valve ports for the desired high speed performance or evaluate the effects of various after market valves on the performance of your suspension. The capability to compute fluid dynamic forces also gives you the capability to scale the suspension damping rates of your current suspension to another bike with a different bike weight and a different shock absorber valve diameter using the fundamentals of spring-mass-damper theory.

All of this capability means nothing if the calculations of ReStackor pro can not be shown to accurately compute damping forces in real world suspension systems. ReStackor pro has been successfully compared to dyno data of real world suspension systems. For results of those comparisons check out the dyno section of the ReStackor web site.

For practical application of ReStackor pro to real world tuning problems check out the sample applications section of the ReStackor web site.

ReStackor Introduces A New Era In Suspension Tuning

Tuning the shim stack in a shock absorber can be very difficult due to the number of parameters involved. Stack stiffness is controlled by the number of shims, shim thickness, shim diameter, stack taper, stack clamp diameter, clamp thickness, the crossover shim diameter and thickness, backing shims, stack float and stack preload. Changing any one of these parameters produces interactions with other parameters in the stack. Controlling shock absorber performance over the range of wheel speeds often requires changing a couple of the stack parameters simultaneously leading to the age old question "Is this stack stiffer or not?"

The 11,000 lines of ReStackor code have been specifically designed to answer this question. ReStackor calculations use a detailed stack force balance capable of accurately accounting for the subtle effects of stack configuration changes allowing you to focus on tuning the stack instead of the stack structure. Results of the calculations are graphically displayed allowing you to instantly recognize the influence of each shim in the stack. This allows you to easily modify the stack to control stiffness. ReStackor calculations reference stack changes in terms of "clicker" flow area. This simple clicker reference allows you to instantly recognize effects of stack changes in terms of real world clicker settings.

The capability of ReStackor to compute complex interactions of stack design parameters and relate stack stiffness to real world clicker settings allows you to fine tune your suspension far beyond levels previously possible. You can now design the stack to be two clicks softer at high speed -and- two clicks stiffer and low speed without spending months shuffling shims. ReStackor creates a new era in suspension tuning.