Motorcycle Front Suspension: Telescopic Fork
Conventional motorcycle front suspension mainly consists of a telescopic fork and damper/shock-absorber. It is a type of suspension design which is common for most bikes. The telescopic fork operates hydraulically. It has fork tubes and sliders which contain the springs and dampers. Telescopic forks sometimes have gaiters to protect the tubes from abrasion and corrosion.
At the top, conventional forks commonly have a pair of fork tubes, or "stanchions". They are clamped to a triple clamp or a yoke. However, the sliders which are at the bottom attach to the front wheel hub. The tubes contain air, fork oil, and spring which collectively perform the shock-absorbing function.
To stiffen the suspension, manufacturers add pressurized air through a valve at the top of the fork. Some forks have a flexible air tube between both fork tubes to balance the air pressure in both forks. Modern fork tubes employ a screw to compress the fork spring to increase or decrease its pre-load.
Telescopic Fork: Damping
Cartridge type telescopic forks provide a regressive damping effect. Self-contained cartridges inside the forks have spring-covered holes for regulating fork oil flow. The springs resist low forces and thus, provide high damping rates. Higher forces compress the springs and allow more oil to flow. This results in less damping. Thus, the fork behaves stiff on small bumps. However, it softens as it encounters bigger bumps/potholes.
Most modern telescopic forks are also suspended on springs. They control the movement through adjustable valves by regulating the flow of fork oil. The larger the hole causes more free flow and less the damped fork. There is a selector at the top of the fork. On compression, it engages the desired hole or valve size and corresponding damping rate. However, usually, the lower tube has a mechanism to control rebound (extension) damping.
The up-down motion of the telescopic fork generates heat which causes variations in the viscosity of the oil. This adversely affects the effect of damping. So, nowadays, manufacturers employ single-action damping technology in the front suspension.