Tuning Basics
Sample Applications
ReStackor Sample Applications
Finally computer software to tune a shim stack
CRF450 Stock Suspension Scaling for a Change in Spring Rate
In what may one day be a classic garage tuning thread "The Dogger" on TumperTalk took on the task of re-valving his crf450r suspension to match his 6' 8", 265 lb, 11% body fat weight. The Dogger tuned his suspension using the old school method of cut and try along with some guesses, intuition, and a bunch of "butt dyno" testing. You can read about the process on the Thumper Talk thread.
ReStackor gives you an easier way. The Dogger did not set out to prove spring-mass-damper theory could be used to determine the damping rates needed for a heaver rider. But in most cases the shim stacks the Dogger developed at after months of "butt dyno" tuning and discussions on the ThumperTalk thread turned out to be close to the theoretical shim stacks determined directly from weight scaling of the stock stack.
Weight scaling the suspension damping using Shim ReStackor is easy. The calculations determine the target stack stiffness. All you need to do is add or remove shims to match the target stack stiffness.
If you have been forced to change spring rates to get your sag right and are unhappy with the performance of the stock damping rates the scaling process worked out from the fundamentals of spring-mass-damper theory may give you a reasonable starting point. The ReStackor weight scaling calculation have become popular enough that a specific ReStackor spreadsheet has been setup for this calculation. The examples below demonstrate application of this weight scaling spreadsheet.
crf450 Shock Compression Shim Stack Scaling
The ReStackor-SR-weight.xls spreadsheet has been specifically setup for weight scaling of shim stacks . Simply enter the stock shim stack, click on the "Run" and "Weight" scale macro buttons and the stiffness of the target shim stack will be plotted. From there you adjust the shim stack configuration by adding shims or adjusting the stack taper to match the stiffness of the target stack.
HOWTO setup a Shim ReStackor input file for a crf450 shock
The Doggers shock compression stack
The Dogger knew he was going to need a stiffer stack for his 265 lb weight. For his baseline stack configuration, given in his thread posted 9/24, he added 8- 44x0.2 shims to the face, softened the crossover and softened the high speed taper of the stock stack. With this combination of simultaneous changes both stiffening and softening the stack the age old question arises "Is this stack stiffer or not?". Running the stack through Shim ReStackor shows the Dogger's 9/24 stack is stiffer and about 5 clicks to stiff at high speed relative to the weight scaled target stack.
HOWTO Shim ReStackor outpus
After riding the baseline stack for awhile The Dogger wanted to soften it up a bit. His stack, posted on 10/10, went back to the stock face shim configuration and put in a stiffer set of backing shims. The configuration kept the softer crossover and high speed stack modifications used in the 9/24 stack. The 10/10 stack modifications, the Dogger's 9/24 baseline stack, and the stock stack are shown below.
The Dogger used a softer crossover and a softer high speed stack for both configurations. The Doggers baseline 9/24 stack added 8 face shims. The 10/10 stack used a stiffer backing shim configuration.
So, with the additional backing shims, the softer crossover and modifications to the high speed stack is the 10/10 stack configuration stiffer or not? Shim ReStackor calculations show the 10/10 stack configuration is softer. In fact, the 10/10 stack turns out to be about 1 click softer than the stock stack.
Due to the softer stack the Dogger ended up running that 10/10 stack with the HSC adjuster about 3 turns in to compensate for the softer compression stack. Later modifications stiffened the compression stack to recover from the modifications of the 10/10 stack.
The Doggers 10/10 stack modifications turn out to be about the same stiffness as the stock stack
The plots above demonstrate some import features Shim ReStackor provides for tuning a shim stack.
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When you modify the stack ReStackor gives you a pretty good idea of how much stiffer or softer the stack will be in terms of clicker settings. This helps prevent you from wasting your time installing and testing stacks that are outside the range of stiffness you want.
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You can make complex stack modifications changing the crossover, stack taper and face shims or the thickness of shims and determine how much stiffer or softer the stack will be. Being able to see the effect of stack modifications over the entire range of stack deflections helps avoid surprises of the stack suddenly becoming too stiff or too soft at extreme deflections.
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You can see the shape of the flow area curve over the range of stack deflection. That curve is linear for crf450r shock compression valve despite the stack taper modifications the Dogger installed. The stack structure plot produced by Shim ReStackor shows you the face shim stack is so thick on the crf that the stack is relatively insensitive to stack taper. Changing the backing shims changes the stiffness of the stack but does little to change the shape of the stack deflection curve.
Fork Rebound Stack
For the mid-valve rebound stack the Dogger added one 20x0.1 shim to slightly stiffen the stack. According to The Doggers notes the clamp shim was also changed to a thinner 11x0.1 shim.
The Doggers addition of a single face shim brings the stack very close to the weight scaled target. To match the target stack stiffness we need to either further increase the face shim thickness or change the stack taper. Doing this in software with Shim ReStackor allows you to test all of those options to arrive at the stack stiffness that provides the best match to the weight scaled target. In this case changing the 18x0.1 shim backing the face shim to a 20x0.1 shim provides the best match.
