 Shimstacks and Valving are probably the least understood topics in the dirt bike community. Not only because it's a complex and subjective topic by itself, but also because there's little information online. That's exactly what I'm talking about today. I'll introduce you to Shimstacks, I'll explain you how Valving works and how tuning them will affect the performance of your suspensions. Shimstacks and Valves were developed by the racing industry around the mid-80s. At the time, the most advanced suspension technology available was Orofistal damping rods, which provided a really progressive damping. A progressive damping curve looks something like this. Orofistal suspensions provide little damping at low speed, making the suspensions kind of mushy and too plush on low-speed compressions, and on the other side, they provide a lot of damping at high speed, making it too harsh and uncomfortable at high-speed compressions. This is why engineers started looking for a different solution. And that's when piston valves and shimstacks came into the picture. The valves have two major components, the piston or valve body and the shimstack. The valves body is, in its essence, a piston with holes or ports where the oil flows through them. If you remove the shims from the valve, the suspension becomes an Orofistal damping rod so just the piston isn't the solution. The shimstack is just a superposition of shims that is blocking the valve's ports. Are you wondering why they have this tapered shape? It's not to provide a more progressive damping. Shims bend just like a spring, they bend according to an axis. If the shims have a different bending axis, the shimstack will less likely suffer distortion after extended use. With the shimstack had a straight stack of shims, every shim would bend according to the same axis and would become permanently deformed more easily after extended use. Let's do a quick recap on how a tapered single-state shimstack works. When the suspensions are quickly compressed, the damping is initially provided by the low-speed damping Orofist. It provides sufficient damping without feeling too stiff on the travel. However, the low-speed Orofist is not enough to prevent oil pressure from building inside the cartridge, until it's high enough to bend up shims from the ports. This allows more oil to go through the valve, reducing damping. A shimstack will always provide damping at a linear rate, with a speed of compression. This is why suspensions have shims. They are in essence a spring, which makes them predictable and easy to work with. But this creates a new problem. Before we had a damping curve too progressive, and now we have a linear curve. If only we could find a middle ground, of course we can. What I showed you is a tapered single-state shimstack, but the most common setup by far is a tapered double-state shimstack. Let's take a look at how it works. Like the name implies, you have two stacks of shims. The shimstack blocking the ports is the low-speed damping shimstack. The second shimstack is the high-speed damping shimstack. During the compression stroke, the low-speed shimstack bends, allowing more oil to flow through the valve, in addition to the oil going through the low-speed orifice. If that opening isn't enough to prevent oil pressure from increasing, the low-speed damping shimstack keeps bending to a point where it starts pushing the stiffer high-speed damping shimstack, opening the ports even more. These phenomenons don't take place step-by-step as I explained. They happen gradually, and that's why suspensions feel so smooth down the stroke. Let's see how changing these components affects your suspensions. Let's not forget that one doesn't fit all. There is no such thing as a great setup for every scenario. Each application demands a unique setup in order to achieve the best performance. You can change two major components in your valves, the piston and the shimstack. To be more accurate, on the piston, only the ports can be changed. For some specific applications and suspensions, the size of the ports out of the factory can prove to be too restrictive, even if the shimstacks are fully open. The ports come in many shapes and sizes, but don't be fooled. A port isn't just a hole in which the oil flows. Engineers spend countless hours working on fluid dynamics to make sure the ports provide the best oil flow while reducing any unwanted phenomenons, such as cavitation. In most cases, changing the shimstack setup will provide the desired results. The most common feedback people hear about suspensions is that they are either too stiff or too soft. This feedback can be translated to the stiffness of the shimstack. You can change the stiffness of your shimstack in three different ways. First, you can add shims. Replacing shims is quite easy to understand the principle. The more shims you have to bend, the harder it is. The opposite happens if you remove shims. Second, you can replace a shim with a different diameter. It is harder to bend a shorter lever than