they say the 17 is 22.5 mm , the old 09 had 23.5mm i put pro circuit 22 mm on the 09 and made a difference in grip and turning , i dont think i would bother with a 17 or 18 250 f until i tested one to see if it needed it , as i heard they handle well , i would spend time on setting up the suspension and revalve to my needs.