Ive an R so dont really know what size the Z is two up but im 6ft4 and 15stone and i know what you mean you need to seat back in the seat. Ive taken my dad/mate on the back both about 6ft and 14-15stone and you did have to give the bike some to keep it going and over takes etc.

My dads got a tiger and its got plenty of power and can cope with the 2 up but my mum finds the pillion seat slopes forward too much causing her to constantly to slide back etc.

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Plough on