a longer lever, right? It's the same principle. The opposite happens if you replace it with a bigger diameter shim. So far so good. However, replacing shims with the same material and diameter, but different thickness isn't as straightforward as it might sound. The stiffness is proportional to the thickness cubed. Meaning that if we need to replace a 0.15 mm shim with a 0.10 mm shim without changing the valve stiffness, we need more than 3.10 mm shims. To replace a 0.20 mm shim, we need 8.10 mm shims. And to replace a 0.30 mm shim, we need 27.10 mm shims. So why even use really thin shims? Like the tapper shape, this is simply to reduce the chances of permanent distortion. It is particularly useful to use thinner shims on the low-speed shim stack, as they bend a lot more both in frequency and amplitude, compared to the shims on the high-speed shim stack. If you bend a sheet of paper and a cardboard piece, the cardboard will show signs of distortion quicker. It's the same principle. Now you know how to change the shim stack. But how does it translate onto the damping of your suspensions? Let's take a look at the damping curve of a double-stage compression shim stack. This is a random double-stage damping curve. We can see it as progressive, since it provides more damping as the velocity of the piston rod increases. The initial rise in damping is provided by the low-speed orifice, before the low-speed shim stack opens. We can see there's a deflection point, which is a transition from the low-speed damping to high-speed damping. In dashed lines, we have the damping curve when the clicker is fully open and when it's fully closed. The clickers have a clear influence on the damping curve, anticipating or delaying the high-speed damping transition on the shim stack for the same compression speed. It also increases, or decreases, the maximum damping provided by the suspension. Now let's make some changes to the low-speed shim stack. If we remove shims, the low-speed damping slope decreases, providing less damping. This means that your suspensions will become softer for low-speed compressions. If we introduce shims, we will increase the slope of the low-speed damping section, meaning it will stiffen the suspension on slower compressions. Changing to a smaller diameter shim will have the same effect. Increasing this slope can be suited for supercross, for example, since it reduces the bike's squat right before a jump or prevents the bike from diving too much during braking. The smallest shim right below the high-speed shim stack is called a crossover shim, and its thickness and diameter are really important. Besides being able to affect the low-speed damping, since all those shims bend according to the crossover shim, its thickness also affects the transition between low-speed and high-speed damping. If we introduce a crossover shim with a bigger diameter, it will stiffen the low-speed shim stack. However, if we increase its thickness without increasing its diameter, we'll just delay the high-speed damping transition, since the low-speed shim stack has to bend further to reach the high-speed shim stack. On the high-speed shim stack, the same principles are applied. If we introduce more shims, replace with a thicker shim, or reduce the diameter, it will stiffen the shim stack, increasing the damping force and making your suspensions stiffer on high-speed compressions. This is ideal if you do a lot of big jumps and you want to increase the bottoming resistance of the suspensions, for example. Decreasing the stiffness can be beneficial for hard enduro, for example, since the rock terrain will translate into bigger travel compressions. It will make the suspension softer and will increase the traction and comfort. However, it will reduce its bottoming resistance. The smallest shim on the high-speed shim stack is called the clamping shim. Its diameter is the most important one, because the whole valve and shim stacks work according to this shim's diameter. If we increase the diameter of the clamping shim, we are stiffening everything up. If we reduce its diameter, the suspensions will provide less damping throughout the whole suspension travel. A good rule of thumb is that a crossover shim should never have a diameter smaller than a clamping shim. It's easy, right? Please keep in mind that any small and isolated modification will have an effect on the whole damping curve, even if it's just a tiny bit. And these modifications will vary according to the suspensions you're working on, whether it's an open cartridge, closed cartridge fork or even a shock. The next time you need to visit the suspension workshop, you will remember that you are not only paying for the extra two shims on your valves, but also the experience and ability in providing you the right setup according to your needs, without disassembling your suspensions over and over again. Also, don't forget to ask what they have done. Knowledge is power. Subscribe and if you have any questions, leave a comment. Thank you for watching.