Fork oil

Fork oil is the hydraulic fluid in front forks and shock absorbers. As the fork compresses and rebounds, the oil is forced through holes and valves, creating the damping that keeps the spring's movement under control.

Viscosity is usually given as a weight (e.g. 5W, 10W, 15W). Thicker oil gives more damping, thinner oil less. These numbers are not comparable between brands; comparing viscosity in cSt at 40 °C is more accurate. Mixing two oils gives an in-between viscosity – work it out with our fork oil mix calculator.

The oil level matters too: a higher level reduces the air gap and stiffens the end of the stroke. Find the right volume with our fork oil level calculator. Fork oil degrades and is changed with the seals or per the service manual.