The Mystical art of Tire Reading

-Or-

It's black, it's round, and it goes on the wheel

Author: Josh Fielek

Tires are a mysterious thing -- think about it. A reinforced bladder with about thirty pounds of air is all that keeps your motorcycle from rolling on it's bare metal rims. That thin bit of rubber is what lets you get out there and enjoy a good ride. Obviously, it pays to know about your tires.

There are a few basic elements to consider when looking at tires. The type of tire, the size, the profile, and the aspect ratio are all elements that can affect the behavior of the tire, and thus the behavior of the bike. We'll take these elements and go through them, and the effect that they can have on your bike.

The most apparent difference between tires is the size and aspect ratio. These are indicated by the numbers on the side of the tire, usually something along the lines of "120/80V-16" or "4.50H-16". There is also a alphanumeric system that would look something like "MR80H-16". So what do all these numbers mean?

Let's take a look at the most common designation, the metric system for tires. This is the one that reads "120/80V-16". This type of designation is the most commonly used nowadays, and would break down as follows:
 

So, we can look at a tire and read a little bit of the code. But how do the numbers interrelate? Here's the scoop --

The nominal width is approximately how wide the tire is at the widest part of the tread. In this case, the width is about 120 mm. There is some variance from tire maker to tire maker, and from brand to brand, so one tire may be 124 mm wide, while another may be 118 mm.

The aspect ratio is approximately how tall the tire is in relationship to its width. Thus, a 120/80 tire is about 95 mm tall, from the bead to the tread surface.

The speed rating indicates at what maximum speed the tire is considered safe for continuous use. In this instance, the V stand for speeds up to 149 mph. A table of speed ratings is listed below.

The last number is the diameter of the wheel that the tire is intended for. In this case, we are talking about a 16-inch rim for the tire.
 

The second example above reads "4.50H-16". This is the so called American system, and reads as follows -
 

The American system is a little quirky when it comes to aspect ratio. Generally, a tire is a high profile tire, with about a 90% aspect ratio, unless the width is indicated with a ".10" or ".60" designation. This indicates a lower profile tire, on the order of 75-85 percent. Thus, a 4.5 inch tire with a 85 percent aspect ratio would be shown as a 4.60. A 4.0 inch low profile tire would be a 4.10.
 

The last method of tire designation is the British system, which we showed above as "MR80S-16". This breaks down like so --
 

Since the British system uses an alpha code for the tire width, a table is included below to indicate what metric and American widths are included. The table also includes the recommended rim widths for those tire widths.

Here's all the tables I've been speaking of:
 

Speed Ratings:

Tire Size Conversions (Courtesy of the Dennis Kirk Catalog, and they probably don't even know it):

Over 4.00 inches, I have no data. I will add it when I find it.
 

Okay, so we've got the codes... but what does it mean? How does a 130/80-16 differ from a 120/80-16, if both can fit on the same size rim?

First of all, the most obvious characteristic is the width of the tire. The width, in concert with the diameter of the wheel, determines the size of the contact patch. The size of the contact patch in turn determines the load that the tire can bear, the amount of traction that will be available, how well the tire disperses water, and how mush it resists steering inputs.

The diameter of the wheel and tire, in addition to the effect on the contact patch, affects the steering and stability of the bike. A larger wheel will be more stable at speed, and more resistant to steering inputs. This is in part due to the greater gyroscopic effect, but another factor is the greater contact patch caused by the larger wheel. In essence, a larger diameter wheel creates a longer contact patch, which requires more leverage to move.

The aspect ratio of the tire is the distance from the bead of the tire to the tread surface. The primary effect of the aspect ratio is on the shape of the tire. In general, a lower aspect ratio results in a flatter profile.

