Originally Posted by The Griz
Oil lubricates too. Being you're so smart, and for some reason you know that the oil isn't for lubrication purposes, then please explain why they even bother with fork oil then??
Bit grouchy, then?
As noted previously:
1. Air spring. Probably the primary reason for fluid in the fork tubes with the Bitubo dampers. All suspension benefits from SOME amount of 'progressiveness', and this is achieved (in forks) by compressing the air above the oil. No oil would make the air spring virtually unnoticeable since you're compressing 30 or so inches of air into 20 or so inches of fully compressed fork. Too much oil (not enough air) makes the air pressure ramp up too quickly (say, 10" of air into 2" or so of space) and the forks become harsh. Proper tuning of oil height provides the optimal mix of compliance in the typically used range (give or take the first half of travel), but the necessary stiffness (and to a degree, resistance to bottoming) deeper in the stroke.
2. Heat dissipation from the damper unit. Probably not the main reason for the oil, but certainly necessary, especially in heavy use. The sealed damper unit has far less fluid (and metal) than a conventional fork, but it does the same amount of work, and all that heat has to go somewhere. Overheated suspension fluid leads to accelerated wear on the hard parts, shortened life of the fluid, and less consistency with the damping.