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Suspension Scaling with ReStackor pro
Scaling suspension systems:
Lets suppose that after riding your old YZ for the last six years and continuously shuffling the shim stack you finally have gotten the suspension perfect. Perfect everywhere, sand, whoops, rocks, everywhere. You bought a new bike. Different forks, different springs, different bike weight. You are going to have start over on the entire suspension tuning process, right?
Maybe not. Based on spring-mass-damper theory if you setup the new bike with the same tau and zeta values the suspension feel and response of the new bike will match the suspension of your old bike:
So the process is to figure out the value of tau and zeta that was working on your old bike and setup your new bike with those same values. Matching tau and zeta automatically corrects for differences in spring rate, bike weight and the differences in the suspension valve geometry and oil flow rates.
Getting the suspension natural frequency (tau) right is pretty straight forward. The value of m/k is basically race sag. So if you setup your new bike with the same race sag as your old bike it will have the same natural frequency.
Matching zeta requires a way to estimate the shock absorber damping coefficient (c). The damping coefficient (c) is defined as the damping force produced by the shock (lbf) divided by the suspension wheel velocity. ReStackor pro can compute this value for you old bike and your new bike.
To maintain the same zeta value the damping rate needs to be scaled by the difference in spring rate between the two bikes.
ReStackor pro scaling setups from one bike to another
Entering the valve geometry and shim stack configuration for your old bike into ReStackor pro gives you the capability to compute the value of the damping coefficient (c) your old custom tuned suspension was producing. Application of the above relationships from spring-mass-damper suspension theory give you the capability to scale that damping rate to your new bike and setup your new bike to have the same suspension feel and behavior including the necessary corrections for differences in bike weigh and spring rate.
ReStackor pro computes damping force as a function of wheel velocities giving you the data necessary to scale suspensions.
The process:
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Determine the shock absorber damping coefficient (c) for the custom stack configuration on your old bike.
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Scale that damping coefficient by the difference in spring rate to determine the damping coefficient needed for the new bike.
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Hack on the shim stack configuration of the new bike using ReStackor pro to determine the stack, and possibly valve port modifications, needed to match the scaled damping coefficient of the old bike.
In this case matching the desired damping coefficient required the addition of there face shims and a small adjustment to the clamp shim diameter. Based on spring-mass-damper theory matching the scaled damping coefficient will give your new bike the same suspension response, feel and behavior as your old bike.
Now you may find that the damping rates that worked well on the old bike aren't the hot tip for the new bike. But the above process will get you back into the ball park of where your old bike was and provide you a familiar suspension feel and behavior. This will allow you to use your old tuning experience to develop the fine tuning tweaks needed to perfect the new setup.
Weight Scaling Damping Rates of the Stock Suspension for a Rider of Different Weight
Spring-mass-damper theory provide two parameters that define the suspension response. Tau defines the oscillation time constant of the suspension and will be the same for two riders of different weight if the bikes are setup to have the same race sag. The zeta coefficient describes the damping characteristics of the suspension. To scale the suspension response, feel and behavior from one bike to another the spring rate and shim stack stiffness must be changed to that:
In the end suspension performance all comes down to "butt dyno" performance. Nothing else really matters. After a couple of rides the "butt dyno" may tell you the high speed rates are two stiff or the low speed rates are too soft. Tuning based on the results of test rides is the central focus of ReStackor. ReStackor gives you the capability to run a few experiments with the clickers and determine the desired high speed and low speed clicker settings and then retune the stack to match those specific clicker settings. The capability to do that in software, match specific clicker settings and retune the stack to simultaneously match the desired low speed and high speed damping rates gives you the capability to fine tune the stack far beyond levels previously possible. This capability of ReStackor defines a new era in suspension tuning.