The Doggers mid-valve rebound stack is close to the value expected based on weight scaling
Shim ReStackor Helps You Tune Complex Stack Structures and Identify Shim Changes Needed to Produce the Desired Effect
Tuning this stack the old fashion way of sticking a shim in there to see what happens would take you months of test riding chaning crossover shim diameters and the stack taper to arrive at the above curve. With Shim ReStackor you can test all of these configurations and in a couple of minutes find the stack that comes closest to the stack stiffness you want. This allows you to confidently install that stack without wasting your time installing and testing stacks that are too soft of too stiff.
For suspensions the bottom line is how it rides. After riding the above stack if you find it is a couple of clicks stiffer than you wanted a few more ReStackor calculations would allow you to retune the stack to hit that specific clicker setting. The capability to tune the stack in software and figure out whether a crossover change or stack taper change is the best option to hit the desired stiffness is a huge streamlining of the entire tuning process. Shim ReStackor removes the guess work out of shim changes by allowing you to relate the shim stack stiffness to clicker settings and confidently tune the stack to the stiffness you want.
Fork Mid-Valve Compression
The Dogger added 4-20x0.1 shims to the stack face and removed one of the 17x0.1 backing shims for the mid-valve compression stack. The additional face shims result in a rigid stack face structure that behaves like a straight stack with a linear force -vs- lift profile.
Shim ReStackor calculations for The Doggers mid-valve compression stack in comparison to the stock stack and weight scaled target stiffness are shown below.
The Doggers mid-valve compression stack closely matches the weight scaled target value. At high deflections the flow becomes restricted by the vale port flow area.
Valve Port Restriction Effects can be Determined by Shim ReStackor
Above 20 lbf the the flow area through the shim stack becomes restricted by the valve ports. When the ports limit the flow the Shim ReStackor results roll over to a constant flow area as shown in the plot above. Having the valve port limit the flow is not necessarily a bad thing. This allows you to use a soft shim stack for plush low speed operation and rely on the valve port flow restriction to keep the suspension from blowing through its stroke at high speed. The crf450r mid-valve ports limit suspension speeds above wheel velocities of about 250 in/sec based on some side calculations with ReStackor pro. For an example of using ReStackor to tune valve port flow restrictions check out the XR650 sample application.
The Dogger stack comes close to matching the weight scaled target stack but matching the high speed behavior requires a way to reduce the effective flow area of the valve port at high stack deflections. From the principals of stack design you could do that by installing a backing shim. By tuning the backing shim gap and thickness you could nail the weight scaled stack target curve over the entire range of stack deflections.
With Shim ReStackor you can design backing shims to restrict shim stack deflection and mimic valve port flow restriction for control of high speed damping.
The crf450 mid-valve example demonstrates an important capability of Shim ReStackor to diagnose valve port restriction problems. You can spend months experimenting with progressively softer shim stacks to try and soften the high speed performance before figuring out that the high speed damping is controlled by the valve port and not the shim stack. With ReStackor pro you would know the suspension velocities where the valve port flow restrictions effect performance at the start of the tuning process. The above example also demonstrate the capability of Shim ReStackor to evaluate numerous stack design options and pick the stack that most closely matches the damping profile you wanted. That can be very difficult when tuning complex stack structures.
Shock Rebound Stack
The Doggers shock rebound stack posted on 9/12 used 5-40x0.2 face shims and a larger crossover diameter. After a few rides on that stack The Dogger went to a slightly softer rebound stack using 4-40x0.2 face shims and a smaller crossover diameter.
The modified 9/12 stack is very close to the theoretical stack stiffness obtained based on weight scaling the stock stack.
The Doggers shock rebound stack is very close to the weight scaled target stack.
Shim ReStackor Can Tune Crossover Shim Effects
The larger crossover diameter used in both the 9/12 stack and 10/10 stack delay the crossover closure point. The point where the crossover closes can be tuned in Shim ReStackor by adjusting the crossover shim diameter, thickness or position in the stack. For a purely weight scaled setup The Dogger would need to use a crossover diameter closer to the stock diameter to get the crossover to close at the suspension velocity indicated by the weight scaled target stack curve.
The "Butt" Dyno
Suspension are ultimately developed on the "butt dyno". In the end, nothing else but the performance on the "butt dyno" really matters.
The problem with relying on the butt dyno to tune a suspension is every time you think you might want to try a shim change you are looking at a couple of hours to pull the shock, change the shim, bleed the shock and get it back on the bike. Due to the time involved few butt dyno tuning experiments are made and the fine tuning changes needed to perfect the stack are rarely completed.
ReStackor makes a huge difference here. With ReStackor, and a little more testing on the butt dyno, you can figure out if it is two clicks or three clicks that are needed to get the whoop performance right and the clickers settings need to get other aspects of the suspension performance dialed in. With that information ReStackor can plot the performance of your existing stack and show you the shim stack changes needed to reproduce those specific clicker settings. That allows you to experiment, in software, with different stack configurations and determine the stack structure and crossover configurations needed to best match your desired clicker map across the entire range of suspension speeds. The ability of ReStackor to do this in software and relate stack stiffness, clicker position and butt dyno test results to the hardware changes needed to tune the suspension is a huge streamlining of the entire tuning process. ReStackor gives you the ability to fine tune the stack far beyond levels previously possible. This capability of ReStackor defines a new era in suspension tuning.
