Tires and tribulations: Touring and sport-touring tire buyer's guide.

Publication: Rider

Publication Date: 01-MAY-02

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COPYRIGHT 2002 Ehlert Publishing Group

YOU'LL HEAR IT OVER and over, and not just from tire company representatives, but from racers, professional mechanics and pretty much anyone who rides a lot--tires are the single most complex part of a motorcycle. Underneath the dull black exterior, sectioned into blocks of tread by grooves, lie mysteries the likes of which few riders will ever understand, much less care about. It's fortunate, then, that an intimate knowledge of tire technology isn't necessary to take advantage of the traction, handling and longevity the modem motorcycle tire offers. But there are things every rider should know, if only to make the process of choosing and buying tires as easy and safe as possible.

To begin with, there are two types of motorcycle tires, bias-ply and radial. They differ only slightly outwardly--bias-ply tires tend to have tall sidewalls, and a round profile, while radials have short sidewalls and a flatter profile--but inside, the differences are not only large, but critical. The layer of material under a tire's tread is called the carcass, and a bias-ply carcass is made of overlapping layers of cords made of nylon, rayon or other synthetics. The layers, called plies, stretch from bead to bead, across the tire, at an angle. Each layer runs at an angle opposite to the ones above and below, so that if you could look through the plies with an X-ray machine, you'd see an "X" pattern. Some tires, called bias-belted, also have a layer of material called a belt laid over the cuter layer of plies, with its cords parallel to the direction of rotation, to stop the tire from growing at speed.

When a tire rotates, a portion of it comes in contact with the ground, flattens out momentarily, then rotates away and resumes its rounded shape. This flexing of the carcass causes the plies to scissor against each other, generating heat. Heat is a tire's greatest enemy, accelerating wear and decreasing performance, so tire engineers looking for a way to make a cooler-running tire came up with a design that lays the plies from bead to bead at or near a 90-degree angle to the direction of rotation. The so-called radial design greatly reduces the scissoring motion, allowing radial tires to run cooler...

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