Out of pure curiosity this afternoon, I measured the heights of my new 120/70 and an old 120/60 on the SS, both on rims.
Measuring from the edge of the rim to a straight edge across the top of the tyre, the difference is indeed 12mm (near as dammit). I was slightly surprised even though the maths says that would be the case.
I've put a 170 on the back of the Monster this time, having only fitted 180s previously. I must say it does look right and I'm very much looking forward to trying the new pair out.
Having taken on board the comments on this thread, I might well even put a 120/70 on the front of the 750SS when I commission it for riding. According to the glyphs on the sidewalls of the current pair, they originate in the Mesolithic period!