The profile of the tire is the cross sectional shape. It is affected by several elements: the aspect ratio, the wheel width, the width of the tire, and the manufacturers intentions for the tire all affect the profile. The profile in turn affects the handling. A triangular profile will cause the bike to turn in more easily, but may make it a little more prone to tucking or falling into the turn. On the other hand, a rounder profile may be a little more difficult to turn in, but might have a little more linear response to steering inputs. This is to a great extent motorcycle dependent -- some bikes may like a particular tire, others may not.

All in all, this means that a wider tire will tend to steer more heavily, but will also offer more traction. A larger rim will also steer slower, as will a broad, relatively flat tire.

So, what does all this mean when it comes time to go out and buy the tire? Well, it means a lot. First of all, in many cases the wheels on the bike, or the age and style of the bike may limit the tire selection. An excellent example of these limitations is found when attempting to find tires for a pre-1994 EX500, or for a Ninja 600 (NOT the ZX-6). Both of these bikes have relatively narrow 16 inch rims, but they are also meant to be more sporting motorcycles. The selection of quality rubber for these bikes is pretty much limited to either Dunlop K591s or Metzler ME33/ME1 tires.

In other cases, while the hardware may be capable of supporting a more current tire, the optimal sizes are not available. A good example of this is the 1986 VF1000R (My personal bike at this time) that has a 2.75x16.0 inch front wheel and a 3.50x17.0 inch rear wheel. The fitment of rear tires is not a problem -- any of the quality 140, 150, or 160 width radials will fir nicely. The slightly narrow front, however, really ors best with a bias-ply tire. (FWIW -- I am currently running Michelin 89 series radials, front and rear, but I had to do a little tweaking to make them work. More on that later...).

And that brings us to an important matter for tire buyers -- should I buy a radial tire? The answer is a firm "maybe". First of all, it helps to know the difference between a radial and a bias ply tire. To understand the difference, it helps to know a little bit about tire construction.

A tire is nothing more than a bladder for air. It is made of rubber reinforced with some sort of fiber (once they were reinforced with canvas, now they're reinforced with Kevlar -- times have changed). The rubber keeps the air in, and the fiber keeps the tire together, in addition to importing shape characteristics to the tire. The fibers are laid out in layers, each layer consisting of fibers laid in parallel, and impregnated with rubber. The final element is the bead, which is simply a wire put around the edge of the tire to help it clamp to the rim.

The way these layers are placed on the tire is what determines whether the tire is a bias-ply or radial tire. In short, a radial is a tire where the threads in the ply are laid perpendicularly to the bead. This gives them the appearance of radiating from the center of the tire, thus the term radial. Bias ply tires are a little different, because the angle of the fibers is at a bias to the bad, usually at an angle of about 70 degrees. For reference, a "zero degree" layer would be a belt around the circumference of the tire.

Okay, so what does it mean? Well, a bias-ply tire requires a minimum of two layers to support the sidewall and the tread. The layers are fairly stiff, and essentially must scissor against each other to flex. This makes the tire run hotter. A radial tire needs only one ply, and thus is not only softer, but runs much cooler, and weighs less to boot. Advantages all around -- lighter, cooler, more compliant. This means that not only can you run a softer compound for radials, but also it will provide better grip since it can conform to the road better. Great!

But wait! There's a catch -- radial tires require wider rims to support the same tire width. That means that a 120/80 bias ply tire would work on a 2.5-inch rim, but a 120/80 radial would need a 3.0-inch rim to support it properly. Why does a radial need a wider rim? In simplest terms, a radial tends to have a softer sidewall, and a wider rim with a similar aspect radial puts less load on the sidewall, causing it to distort less under load. Most radials are designed for wider wheels, and putting them on a narrower rim will distort the cross-section of the tire and affect the handling of the bike -- almost always negatively.

Mixing tires itself, whether mixing radials with bias ply or mixing brands or makes of tires with similar construction is a black art. The standing rule is "caveat emptor" -- the buyers beware. Some combinations on some bikes work quite well, others are ill advised if not outright dangerous.