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Note: The goal of this brief document is to help you understand the simple basics of shim-valved systems.
At FOX Racing Shox we use several methods to control the damping in our suspension products.
The following method is a common way to increase or decrease the overall damping of the shock. By changing the quantity, thickness, and arrangement of the valve shims, you can tune the shock for different operating characteristics. There is a valve stack on both sides of the damper piston. One side of the piston controls compression damping, and the other side controls rebound damping.
The rebound valve shim stack, in most cases, will have a smaller outside diameter (OD) than compression valve shims do and the valve shims will usually have a thicker cross section when compared to the compression valve shims. This is due to rebound damping having to resist the force of the spring, and because piston speeds are much slower during rebound.
In the case of the Vanilla and Float shocks, the damper piston will have a free-bleed hole drilled into the damper piston. This free-bleed hole controls the rebound speed of the damper by changing the size of the hole, which directly affects rebound speed (i.e., smaller equals slower, larger equals faster).
There is an unlimited amount of valving combinations, so many different variations will achieve very similar results. Many suspension technicians that routinely tune suspensions will have different ideas regarding how to increase or decrease damping with valve shims. As long as the rider is happy with the changes, this is what matters most, not how one achieved the results. We think of it as great guitar or bass soloing; musicians have different styles and approaches to making their music. This is not much different than shock tuners, in a metaphorical sense.
The end result is what ultimately matters; you either like it or you don't!
You should consider working on shock valving only after you have determined the correct rear shock sag setting for the application. Also, make sure that everything in your suspension is well lubricated and is moving freely.
If the compression damping adjustments are full in (“closed”), you should be thinking about adding compression valve shims.If your compression damping adjustment is full out (“open”), and your suspension is still feeling over-damped, you should be thinking about removing compression valve shims.
In general, the rebound damping does not need to be changed unless the spring rate is changed. If you increase the spring rate, a corresponding increase in valve resistance is recommended.
It is better to use more thin shims than a few thicker shims as the thin shims will provide better, smoother damping.
However, two .006" thick shims do not equal one .012" thick shim, as the two thinner shims would deflect more (be softer) than the single thick shim.
If you are using a progressive stack with a gap shim in place, by making the gap shim thicker, the shock becomes softer. You would think that a thicker shim would make the shock stiffer, yet what really happens is the thicker (small diameter) gap shim lets the large shims deflect further before hitting the next line of progressive shims in the second stage stack, thus making the shock softer.
Another alternative is to go to a different diameter gap shim. As mentioned previously, the valving combinations are endless.
After changing springs and oil seals, your results are documented with valve coding, to:
A valve code is the sequence and the count of shims in a valve shim stack, or in other words, "recording the damping valve shim setup".
The term “speed” in damping (i.e., Low, Medium and High) relates to shaft velocity of the shock shaft, not how fast the bicycle is moving.
Note: Now that you know the basics of damper tuning, you can appreciate the man-hours we have spent working with our OEM customers, to assure that damper valving is fully optimized at the factory.
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