ReStackor User Manual
Finally computer software to tune a shim stack
Weight Scale Damping to Correct for a Spring Rate Change
The dynamic response of a suspension depends on spring rate, weight and damping. If you change the spring rate to get race sag right, you need to also change damping to maintain the original suspension response, feel and performance the manufacturer intended for your suspension. The change in damping force needed to restore the balance of spring rate, weight and damping is defined by spring-mass-damper theory.
The ReStackor-weight.xls spreadsheet figures out the damping correction needed for changes in weight and spring rate to restore the original dynamic response of the suspension. The spring rate table in the spreadsheet contains the inputs needed for weight scaling.
Weight Scaling Spreadsheet Operation
Run Button: Writes the spreadsheet inputs to a file and runs ReStackor calculations.
Load Output: Loads the calculation results into the ReStackor tab.
Weight Scale: Loads the current calculation results (assumed to be for the stock stack) into the "Wt_Scale" tab and computes the damping force needed for the target custom spring rate. Stack stiffness and damping force requirements for the target spring rate are added to the plots using dashed blue lines.
The plots include the stock stack with "Open Clickers, Setting and Closed Clickers". These stock curves give you a feel for how many clicks stiffer or softer the recommended weight scaled stack will be.
The orange curve in the Stack Flow Area plot is the stack stiffness for the current stack and is updated every time you hit the "Run" and "Load_Output" buttons. The goal is to adjust the shim stack configuration, hit the "Run" and "Load_Output" buttons, inspect the orange line to see what additional stack modifications may be needed to get the orange curve to match up with the dashed blue lines of the target weight scaled stack. An example is worked out below.
Weight Scaling A Stock Shim Stack
So here is the process:
The example below demonstrates use of ReStackor pro to weight scale a shim stack. Examples using the baseline Shim ReStackor code to weight scale a shim stack are on the ReStackor web site . As a first attempt to match the stiffness of the target shim stack a 8x0.05 shim was added to the stack taper. Results of the calculations show the stack matches the needed high speed damping but the low speed damping is still too low.
To increase the low speed damping the clicker was reset from 10 to 8. Running the clickers at 8 provides the needed low speed damping and simultaneously match the high speed damping curve for the weight scaled target stack.
You could also keep the clickers at 10 and increase the oil viscosity from SAE 5 wt to SAE 6 wt. For the small 10 mm valves used in the ReStackor demo small changes in oil viscosity create large damping effects due to the small passages and ports in the 10 mm valve.
Weight scaling the stock stack to correct for spring rate changes gives you the capability to create a baseline suspension setup using custom spring rates that maintains the suspension response, feel and performance the manufacturer intended for the ideal weight of a stock rider with stock spring rates. The process lets you scale the stock suspension setup to a heaver or lighter rider while maintaining the suspension response the magazines rave about for your bike.
This same process can also be used to scale the suspension setup from one rider to another, correcting for changes in weight and spring rate, while maintaining the suspension response of the original setup. Creating the baseline setup for a new rider weight in software lets you experiment with the number of face shims, stack taper and clamp shims to create a solid baseline setup. The tuning process can then focus on refinements instead of the endless hackery searching for a baseline setup.
Contents of "Wt_Scale" tab
Clicking the "Weight Scale" macro button simply loads the current calculation results into the "Wt_Scale" tab of the worksheet. Both Shim ReStackor and ReStackor pro results are loaded. Parameter definitions are given in the Output file link.
From spring-mass-damper theory low speed damping needs to be adjusted by the ratio of the spring rates and high speed damping by the square root of the change in spring rate ratio. The scaling process is all handled internally when you hit the weight scale macro button and the plots show you the stiffness of the target weight scaled shim stack.
ReStackor pro calculations are capable of scaling damping coefficients directly. Shim ReStackor calculations require scaling of the shim stack stiffness based on a Bernoulli assumption detailed on the ReStackor web site. Stack scaling in Shim ReStackor does not account for oil viscosity or cavitation effects. For that reason shim stacks scaled using Shim ReStackor will be slightly different from the stacks scaled using the more thorough ReStackor pro scaling approach.