What's that with that fork oil level?

On some pages around here you'll find remarks like "fork oil level has to be xxx mm" or "fork oil level has to be exactly the same in both fork tubes", and you may wonder, why this has to be so. I'll try to explain.

Things being important for suspension are:

• the spring
• the damping unit
• the oil (brown)
• an air volume (blue)

As you may have noticed, when changing fork oil, fork springs normally have a "progressive" winding, meaning, on most of the lenght of the spring, windings have a constant width, while towards one end of the spring the windings become more narrow (see the difference between a stock fork spring (left) and the Wirth aftermarket spring (right)). This makes the suspension getting harder, the more it is engaged. This is called progression and prevents the fork from hitting through. Well, it shall do, anyway. The air volume additionally supports progression. Air is, unlike any fluid, a compressible medium. Thus, the more the suspension is engaged, the more the air is compressed, the harder the complete suspension system will become. The more of air volume you have, the more you can compress it, and the less you have, the less you can compress it. Thus, if you fill in more fork oil, the air volume will become smaller, and the suspension will become harder, when coming into progression. Now it's clear, I guess, why the oil level must be the same in both tubes. BTW, if ever you have heard of an "air supported telefork" (of the Africa Twin, f.e.), it's just half of the truth. According to what I said above, any telefork is air supported. The difference is: On an "air supported telefork" you can influence the progression in an easier way, by pumping an additional volume of air into the fork and thus precompressing the given volume. The effect is the same: suspension will get harder when coming into progression. Of course, air pressure and oil level must be the same in both tubes. BTW, to be sure, that the air pressure is the same in both tubes, when using such an air supported telefork, you may want to install a special three valves hose: two valves for connecting the tubes' valves and one valve for pumping air in, or releasing it, respectively.