Hysteresis and Cavitation
Hysteresis - The force difference between the open/close phases of the compression and rebound curves. Also known as "valving lag", in its simplest form, this is determined by three variables: 1) Air Pockets - A monotube damper works on the premise of 3 individual chambers: Rebound Chamber, Compression Chamber and Gas Chamber. The Rebound & Compression Chamber must only be filled with oil, which is, for all intents and purposes, non-compressible. The Gas Chamber must only be filled with gas (usually Nitrogen) and is compressible. If there are air pockets inside the Rebound and/or Compression Chambers, then these turn from a non-compressible state to compressible state. As mentioned above, it is the piston that flows through the oil and not the other way around. The non-compressible oil acting upon the shim stacks is what creates the damping pressure. If the piston/shims interact with compressible air, then there is very little damping pressure created; not what you want. This usually manifests itself as hysteresis during the low-speed section of the damping curve Unfortunately, 20% of the oil in a bottle/drum is dissolved air. If, during assembly, this air is not removed from the oil, then you will have "compressible oil" inside your Rebound/Compression Chambers. As explained above, not desirable. How to solve this issue? The proper way is to use a damper vacuum during assembly. This basically sucks 100% of the air out of the internal chambers as well as filtering the oil of dissolved air. Very few manufacturers use these machines, and most haven't even heard of it. It is typically only your larger companies that also make bike shocks that know about this technology, which is a pity. The other way is to manually pump the air out by spending 20+ minutes pumping the piston rod up/down the damper body. This is what most good manufacturers will do; however, this will only get you 90% of the way there and can be quite an intensive work out! hah 2) Seal Drag - Can be separated into two variables: Friction and Stiction. Friction is how much of an opposing force there is between 2 surfaces. Stiction is how much force is required to overcome the static friction force and enable motion. Lower friction/stiction means smoother movements as the rod and IFP moves up/down the damper body and less of an opposing damping force, which is why only the highest quality o-rings and bushings should be used. This usually manifests itself as bumps/anomalies anywhere along the damping curve 3) Internal Pressure - Besides the fact that higher internal pressures (and higher temperatures) will increase seal drag, to understand why high internal pressures are undesirable, you must first know the stages of motion of a damper. There are 4 stages: - Stage 1: Compression Open. This is the acceleration phase of the compression stroke - Stage 2: Compression Closed. This is the deceleration phase of the compression stroke - Stage 3: Rebound Open: This is the acceleration phase of the rebound stroke - Stage 4: Rebound Closed: This is the deceleration phase of the rebound stroke Unlike the damping forces generated by the valving which is velocity dependent, the internal pressure of the gas chamber (which subsequently sets the internal pressure of the other chambers) is displacement dependent i.e. The more the gas is compressed, the higher the pressure So keeping this in mind, if you run through the 4 stages of motion: - Stage 1: Starts at preset gas chamber pressure, and increases the more the chamber is compressed - Stage 2: Rod starts slowing down, but as pressure is displacement dependent, pressure still increases. This is why the Compression Closed curve is always greater than Compression Open as it takes more and more damping force to compress the chamber - Stage 3: Rod stops (for a very brief moment) and reverses direction. Chamber starts to expand, and pressure starts to decrease over the rebound stroke - Stage 4: Rod starts slowing down, and because the chamber pressure is decreasing, there is less “assistance” to extend the rod. This is why the Rebound Close curve is always greater than Rebound Open as it takes more and more effort to extend the rod. Now, as stated at the start, Hysteresis is simply the force difference between the Open & Close phases of the Compression and Rebound curves and, now knowing the 4 stages of motion, it is now clear why we come to the conclusion that higher internal pressures leads to higher Hysteresis Hysteresis is unavoidable, but we can keep it to a minimum by pressure tuning. Cavitation - This is when pressures inside the chambers drops to the point that the oil starts to vaporize. As mentioned above, compressible oil is not desirable. While the vapor will return to liquid relatively quickly, the more this happens, the faster the oil will breakdown which will decrease damping forces. This is known as “fade”. So how does this happen in the first place? during the compression stroke, piston face pressure on the compression chamber side increases, so the pressure on the rebound side of the piston face must decrease by the same amount (Newtons 3rd law). If the pressure on the rebound piston face drops to zero, this is called the flash point, and the oil vaporizes. The higher the compression damping forces, the larger the pressure drop. I emphasize this point as one of the reasons why you should not adjust compression using the piston rod bleed adjuster. when the rod reverses (i.e goes into rebound), the air pockets in the rebound chamber do not flow through the piston. Instead, they compress (oil does not compress. Air does), which means as the piston moves back up the damper body, it encounters minimal resistance as there is no oil trying to force its way through the shims. This also has the side effect of creating a vacuum in the compression chamber i.e volume increases, but there is no oil to fill the void. So, what happens then? Worst case, nitrogen starts getting sucked past the IFP seal and into the compression chamber. This then causes aeration (nitrogen mixing with the oil). Cavitation is avoidable (to an extent) and, like Hysteresis, can be controlled by pressure tuning.