Bike Tire Size - Answers - MC Garage

Tired Again
I was hoping you guys could shed some light on the science of sizing tires. What makes a certain tire size right for a bike? And can you please explain why slapping a big, fat, wide-as-will-fit tire on any bike won't always make it handle better? Can't oversized tires have an adverse effect? I get the feeling when guys say "handle" they really mean "look." Oh, and what do all the numbers mean, such as 100/90-19? Thanks.
Justin Illg
Geneva, IL

Mike Manning, Dunlop Motorcycle Tires' product and marketing manager, replies, "There are many reasons why certain-size tires are selected for a specific motorcycle. In general, the intended performance and use of the bike dictates the size. For example, a touring bike that's ridden long distances with lots of weight and isn't leaned over a great deal has a drastically different-size tire than a sportbike.

"Whatever size is intended for the bike is the size which should be fit again, for many reasons. Certain-size tires fit on certain-size rims, so you need to replace the correct-size tire onto the wheel you have. The rake and trail of the bike and front/rear weight bias are affected by the front and rear ride height, so you need tires with the same circumference. Anything else can have very negative effects on handling or grip. If you try to put a wider tire onto the same rim, either for looks or performance, you are pulling that tire into a narrower wheel than it was meant for, and you often lose footprint because the tire profile is now wrong. Plus, the wrong size tire can rub against the motorcycle and get damaged during use.

"These explanations are all very general because the actual sizing of tires to bikes is extremely specific model-to-model, and tire engineers and bike engineers alike spend years on each model working it out.

"The numbering system can seem a bit odd. In a 100/90-19 tire, the 100 is the nominal width in millimeters (and there is a little allowance in the standards for some variance from the nominal width). the 90 is the aspect ratio, and refers to the height of the tire as a percentage of the width (in this case 90 percent of 100mm). And the 19 is the wheel diameter in inches which the tire is intended to fit.

"Just remember: You are best off replacing your tires with the stock sizes; a lot of engineers worked hard to make sure they suited your bike best."