 The rear shock is, without a doubt, one of the most important components of a dirt bike. But do you know how it works? What is the linkage for after all? Stay tuned to find out why. If you watch the last off-road engineered videos, you'll easily understand how a shock works. If you haven't, check them out before watching this one. A shock is probably the most demanding and sensitive component to work with on a bike for three reasons. First, there are several components between the wheel and the shock that can interfere with its proper functioning. The swing arm, the linkage system and all the bearings in between. Second, since the shock is not directly connected to the wheel, the forces applied to the shock are greatly dependent on the bike's setup, as opposed to the front forks, which are directly connected to the wheel. And third, the rear shock has half the size of a front fork and has twice the job. Besides having to absorb a major part of the obstacles, the rear shock is also responsible for transferring efficiently the power of the engine to the ground. The shock has pretty much the same components as a front fork, but given the lack of space on the rear of the bike, things have to change a bit in order to fit. On the outside, we'll find the spring, with a piston rod and a bumper on the bottom and the shock's body on top, and a reservoir on the side of the shock's body. The low speed rebound adjuster is placed at the bottom near the linkage, and the low speed compression and high speed compression adjusters are placed on top. If we take it apart, we'll find the piston rod assembly, with the main compression shim stack below the piston, and the rebound shim stack above the piston. These two shim stacks are going to decide the damping character of the shock, whether it has a more linear or a more progressive damping, depending on its setup. Inside the oil reservoir, we can find the bladder, responsible for pressurizing the oil inside the shock. In shocks from other manufacturers, a diaphragm can be found doing the same job as the bladder. The differences between both technologies follow the same principles as in closed cartridge forks. Behind the adjusters, there's a compression valve. The oil flows through the low speed orifice, and the oil flow is determined by how much the low speed compression adjuster is tightened. Besides the orifice, there's a high speed compression shim stack. The preload of the spring is determined by how much the high speed adjuster is tightened, and will in turn affect the preload on the shim stack. At the bottom of the valve, we can find the check valve, which allows the oil to freely fill the shock's body during rebound. Let's find out how all of this works together. On low speed and smaller compressions, the same volume of the piston rod entering the shock's body is pushed through the low speed adjuster, creating low speed compression damping. Besides the low speed adjuster, the oil above the piston has to fill the chamber below. The oil goes through the low speed rebound orifice on the piston rod, also creating low speed compression damping. Yes, it's shocking! The low speed rebound orifice creates damping both in rebound and compression, since you don't have a check valve to freely fill the chamber. On rebound, after a small compression, the oil below the piston is pushed through the low speed orifice, creating low speed rebound damping. Since the piston rod is exiting the body, the oil is drawn from the reservoir to fill the shock's body through the adjuster's check valve. Let's see what happens when you compress faster or deeper into the stroke. On quicker or bigger compressions, the same volume of the piston rod entering the shock's body is pushed through the low speed adjuster. However, in this case, the shim stack opens in the high speed compression adjuster, since the adjuster's orifice is not enough to prevent the pressure from building. This allows a greater volume of oil to get into the reservoir, creating high speed compression damping. While this happens, the oil flows through the piston rod's low speed orifice, but like in the adjuster, the oil pressure keeps building, and the main compression shim stack opens. This allows more oil to fill the chamber below, providing the most amount of high speed compression damping. On rebound, after a deep compression, the spring makes the shock extend faster. The oil below the piston is pushed through the low speed orifice, but like in the compression stroke, the oil pressure keeps building, and the rebound shim stack opens, allowing more oil to exit the chamber below, creating high speed rebound damping. While this happens, the check valve in the adjuster opens, allowing the oil inside the reservoir to fill up the shock's body. What about the linkage system? What do these guys do in all of this? The shock has a small stroke length compared to the front forks, so it's a lot harder for the shock to provide a progressive damping when compared to the front forks. This is where the linkage system comes into play. The linkage system allows the shock to have a speed and position-sensitive damping. Without them, most shocks would lack bottoming resistance or lack of traction. This is a random linkage ratio curve, and what it tells us is that in the beginning of the compression stroke of the back wheel, the wheel will travel three times more stroke length than the shock. More down the middle of the stroke, the wheel will travel twice as much stroke compared to the shock. And at the end of the wheel stroke, the shock will compress pretty much the same stroke length compared to the back wheel. This is quite fine and dandy, but how does the linkage system enable that? This is a random linkage system. The blue circle represents the angular travel of the swing arm on the linkage. We assume the back wheel has vertical travel, but it has, indeed, angular travel. The green circle represents the angular travel of the linkage pull rod. To note that the green point is further back than the blue point. During compression, the pull rod will rotate with the swing arm, which will, in turn, make the shock rotate slightly, greatly reducing the compression rate of the shock for small stroke compressions. For mid-stroke compressions, the pull rod can no longer keep up with the rotation of the swing arm, so it starts pulling the triangular link, which will, in turn, rotate and make the shock compress at a faster rate. At the end of the stroke, the linkage will be fully locked with the triangular link drawn forward, which will make the shock compress at almost the same rate as the wheel. This means that for smaller compressions of the back wheel, even if it's a high speed compression, the oil will flow through the low speed rebound orifice and the low speed compression adjuster. And on bigger compressions, the oil will mostly flow through the main compression shim stack and the adjuster's high speed compression shim stack. Going with the flow, let's talk about adjustments. The shock's adjustments clearly aren't as straightforward to fiddle with as in front forks, but the same principles are applied to it. The biggest challenge on the shock is the fact that you can adjust both low speed and high speed compression, but let's take it step by step. If the low speed compression adjuster is fully open, the low speed orifice will restrict the oil's flow the least possible and allow a plusher feel on smaller compressions. If the low speed compression adjuster is fully tightened, the low speed orifice will restrict the oil's flow the most possible and will stiffen the shock on smaller compressions. If the high speed compression adjuster is fully open, the high speed compression shim stack on the adjuster will open more easily on mid-stroke compressions. This means it will provide a plusher feel, but provide less bottoming resistance. If the high speed compression adjuster is fully preloaded, the high speed compression shim stack will only open on bigger compressions or really high speed compressions. This means that on high speed compressions, the shock will be stiffer, but will provide more bottoming resistance. Bear with me, it's almost over. If the rebound adjuster is fully open, it will create the least amount of low speed rebound damping and low speed compression damping on the piston rod, and allows the shock to have a plusher feel. It will also delay to a certain degree the high speed transition of the shim stacks. If the rebound adjuster is fully tightened, it will create the most amount of low speed rebound damping and low speed compression damping on the piston rod, stiffening the shock. It will also anticipate to a certain degree the high speed transition of the shim stacks. The way we set the adjusters will create damping at different rates and at different moments of compression. It all comes down to how you want the shock to react to the inputs of the terrain. Don't forget these adjustments are supposed to fine tune your suspensions. If the base setting is off, the adjustments won't compensate for that. Putting some sprinkles over a poop won't make the poop any better. Same thing with suspensions. Hope you enjoyed the video, subscribe and leave your comments below. Thank you for watching.