Spreadsheets

ReStackor User Manual

Finally computer software to tune a shim stack

Weight Scaling of Stock Damping Rates to Correct for a Spring Rate Change

The fundamentals of spring-mass-damper theory define the relationship of suspension response, spring rate and rider weight. If you change the suspension spring rate to get your race sag right the damping rate will need to be adjusted to restore the suspension response, feel and performance the manufacture intended for your suspension. While weight scaling may not give you the ultimate suspension setup for the speed and terrain that you ride it will provide a solid starting point and preserve the suspension performance the manufacture built into the bike.

The ReStackor-weight.xls spreadsheet is specifically setup for weight scaling of stock suspensions. The table on the right-hand side of the input block contains the spring rate information needed for scaling.

The inputs are:

Stock Bike Spring rate: The spring rate of the stock fork or shock in kg/mm.

Custom Spring rate: The spring rate you are using on the fork or shock in kg/mm. The scaling process is built around the assumption that this spring rate gives you the race sag and suspension setup recommended by the manufacture.

Shim ReStackor Weight Scaling Spreadsheet

Run Button: Writes the spreadsheet inputs to a file and runs the ReStackor calculations.

Load Output: Loads the current 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 target shim stack stiffness needed to weight scale the suspension to the custom spring rate. The target stack stiffness is plotted in the Stack Flow Area -vs- Force plot using dashed blue lines. The plot also includes the stock stack with "Open Clickers, Setting and Closed Clickers". These stock curves are on the plot to give you a feel for how many clicks stiffer or softer the recommended weight scaled stack is.

The orange curve in the Stack Flow Area plot is the stack stiffness for the current stack and gets updated every time you hit the "Run" and "Load_Output" buttons. For weight scaling the goal is to try various stack configurations in columns C and D, hit the "Run" and "Load_Output" buttons, and inspect the Stack Flow Area plot to see what additional modifications are needed to get the orange curve for the current stack to match the blue dashed line of the weight scaled target stack.

Shim ReStackor SR Weight 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. Explanation of the parameters in the output table are in the Output file description.

From spring-mass-damper theory we need to change the low speed damping by the ratio of the spring rates and by the square root of the ratio for high speed damping. That scaling process is all handled internally when you hit the weight scale macro button.

ReStackor pro calculations are capable of scaling damping coefficient directly. Shim ReStackor calculations scale damping rates in terms of stack face flow area and the Bernoulli equations using the relationships detailed on the ReStackor web site . Stack face area scaling using Shim ReStackor does not account for oil viscosity effects and for that reason shim stacks scaled using the baseline Shim ReStackor code will be slightly different from the stacks arrived at using the more thorough ReStackor pro approach.

Results of the above scaling relationships are in columns S through Z of the "ReStackor-weight.xls" spreadsheet.

Weight Scaling A Stock Shim Stack

So here is the procedure:

Setup the ReStackor-weight.xls spread sheet with the stock stack and the spring rates you will be using.
Click the Run and Load_Output buttons to compute the performance of the stock stack
Click the Weight Scale button to determine the stack stiffness and damping performance needed for the weight scaled shim stack.
Modify the shim stack. Click on the Run and Load Output buttons to compute the performance of the modified stack.
- Inspect the stack flow area and damping performance plots. Modify the stack configuration and recompute until you find a shim stack that matches the dashed blue lines of the target weight scaled stack.

Example:

The example below demonstrates use of ReStackor pro in weight scaling 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 too low.

To increase the low speed damping the clicker was reset from 10 to 8. Running the clickers at 8 matches the needed low speed damping curve and the high speed damping curve for the weight scaled custom 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.

Instead of randomly changing the stack and test riding the bike to see what happens ReStackor gives you the capability to evaluate several stack configurations and pick the best option to produce the changes you want. You can experiment with the number of face shims, changes in stack taper and clamp diameter to determine the stack structure that comes closest to the change you want. The capability to do that in software with predictable results gives you the capability to tune your suspension far beyond the limits previously possible.