It's One Of The First Things Any Race Team Changes And You Can Pay Over $2000 For A Good One, But JustA rear shock is generally made up of two components-a spring and a damping unit. The spring is there to allow wheel movement and to support the weight of the bike and rider. The damping unit is there to absorb the energy in these movements, slowing them down and preventing the spring from simply rebounding up and down continuously after each bump.
Most bikes use a coil-type spring wrapped around a hydraulic damping unit. This combined spring/damper design is compact, easy to site on a bike and relatively simple to manufacture. The spring is the simplest part of the unit, being basically just a length of thick metal wire-usually steel-wound into a coil shape. The dimensions and material of the spring will define its main characteristic: the spring rate. This is a measure of how much force is needed to compress the spring a certain amount. So a spring with a rate of 10kg/mm would compress by one millimeter if you put a weight of 10kg on it. Put 200kg on it, and it will compress 20mm. A "stiffer" spring (or higher spring rate) will deflect less for a given force, while a "softer" spring will deflect more.
Most units have a facility to preload the spring, usually by means of a threaded or stepped collar. If you turn the collar so that it compresses the spring, it will affect how the suspension unit moves when you put a force on it (i.e. the weight of the bike). If you had our example 10kg/mm spring on your rear shock with no preload, then putting 150kg of bike and rider onto it would compress it by 15mm. But if you used a preload collar to compress the spring by 10mm to start with, then our 150kg load would only compress the spring by 5mm, reducing the amount of initial travel on the unit.
It's important to realize that preloading a spring doesn't change its spring rate, so once the preload has been worked through, the spring operates exactly as it did before.
Ohlins is a highly regarded...
Ohlins is a highly regarded authority on aftermarket suspension, and its components are found on the highest-spec racebikes.
The damping unit is also a pretty simple principle at work. The damper body has a chamber filled with oil. Inside is a piston with small holes in it that seals against the inner walls of the chamber. As the shock compresses and extends, the piston is pushed through the oil and the oil passes through the small holes in the piston. The act of forcing the oil through tiny holes puts a drag on the shock's movement, absorbing the kinetic energy of the suspension movement and converting it into heat.
This simple principle has been fine-tuned over the years to the point that a modern monoshock unit has several sophisticated, adjustable, damping circuits. The holes that the oil has to pass through can be made larger or smaller by means of tapered needles screwed in or out of the hole, altering its area and the rate at which oil can pass through. Stacks of thin metal washers, or "shims," are used to cover extra holes in the piston, only opening them up when the movement of the shock is very rapid.
Shim stacks can also serve to separate compression and rebound damping. Compression damping comes into play when the wheel is traveling upwards (the suspension unit is compressing), and rebound is when the wheel is moving downwards (the unit rebounds). The shims can only open when the motion of the piston is such that the oil is pushing the shim away from the piston, opening the hole. When the piston moves back the other way, the oil pushes the shim closed even harder, preventing any oil passing through. So by having two stacks of shims facing either way, one on either side of the piston, we can fine-tune the compression and rebound damping characteristics to suit the bike.