Fill fork with oil via air cushion / oil level

Fill fork with oil after maintenance, shortening or machining.

Something basic: the fork should be disassembled even for simple maintenance or oil change.

Why? Quite simply, it lagerts itself abrasion in the inside of the fork. This can not come out over 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, Monday cleaner from the spray can or WD40.  

Another important reason: the fork oil seals need to be greased from time to time. The grease in the space between the two sealing lips ensures a low breakaway torque and a sensitive response of the fork. This sounds profarm, but is imens important for a comfortable and relaxed motorcycling. 

Why not just fill in a certain amount? Because it is simply not accurate. If the oil was then only drained via the drain plug, the question arises as to whether everything has really leaked 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? The determination via the oil level/air cushion always and for each case results in the actual value. 

This is how it works:
Fill enough oil into each fork tube so that you can easily see the fluid level in the tube. Pull the fork tubes apart several times by about 100mm and push them together again to bleed the system. Then completely drain the front of the motorcycle, or if the fork is removed, push the outer and inner tubes together completely.
Measure air cushion / oil level: The oil level is the measurement between the top of the upper fork tube and the oil level inside the fork. This can be easily measured with a folding ruler or tape measure.
Adjust air cushion / oil level: If the measurement is too high top up the oil, if it is too low drain the oil. Tip: If there is too much oil in the fork, the easiest way to suck it out is with a syringe and a small hose.

The progression of the fork can be changed via the size of the air cushion (oil level). A higher progression means a harder fork towards the end of the suspension travel. This has an effect, for example, during hard braking maneuvers and or when the fork sags (fork goes into the end stop).

Recommendation air cushion / oil level:

 more comfort 150mm
 more reserves 120mm (higher braking stability)
or a compromise between these two values.

The diagram illustrates the development of the forces over the course of the compression travel of a suspension fork. You can see well how this force changes towards the end of travel, with change in air chamber.