Skinny:
Modern suspension has come a long way. In the old days you did not have any external adjustments. The internals were not all that sophisticated either. It was a hydraulic system with a hole in a tube & a certain amount of oil was forced through these holes. Nowadays, there is a lot more stuff going on inside your forks & shocks. Basically it is like this: your shocks & forks hit bumps & dips on the trail & they move in & out with a certain amount of resistance to keep your tires on the ground which gets you good traction. Suspension is always 'compressing' & then 'rebounding'. Ideally you want the suspension to compress easy enough so you don't get beat up by every little bump on the trail. But you don’t want it so soft that is wallows or bottoms out everywhere. After you pass the bump you want the suspension to rebound & stick to the ground & be ready for the next bump. So the suspension is compressing & rebounding fast or slow depending on the obstacles you are hitting. When somebody says “high speed” or “low speed” they are talking about the suspension movement (in & out). Not the speed of the motorcycle. It is the speed of the suspension. If you are in the garage pushing up & down on your suspension, this is “low speed”. When you are out on the trail & you hit a log or root, the suspension moves very quickly. This is “high speed” damping. Inside your suspension oil is being forced through many different channels, holes & past small steel washers called 'shims'. The holes can be large or small but will ultimately flow only as much oil as can fit through that particular hole. Oil flowing through a hole is usually low speed action. As the flow of oil reaches its limit it will then open up a secondary shim stack and/or spring loaded shim ‘pack’. Inside modern suspension systems there are damping pistons. These pistons have holes or slots which are covered up by a set of stacked steel washers or shims. With different forces of oil against these shims they will flex back a certain distance & allow oil to flow through. The arrangement of these shims, their stacked height & individual thickness all dictate how much oil will flow. Many companies have sprung up that will “re-valve” your suspension. These companies are rearranging the shim stack inside to better suit the individual rider’s taste. MX, off road & street bikes all use different shim packs & the changes are infinite. Beyond this internal damping system most modern suspension systems have external adjusters. These are (for the most part) convenience items intended to allow the user a broad choice of movement control. MX riders like stiff settings, off road riders like soft suspension, etc. Theoretically with the simple turn of a knob both riders can ride the same bike.
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