OK, I have an Excel rim that’s 2.15 X 18. Through searching this forum I have gathered that means its 2.15 Inch’s wide with an 18 inch diameter. Right? The bike came stock with a 140/80-18 tire Again I believe that means the tire is 140 MM wide, the sidewall is 80% of the height, and an 18 Diameter. Right? Here’s where I get lost. I need a new tire; however there is a very limited selection of tires in the stock size. So, how do you know what size tire will fit on what rim? Searching this forum I found multiple posts suggesting that you could use different width tires, (IE 130/80-18) how is this possible? How does a 130mm wide tire fit on the same rim that a 140mm tire did? And how would you know?