Oil height is often used to tune bottoming resistance without making the first part of the stroke harsh.
The air-spring effect
Fork oil height changes the air volume trapped above the oil. A higher oil level leaves less air volume, so pressure ramps up faster near the end of the stroke and adds bottoming resistance.
When to change it
Oil height is useful when the fork feels acceptable early in the stroke but uses the last part too easily. It is not a substitute for correct spring rate, worn bushings, contaminated oil or a poor compression-valving baseline.
Workshop checklist
- Measure oil height with springs out and the fork fully compressed.
- Change oil height in small, equal steps on both fork legs.
- Use a travel marker to confirm whether end-stroke use actually changes.
- Keep the oil volume within the fork manufacturer's safe range.
Next step
Use the calculator for stack comparison, the handbook for deeper theory, or the workshop booking form when the bike needs service or valving work.