Fill motorcycle fork oil - adjust oil level air cushion
Something fundamental: The fork should also be disassembled for simple maintenance or for an oil change.
Why? Simply put, abrasion accumulates inside the fork. This cannot escape through the drain plug, if present. It is therefore better to disassemble the fork and clean the components. This is easily done in a simple tub with some cold cleaner, assembly cleaner from a spray can or WD40.
Another important reason: The fork seals must be greased from time to time. The grease in the space between the two sealing lips ensures a low breakaway torque and sensitive response of the fork. This sounds professional, but it is immensely important for comfortable and relaxed motorcycle riding.
Why not just fill a certain amount? Because it is simply imprecise. If the oil was only drained via the drain plug, the question arises whether everything has really flowed out. In the case of shortened forks, different values result depending on the extent of the shortening. Are only fork springs installed or a fork damping system? Determining the oil level/air cushion always gives the actual value for every case.
Here's how:
Pour enough oil into each fork tube so that the fluid level in the tube can be clearly seen. Pull the fork tubes apart several times by approx. 100mm and push them back together to bleed the system. Then completely lower the front of the motorcycle, or, if the fork is removed, push the outer and inner tubes completely together.
Measure air cushion / oil level: The oil level is the distance between the top edge of the upper fork tube and the oil level inside the fork. This can be easily measured with a folding rule or a tape measure.
Adjust air cushion / oil level: If the measurement is too large, add oil; if it is too small, drain oil. Tip: If there is too much oil in the fork, it can be most easily sucked out with a syringe and a small hose.
The progression of the fork can be changed by adjusting the size of the air cushion (oil level). Higher progression means a harder fork towards the end of the suspension travel. This affects, for example, hard braking maneuvers and/or when the fork bottoms out (fork goes to the end stop).
Recommended air cushion / oil level:
more comfort 150 mm
more reserves 120mm (higher braking stability)
or a compromise between these two values.
The diagram illustrates the development of forces over the compression travel of a suspension fork. It can be clearly seen how this force changes towards the end of the suspension travel, with a change in the air chamber.