 The ZF-SAC's separate function front forks have been on a market for a couple of years now, but there is still little understanding on how these forks actually work. Well, not anymore. After reaching out and talking to one of ZF-SAC's engineers, I managed to better understand how these suspensions work and how the adjustments affect their performance. But first, let's do a quick recap. Do you still remember how a traditional open cartridge fork works? On compression, the same volume of oil as the volume of the piston rod entering the cartridge is pushed through the compression base valve through the low-speed orifice. The mid-speed valve opens allowing the oil to more easily fill the chamber above, while still providing compression damping. While this happens, the low-speed shim stack opens on the base valve followed by the high-speed shim stack, allowing more oil to exit the cartridge. On rebound, the piston rod slides up the cartridge and the volume of oil that is in the top chamber is forced to go through the low-speed rebound orifice in the first instance, then the low-speed shim stack opens and finally the high-speed shim stack to allow more oil flow. The check valve in the compression base valve opens allowing the cartridge to get easily refilled. We can note that the compression clicker is placed at the bottom and the rebound clicker is placed on top. Although I use the example of only one suspension fork leg, it's exactly the same on both sides on a traditional open cartridge fork. However, ZF Sachs decided to simplify the adjustability of the forks in order to have both the compression and rebound adjusters on top. Like the name implies, the separate function forks have one fork leg focused only on rebound damping and the other fork leg focused only on compression damping. On the left top cap, you can adjust the low-speed rebound damping. On the right top cap, you can adjust the low-speed compression damping and the preload of the spring, which will affect the front fork sag. If we take the forks apart and compare both cartridges, the compression and rebound legs only have one difference. The compression leg has four holes on top of the cartridge, but more on this later. Although ZF calls it a separate function fork, both legs have a compression base valve, but without a typical low-speed damping orifice and its adjuster. The biggest difference is in the piston rod assembly. The compression leg has a piston rod with a mid-speed valve, but no rebound damping shim stack. While the rebound leg has a piston rod with a rebound damping shim stack, but no mid-speed valve. Let's understand how all these changes affect the damping performance starting with the compression leg. On compression, the piston rod enters the cartridge. The oil pressure increases in the chambers between the valves and the initial damping is provided by the low-speed orifice on a mid-speed valve. As the pressure increases, the mid-speed valve opens while still providing compression damping. The low-speed shim stack on the base valve quickly follows and if the pressure between the valves keeps increasing, the high-speed shim stack opens, allowing even more oil flow and reducing damping. On rebound, the spring makes the fork extend. Since the piston rod assembly has no rebound shim stack, there isn't any rebound damping provided by the piston rod. The holes on top of the cartridge prevent the top chamber from pressurizing, eliminating any rebound damping. This will make the compression damping provided by the compression base valve dependent essentially on the oil pressure between the valves instead of the volume of the piston rod. Now let's check the rebound leg, which is similar to a traditional open cartridge fork. On compression, the same volume of oil as the volume of the piston rod entering the cartridge is pushed through the compression base valve. At first, there's no damping, since there's no mid-speed valve and the low-speed shim stack on the compression base valve doesn't fully block the valve's ports. Only on bigger compressions, the high-speed shim stack is able to provide some damping to keep the rider safe and maintain the reliability of the forks. On rebound, the spring makes the fork extend and forces the oil on the upper chamber to flow through the piston. The damping is initially provided by the low-speed orifice and as the pressure increases, the low-speed rebound shim stack opens followed by the high-speed shim stack. The compression base valve will allow the cartridge to get refilled more easily, since the ports aren't fully blocked. A common question regarding separate function forks is, will I feel any difference between the compression and rebound legs? Absolutely none. Once the forks are fastened to the triple clamps and the wheel, they work as one. However, this design has some limitations. It means that the compression adjuster will only affect the mid-speed compression damping and no longer the damping provided by the compression base valve. Having the compression adjustment on the mid-speed valve will provide a good amount of tuneability on low-speed and smaller compressions, but little adjustability on high-speed and deeper compressions, since the clicker will have no effect from the moment the mid-speed valve is fully open. Funny how this technology doesn't seem so weird after all, right? Let's all give a huge shout-out to Luigi from ZF Sachs for his time and help in the making of this video. If you have more questions, leave them on the comments below and thank you for watching. Knowledge is power.