In a suspension, the springs absorb energy during compression and also return the suspension to the ride height when the compressive load is removed. If a suspension consisted of only springs, the resulting action would be like that of a pogo stick. Obviously, this would not be good and controlling a dirt bike would be nearly impossible. Here lies the purpose of damping.
Damping refers to the controlling of the spring action by means of oil flow through various orifices and valve plates from one side of a moving piston to the other side. Because oil is incompressible and viscous, the capability of it to flow through a given size opening is limited. This fact is exploited to provide damping so the velocity of compression and rebound is in control. At slow suspension velocities, oil flow is at a minimum, relatively speaking. The higher the velocity, the greater the oil flow. Of course, the resulting velocity is a function of the magnitude of force applied to the suspension.
Suspension velocities vary significantly depending on riding conditions. When a bike is casually ridden on smooth terrain, suspension movement is minimal and velocities are slow because forces acting on the suspension are very small. Contrast this to coming up short on a double jump. Slamming the face of a big double jump is obviously undesirable, but it highlights extremely high suspension velocity. So much force is applied to the suspension in such a short time that the suspension fully compresses seemingly instantly. And yes, it hurts.
Because suspension velocities vary so much, there is need to employ damping in various ways. This sheds light on the intricacies of suspension design and why designs evolve from year to year. Achieving excellent suspension design is a combination of science, common sense, careful engineering and exacting construction. Damping design involves the use of orifices, needle valves, check valves and spring loaded ported pistons. The point here is to reveal the basic idea of damping, not to expose a specific suspension design. Designs are indeed different and will continue to evolve.
No matter the suspension velocity, during compression or rebound, a piston within a fork or shock must move through an oil filled cylinder. For this to be possible, the oil must be able to move from one side of the piston to the other, otherwise the suspension movement would be impossible. At the slowest suspension velocities, oil typically will flow through fixed orifices. These orifices are typically associated with the low speed adjusters (if used), whether on the rebound side or the compression side. Oil will flow through the orifices at most if not all suspension velocities. Technically, slow speed adjusters affect the whole of suspension damping but within a very small range at higher velocities.
At higher suspension velocities, the orifices cannot accommodate the required larger flow of oil. For larger oil flows, a valve body (or bodies) is typically used that has very large ports. The valve body may or may not be an integral part of the moving piston. The ports are kept closed by spring washers and are stacked in a manner as to tune the suspension for a particular rider and riding style. The stack is designed to flex open as oil is forced through the ports in the valve. In this way, the valve stack opens variably depending on load, thus controlling oil flow at varying suspension velocities. This damping action occurs on both the rebound and compression strokes. Often, there are separate valve bodies for rebound and compression, each having unique valve stack arrangements. In some cases, only a single valve body is used. Valve ports are typically separate for compression and rebound and ports for each direction of movement will exist in each valve body. Compression ports flow during compression and rebound ports flow during rebound.
Using a basic cross section representation of a shock absorber, the oil flow within a shock can be illustrated. The first image below shows the oil flow during compression. In this illustration, the shock rod would be moving into the shock body during compression. The resulting force pushes oil against the rebound stack and no oil can flow through the sealed rebound ports. On the rebound stack side, the separate compression ports are fully open so the force of the oil within these ports pushes against the compression stack. The compression stack, made of spring washers, flexes open allowing oil to move from one side of the piston to the other. The amount the compression stack flexes open is proportional to the force exerted against the stack. Because the rod of the shock occupies space within the shock body, the gas accumulator volume proportionately decreases.
The next illustration shows the oil flow during rebound, which is when the shock is extending. Oil flow now occurs in the same way as described for compression but in the reverse direction. During rebound, the compressed spring releases stored energy and the oil is consequently forced through the open rebound ports, thus flexing open the rebound stack. The rebound stack will open more or less depending on the energy stored in the spring and the compression stack will remain closed. As the shock rod extends out of the shock body, the gas accumulator will increase in volume to compensate for the increase in volume left by the exiting shock rod.
The stacks of spring washers being discussed here are critical components in how damping occurs at suspension velocities greater than can be managed by the slow speed circuits composed of orifices. Additionally, the spring washer concept is advantageous for tuning purposes, commonly known as "revalving". The number of washers, the diameter of each and the thicknesses of each are characteristics of what are often referred to as valve shims. Using these characteristics, damping at high velocities can be tuned for rider weight and riding style. For example, a two stage stack may be preferred for motocross applications and a single stage stack for supercross.
In a two stage stack, part of the stack closest to the valve body is considered the slow speed stack and the remaining part of the stack is the high speed stack. The sections are separated by a small diameter shim called a crossover shim. This allows the low speed stack to open somewhat without opening the entire stack. Of course, it all depends on the suspension velocity. When the velocity is high enough, the low speed stack will "cross over" and deflect the high speed stack. The two stage stack helps provide damping that can manage varied terrain and jumps. A single stage stack lends more to riding supercross where the bulk of the riding is big jumps. Plushness on bumpy terrain is less likely a concern compared to motocross and trail riding.
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