The internal dim.n of my rim is 20mm what is the max tyre width I can run?

2.4″ ish…maybe

Yeah usual 2.3-2.4s will be fine, it’s just that you’ll need more pressure to keep it under control in corners than a wider rim needs. Otherwise no bother.

A 2.4″ will start heading towards instability in hard cornering, at steep lean angles (20 degres) your contact patch is likely be outside a vertical line defined by the tyre bead. So no matter where you put your weight on the bike you can’t put it straight down via the tread.

Unless you like running 35psi+ I’d stick with 2.25-2.3″ as an absolute max.

Depends who’s measuring too. Conti MK2 2.4 is about the same as a Schwalbe 2.25.