Friction in shock absorber with variable preload

Hello

I have an issue with defining ftiction in a shock absorber.

I modelled the airspring and damping forces as SpringDamper element.

Then I wanted to include sealing friction. So I created 2 bodies and places a translation joint.

My idea was to use the friction tab panel in the joint to include a friction which depends on the airpring value during compression.

However, there is no way to define as an expression the preload field.

Any idea or workaround?