Changing the oil level in your forks can have a significant effect on fork action especially in the later stage of fork compression. Before considering changing the oil volume you should make sure that
Once your set up is right changing the oil volume can be used as a tool
to fine tune the compression towards the end of the stroke e.g.
Always check in your manual the minimum and maximum oil volume allowed in your specific fork to avoid any of the following scenarios:
Air has a progressive compression effect under pressure. The air gab in the forks therefore helps prevent the fork from being too soft and bottoming too easily. The smaller your air gap (the more oil you put in the fork) the harder the fork becomes towards the end of the stroke.
No matter what the air gap the initial part of fork stroke differs very little. Normally the initial pressure in the fork is atmospheric at full extension regardless of the oil volume. This is why the air bleeders should only be pushed with the front wheel off the ground. It is only when the fork compresses that the pressure build up differs substantially depending on the air volume and size of the air chamber.
Understanding the exact relationship between pressure and volume is complex. All you need to know is that the smaller you make the size of the air chamber when the fork is fully extended the more progressive (harder) your fork will become as it compresses.
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders