Few components have as much impact on your motorcycle’s overall performance as the tires. It doesn’t matter how much horsepower you’ve got, how trick your suspension is, or how powerful the brakes are, if the tires can’t do their job, sooner or later you’re going to find yourself riding the world’s fastest shopping cart.
How Tires Work
As the wheel rotates, an elliptical area known as the contact patch forms where the tire flattens against the road. A number of factors determine the patch’s size; the tire’s dimensions, construction, and amount of weight it’s bearing all play major roles, but on average, only about 10 square inches of rubber (per tire) provide traction. That’s roughly that same size as a regulation Ace of Spades, and it’s important to understand that all of the motorcycle’s driving, braking and turning forces are transmitted through those relatively small footprints to the pavement.
Tires provide traction because an amount of friction, which fluctuates according to circumstances, exists between the tire and the road. What creates that friction? As you might expect, tread compound plays a huge part. As a rule, the softer the compound, the more traction it provides. At the molecular level, peaks and valleys on the surface of the tire and the road engage each other, so the softer the tread compound, the more it deforms to match the harder road surface. This allows the road to "bite" into the tread, enhancing grip. Think of both the tire and the road surface as a meshing pair of fine-toothed gears, and you won’t be far off the mark.
The tire’s hysteresis is also a determining factor, especially when the tire is sliding. Among other things, hysteresis determines how readily the tire deforms and returns to its original shape; a high hysteresis tire deforms easily and rebounds slowly. This characteristic allows the molecular "teeth" of the tire tread, which are elastic, to more easily conform to their rigid counterparts on the pavement, while preventing them from returning to their original state too quickly. Consequently, the higher the hysteresis, the more fully the teeth engage and the longer they remain that way.
The size of the contact patch also influences traction. The larger the patch, the more traction the tire generates, for the simple reason that there’s more mechanical interaction between the tire and the road.
Other factors that influence traction include the type of surface you’re riding over, the tire profile and carcass construction, and the sidewall stiffness.
Tire Construction
Tire construction starts with the bead or "bead bundle," which has the job of holding the tire to the wheel rim. Accordingly, it needs to be strong and flexible, so it’s typically constructed of steel or composite wires impregnated with rubber and encased in more rubber, called the bead wrap or filler. As you’d expect there are two separate bundles, one for each side of the tire.
A piece of very strong fabric or brass-coated steel belting, called a body ply or cord, is saturated with rubber and an adhesive bonding agent, then looped in layers over each bead, creating a structure that will eventually support the tire’s tread.
Once the belts are in place, an inner liner is added to seal out moisture and prevent air leaks, then another series of belts and "cap plies" are installed to reinforce the tread and fine-tune the tire’s handling characteristics. Collectively, these parts make up the tire’s carcass. Once that’s complete, the tread and sidewalls are bonded to it, finishing the tire.
Radials vs Bias Belts
Envision a rotating tire. As the tread contacts the pavement, the sidewall flexes outward, causing it to flatten and form the contact patch. As the tire revolves, that patch is lifted, the sidewall rebounds to its original shape and the next portion of the tread forms a new contact patch. This process is repeated hundreds of times a minute, which keeps the sidewall in constant motion.
Bias-belted tires use overlapping cords that are angled at between 27 and 45 degrees to the tire centerline, creating a thick, stiff sidewall. This makes the tire stable and strong, though in some instances maybe not as flexible as we might like.
Radial tires use cords arranged at 90-degrees to the centerline. This imparts the sidewall with a lot more flexibility, and it’s that flexibility that gives the radial its advantage. It takes a lot of energy to flex a bias belted tire’s stiff sidewall, which creates a fair amount of heat (and more is added as the belts scissor across each other). The problem here is that heat can be a tire’s worst enemy: while some heat is in fact necessary, too much creates real problems, and bias belted tires can generate some seriously high temperatures when pushed hard. (As an aside, this is one reason tubeless tires have become popular without a tube to flex and create heat, the tire runs cooler.)
The converse is true of the radial. Its sidewall flexes easily and there’s little or no friction between the belts, so less heat is created, which, if nothing else, makes the radial a more durable tire. But radials enjoy other advantages as well. Because the radial is more flexible, it creates a larger contact patch, which increases traction. Furthermore, because the belts of a bias tread crisscross each other, they tend to make the tread squirm slightly as the contact patch impacts the pavement. This contributes heat and increases wear on the tire. Because the tread of a radial tire is braced by reinforcing belts, it unrolls in a straight line, much as a rigid caterpillar track does, which decreases temperature and wear. So while there’s nothing inherently wrong with a bias belted tire (overall, they work just fine), the radial is simply a more efficient design that works better.
The Radial Question
So can you replace bias belted tires with radials? The answer is a qualified "maybe." Due to their soft sidewalls, radial tires normally require wider rims than equivalently sized bias tires, so a direct swap may not be possible. However, as long as the radials are mounted to the appropriate rims, there shouldn’t be any problems. I’ll also point out that while mixing and matching tire types has always been considered a bad thing, there is at least one manufacturer out there that’s achieved very good results doing just that. Informally, I also know a few guys that have mixed radials and bias tires without problems. As with anything of this nature, though, your results may vary.
Everything you really need to know (with the exception of the price), is listed on the tire’s sidewall. The most pertinent information is the tire’s size, its construction details and speed rating. Formats vary slightly, but on most tires you’ll see something like 130/90-16, or less often, MT90-16, or even 5.00-16. The numbers indicate the tire’s nominal dimensions, and in this instance, the tires are all the same size. Here’s how it breaks down.
The first description is the most commonly used metric format. The 130 represents the tire’s width in millimeters, measured in a straight line from one edge of the tread to the other, while the 90 represents the tire’s aspect ratio, which is ratio between the tire’s width and its height (in this case, the tire is 90% as high as it is wide).
If the tire’s a radial, the letter code R will also be present; for example, 130/90 R 16, or 130/90 16R. If it isn’t marked as such, the tire is bias belted, and the word(s) BIAS or Bias Belted will be printed on the sidewall.
The 16 represents the rim’s diameter in inches, and I should mention that rim diameters are traditionally described in inches largely because of, uh, tradition.
Our second example uses the alphanumeric system; in this case, 'M’ means it’s a motorcycle tire; 'T’ represents its width in inches (here, it’s between 5.00 and 5.10 inches), and '90’ indicates the aspect ratio. Since there’s no R, it’s a bias belted tire.
Our last example is the way they measured tires prior to the advent of low profile tires. In this case, the 5.00 represents the tire’s width in inches, and since there is no aspect ratio listed, it’s assumed (sometimes wrongly) that sidewall height is equal to 100% of the width.
