ReStackor User Manual

Finally computer software to tune a shim stack

Weight Scale Damping to Correct for a Spring Rate Change

Suspension response depends on spring rate, weight and damping. Changing spring rate to get race sag right also requires a change in damping to maintain the suspension response, feel and behavior the manufacturer intended for your bike. Spring-mass-damper theory describes the damping force correction needed and the relationship of spring rate, weight and damping on suspension response.

The ReStackor-weight.xls spreadsheet applies those fundamentals to determine the damping force correction needed to compensate for changes in spring rate. The inputs needed are simply the stock and modified (custom) spring rate.

The inputs are:

Stock bike spring rate in kg/mm.

Custom spring rate you are scaling to in kg/mm. The scaling process is built around the assumption the custom spring rate entered gives the correct free sag and race sag recommended by the manufacture.

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 adds the target damping force curves needed to correct of the custom spring rate. The target damping force curves are shown by the blue dashed 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 blue dashed lines of the target weight scaled stack. The example below steps through the process.

Weight Scaling A Stock Shim Stack

So here is the process:

Setup the ReStackor-weight.xls spreadsheet with the stock stack, stock spring rate and the custom spring rate you want to scale to.
Click the Run and Load_Output buttons to compute the performance of the stock stack
Click the Weight Scale and Load_Wtscale buttons to determine the damping force correction needed for the custom spring rate. The Load_Wtscale button adds the target damping force curves to the plots.
Modify the shim stack. Click the Run and Load Output buttons to compute the damping force of the modified stack. Load Output updates the orange line curves.
- Inspect the stack flow area and damping force plots. Modify the stack until you find a configuration that matches the dashed blue lines of the target weight scaled stack.

There are two parts to weight scaling:

If you are a stand-up rider then you are hitting bumps standing on the pegs. In that case the shock is damping the weight of the wheels at high speed and the weight of the chassis at low speed and those weights are constant. The only damping correction needed is for the change in spring rate and that is the high speed damping correction.
If you are a sit-down rider then the shock has to damp the weight of the chassis plus rider at low speed. In that case low speed damping has to be increased to compensate for the change in spring rate and rider weight. That makes the low speed correction larger for a sit-down rider.

Example:

As a first attempt to match the target curves a 8x0.05 shim was added to the stack taper. Results of the calculations show the stack matches the target at high speed, but low speed damping is still too low.

For a stand-up rider the only correction needed is high speed so the single 8x0.05 shim fixes the setup.
For a sit-down rider further stacks mods are needed to get the damping force curve to match the low speed target.

To increase the low speed damping the clicker was reset from 10 to 8. Running the clickers at 8 provides the low speed damping increase needed and simultaneously match the high speed damping curve of the weight scaled target stack.

Instead of changing the clickers you could 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 changes in damping due to the small passages and ports in the 10 mm valve.

Weight scaling setups

Manufactures spend millions setting up the suspensions on stock bikes for the simple purpose of winning MX shootouts in the mags. Winning those shootouts makes a big difference in sales and manufactures take sales seriously. Weight scaling the stock setup corrects damping for a change in spring rate and restores the suspension response, feel and behavior the magazines raved about for your bike. Weight scaling may not be the ultimate setup but it is the fast track to a really good baseline setup.

The same weight scaling process can be used to scale suspension setups from one bike to another. Assuming you had perfected the setup on your old bike you could enter that as the baseline and weight scale the setup to the new spring rate. Then enter the valve port geometry of the new bike and hack around to match the target curve experimenting with the number of face shims, stack taper and clamp shims. That gives a solid baseline setup so the tuning process can then focus on refinements instead of the endless hackery trying to get to a workable baseline.

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. Definitions of the parameters are given in the Output file link.