 There's clearly a difference between open cartridge forks and closed cartridge forks, and most people are aware that closed cartridge forks are better. But why are they better? What makes them so different? Let's figure this out. The design of an open cartridge fork is quite simple. On the outside you have the outer tube and the inner tube. On the inside you have the cartridge at the bottom and the spring on top. The piston rod has the rebound base valve on the tip and is attached to the top cap, while the compression base valve is fixed at the bottom. An open cartridge fork has enough oil to cover the cartridge, so you end up with a considerable amount of space inside the suspension where you just have air at ambient pressure. This volume of air is particularly useful to provide bottoming resistance in huge compressions for example. However, with the evolution of motocross and specially supercross, which have bigger and bigger jumps, open cartridge forks started losing performance during races due to cavitation. Cavitation is the phenomenon where you have the formation of vapor bubbles in the oil much like boiling. If the oil suffers cavitation over and over again, you end up with a milkshake inside your forks instead of a clean liquid. As you can imagine, oil mixed with air doesn't provide great damping nor keeps your forks well lubricated. When repeatedly doing large jumps, two things will contribute for cavitation to happen. The piston rod inside the cartridge travels at a really high velocity, not giving enough time for the piston rod chamber to get filled with oil, causing the oil pressure to drop significantly. The oil inside the forks starts heating up. To understand why this is important, we have to take a look at some curves. This is the vapor pressure curve for a random fork oil and every liquid has a vapor pressure curve. This line represents the pressure at which a liquid boils or changes to its vapor state at a given temperature. If a fluid's temperature and pressure place it above the line, it will be in its liquid state. If it's below, it will be in its vapor state. Simple. In an open cartridge fork, the oil is at ambient pressure and is at a mild 20°C. For the oil to boil at ambient pressure, the oil would have to be heated to 200°C, so far so good. However, during a supercross race, the oil inside the forks heats up and it can reach 50°C. On big jumps, the piston rod travels so fast inside a cartridge on the landing that for a split second, a near vacuum is created, which means the oil pressure drops to well below ambient pressure. When the oil is at 50°C and the pressure drops, let's say, to below 20 kPa or 0.2 bar, the oil cavitates when it goes through the rebound check valve. If this happens just once, most pro riders won't even feel any difference, but when it happens every jump, everyone can feel the decrease in damping performance. So how can suspensions still perform at the highest level, regardless of how many jumps they go through? This is where the closed cartridge technology stepped in. This is a closed cartridge fork, and although it may seem completely the same as an open cartridge fork on the outside, on the inside, everything gets literally upside down. The cartridge is no longer at the bottom, but on top, and the spring is on the bottom. This switch is particularly useful to reduce the unsprung mass of the suspensions. The compression and rebound base valves also switch position, with the compression base valve placed on top, while the piston rod with the rebound base valve are attached to the bottom. The biggest difference in terms of performance is in the cartridge. The oil doesn't flow in and out of the cartridge anymore like in an open cartridge fork. You just have around 200 ml of oil responsible for the whole damping. Obviously, the great breakthrough isn't turning the cartridge upside down, but how they close it. The compression base valve is attached to the top cap. Between the top cap and the base valve, you have a free piston, which is this white piece over here, and a spring. This system is responsible for the biggest breakthrough in suspension technology of the last two decades. The simple system is responsible for two things. Number one, it compensates for the extra volume of the piston rod entering the cartridge. The free piston can rise to increase internal volume of the cartridge, keeping all the oil inside. And number two, it pressurizes the oil inside the cartridge, according to the compression. The more the free piston spring gets compressed, the more pressurized the oil becomes. The million dollar question here is, why would you need to pressurize the oil? Let's take a look at the vapor pressure curve once more. With the oil under pressure, the lowest achievable pressure inside the cartridge increases, greatly reducing the risk of cavitation. Genius, isn't it? What's great about this system is that you only need to pressurize the cartridge enough in order to eliminate cavitation, and no more. And obviously, you can adjust how much you want to pressurize the cartridge. You simply need to change the spring rate of the spring. You can also find a nitrogen-filled bladder doing the same job in forks from other manufacturers, such as WP for example. Instead of changing the spring rate, you change the nitrogen pressure inside the bladder. Although the bladder is more pressure sensitive, since you don't have the friction from the free piston seals, for example, it is usually not as reliable as a free piston and a spring. Besides greatly reducing cavitation, the closed cartridge also reduces another phenomenon from happening. Its name is hysteresis. No, it's not your hysterical cousin. Hysteresis is the damping delay of the forks when there's a change from compression to rebound or vice versa. Any millimetre of suspension stroke not being used for damping is wasted suspension stroke. Although we assume the oil is incompressible, it is actually just a tiny bit compressible and besides that, the cartridge can also expand a little bit. That delay in compressibility can affect the bike's performance for the best riders in the sport. But if you are a common mortal like myself, don't mind about that. When you pre-compress the oil inside a cartridge, it won't suffer that delay in compressibility anymore. Also, the cartridge will be already dilated, which will eliminate most of the hysteresis. Another detail I didn't mention earlier is the spring collar, which is responsible for having extra damping during big compressions. Since the cartridge has its own oil inside and is placed on top, the oil that lubricates the inner and outer tube only reaches the spring collar on bigger compressions. When that happens, the oil is forced to go through the smaller ports on the collar, restricting the oil's flow and providing extra damping. These guys can find damping everywhere. In theory, the cartridge is closed, hence closed cartridge, but it isn't completely sealed as the name implies. In certain circumstances, such as bottoming out the forks, the pressure outside the cartridge will push some oil inside of it through the cartridge seal at the bottom. This only happens because the cartridge seals do seal the oil from inside out, but not the other way around. This implies that there is an excess of oil inside the cartridge, so when you re-compress the forks, the free piston rises above the orifices on top. This allows the excess of oil to exit the cartridge, self-regulating the volume of oil inside. However, this is not a frequent event. Besides that, the amount of oil that enters the cartridge is quite small, but it explains why the oil inside the cartridge isn't perfectly clean when you service your forks after a long period of time. It almost feels like the closed cartridge technology should get the Nobel Prize, right? Well, not so fast. Having a pressurized cartridge that is almost completely sealed brings two downsides. If one of the seals fail, you'll lose considerable amount of damping performance immediately. Second, having a pressurized cartridge and more contact surfaces between the different components means you'll need a greater initial force to overcome the static friction, which can give a harsher feel, especially for off-road use. So how do dirt bike manufacturers end up choosing which one to use? Performance-wise, open cartridge forks tend to provide a more progressive damping, which makes sense for off-road, and it will be more than enough for the large majority of us. In terms of manufacturing, they are less expensive to produce. And since it is a quite simple system, it will also be a lot easier to service. It will also have fewer components and wear parts to replace. On the other hand, closed cartridge forks provide a less progressive damping and are a lot more focused for racing, since they provide a more stable performance. However, it is more expensive to produce, since it is more complex, require more maintenance and require more experience to service them. Like everything in the off-road world, it always comes down to what is more suited for each application and the balance between performance and maintenance. I hope you enjoyed the video and if you have any questions, feel free to leave on the comments below. Thank you for watching.