The Mystical art of Tyre Reading

-Or-

It's black, it's round, it’s rubbery and it goes on a wheel

Tyres 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 tyres.

There are a few basic elements to consider when looking at tyres. The type of tyre, the size, the profile, and the aspect ratio are all elements that can affect the behaviour of the tyre, and thus the behaviour 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 tyres is the size and aspect ratio. These are indicated by the numbers on the side of the tyre, 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 tyres. 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:
 

120/80V-16	The "120" is the nominal width of the tyre, in millimeters.
	The "80" is the aspect ratio, expressed as a percentage of the width.
	The "V" is the speed rating for the tyre.
	The "16" is the diameter of the wheel that the tyre is constructed for.

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

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

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

The speed rating indicates at what maximum speed the tyre is considered safe for continuous use. In this instance, the V stand for speeds up to 240kmh. A table of speed ratings is listed below.

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

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

4.50H-16	4.50 -- the nominal width of the tyre in inches. In this case, about 4.5 inches, or about 120 mm.
	H -- The speed rating, in this case indicating a tyre safe up to 130 mph.
	16 -- This tyre is intended for a 16 inch rim.

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

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

MR80S-16	MR -- This is a letter code indicating the width of the tyre. In this instance, MR indicates a 120mm tyre.
	80 -- Once again, the aspect ration, expressed as a percentage of the width.
	H -- The speed rating, once again for 130mph sustained running.
	16 -- The diameter of the wheel that this tyre is intended for.

  Since the British system uses an alpha code for the tyre 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 tyre widths.

Speed Ratings:

Okay

Unrated	95mph(150kmh)
S	112mph(180kmh)
T	118mph(200kmh)
H	130mph(220kmh)
V	149mph(240kmh)
Z	above 149mph(+240kmh)

, 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 tyre. 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 tyre can bear, the amount of traction that will be available, how well the tyre disperses water, and how mush it resists steering inputs.

The diameter of the wheel and tyre, 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 larger contact patch, which requires more leverage to move.

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

The profile of the tyre is the cross sectional shape. It is affected by several elements: the aspect ratio, the wheel width, the width of the tyre, and the manufacturers intentions for the tyre 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 tyre, others may not.

All in all, this means that a wider tyre 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 tyre.

So, what does all this mean when it comes time to go out and buy the tyre? Well, it means a lot. First of all, in many cases the tyre selection may be limited by the wheels on the bike, or the age and style of the bike. An excellent example of these limitations is found when attempting to find tyres for a pre-1994 EX500, or for a GPx 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 because of the narrow small wheel..

In other cases, while the hardware may be capable of supporting a more current tyre, the optimal sizes are not available. A good example of this is the 1986 VF1000R that has a 2.75x16.0 inch front wheel and a 3.50x17.0 inch rear wheel. The fitment of rear tyres is not a problem -- any of the quality 140, 150, or 160 width radials will fit nicely. The slightly narrow front, however, really is best with a bias-ply tyre.

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

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

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

Okay, so what does it mean? Well, a bias-ply tyre 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 tyre run hotter. A radial tyre 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 it will provide better grip since it can conform to the road better. 

But wait! There's a catch -- radial tyres require wider rims to support the same tyre width. That means that a 120/80 bias ply tyre 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 tyre and affect the handling of the bike -- almost always negatively.

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

You may have to rely on what your tyre dealer or bike dealer recommends….they are the experts…on round black rubbery things called tyres…