Motorcycle Fork Oil Filling - Adjusting Oil Level and 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, spray-on assembly cleaner, 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 fork response. This sounds professional, but it is immensely important for comfortable and relaxed motorcycle riding.
Why not just fill a certain amount? Because it's simply inaccurate. If the oil was only drained via the drain plug, the question arises whether everything 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.
How to do it:
Pour enough oil into each fork leg so that the fluid level in the tube is clearly visible. Pull the fork tubes apart about 100mm several times and push them back together to bleed the system. Then lower the front of the motorcycle completely, or, if the fork is removed, push the outer and inner tubes completely together.
Measure air cushion / oil level: The oil level is the measurement 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 high, add oil; if it is too low, drain oil. Tip: If there is too much oil in the fork, it is easiest to suck it 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). A 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 reaches its 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 during the compression travel of a suspension fork. One can clearly see how this force changes towards the end of the suspension travel, with a change in the air chamber.
