Join Date: Mar 2000
Scott is right about the 5" rim, but a 120/60 is the same width as a 120/70, but it has a shorter sidewall. The first number is the width of the tire in millimeters, and the second number is the aspect ratio, meaning the proportion of width vs height. That means that a 120/60 has a height from bead to crown that is 60 percent of the width, while the 70 is 70% tall as it is wide.
You can mount a 170 on a 5" rim, but the tire will be deformed, because it is being pinched together more than the mfgs had in mind. This will affect your handling, and I really wouldn't recommend it. You might could live with a 170, but definitely not a 180. 160 is really as big as you should go on a 5" rim. You might could get a 5.5 inch wheel from somewhere, but I wouldn't buy it until I was sure that the wheel would fit inside the swingarm, and that the bearings and spacers matched your axle. Hope this helps.