What is the method behind the sizing of motorcycle tires? Why do certain motorcycles have certain tire sizes? Do the fatter tires you see on most bikes make the motorcycle handle better? How does one read the tire specification numbers on the tire sidewalls?

These are some of the basic questions I had when I first got into motorcycling. I will address each of these questions based on my research and personal experience.

There are numerous reasons why certain motorcycles have certain tires. As any sales person will tell you in their showroom, the intended use and desired performance of the motorcycle help determine the tire size. For example, most dual-purpose motorcycles like the BMW GS 1200 R and Kawasaki KLX 250 and KLR 650 will have tires suited for both off-road and street use. The KLX 250 tires will be more aggressive on the knobby side than the other dual-purpose motorcycles. Street bikes like the Kawasaki ZZR1200, will have a differently size tire than a touring and dual-purpose bike. Again, it all comes down to the intended use of the motorcycle.

When it comes to replacing the OEM stock tires, it is always wise to use a replacement tire with the same size as the stock tire for which you are replacing. The main reason for this is that the rims on your bike were designed to be outfitted with certain sized tires. Therefore, you need the correct size to fit on the stock rim.

Installing the correct size also implies that the tires have the same circumference. Having two different tire circumferences will have negative effects on your motorcycle’s tire grip and overall handling. Why is this so? A motorcycle’s rake and trail and the front and rear weight bias are affected by the ride height of the front and rear ride height. This is why tire circumference is important.

Recently with motorcycles like the Victory Hammer, you see more fat and wider tires being installed on motorcycles for both performance and style. Be sure the stock rim is designed to handle these wider tires. If you are installing a wider tire on a narrower rim for which it was not designed to handle, the footprint of the tire diminishes due to the profile being wrong. Another thing to be cautious of is that a wider tire may not have enough clearance once installed and rub against the fender or swing arm.

The bottom line is that the sizing of tires for certain motorcycles is determined by the factory motorcycle and tire engineers. They know their science well and know what works best on certain motorcycles. They have devised a numbering system which is used when replacing your tire. It is not a user-friendly numbering system but it makes sense to the tire manufacturers. For example a 100/80-18 tire means the 100 is the nominal width in millimeters. The 80 is the aspect ratio, and refers to the height of the tire as a percentage of the width (in this case 80 percent of 100mm). The 18 is the wheel’s diameter in inches for which the tire is designed to fit. So replace your tires based on what your manual says. Why risk putting on a different size tire just for looks at the risk of safety and performance.