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A round black thing. There is usually one on each corner of your car and/or pickup truck

The English spell this word “tyre” and that is what I am going to do.

Why you should care about tyres

There are four pieces of rubber on your car which keep you in control of your vehicle. If they lose grip for any reason, then you and your vehicle are at the mercy of Newton’s first law. Those tyres are the only things that will drive the car forward, allow it to turn corners and bring it to a halt before you hit the tree.

Motorcyclists know this, and most bikers pay a lot more attention to their tyres than car drivers. Trust me on this one.

Most car drivers pay little or no attention to their tyres. Many are content to drive on bald and under-inflated tyres. Either condition significantly reduces the safety margin when manouvering--especially during hard cornering. Even more so in the wet.

A bald tyre is not especially dangerous in the dry: but as soon as the rain comes down, it becomes lethal. Here's why.

When there is standing water on the road, the smooth, round rubber surface of a bald tyre acts like a bearing, and lifts you off the road. Once you are aquaplaning, there is absolutely nothing you can do to control the car. It will sail forward at almost unchanged speed and direction until the water pressure under the tyre reduces once more. Then control returns very quickly. That leads to bad accidents if the tyres are no longer pointing in the direction of travel.

Under inflation can lead to the tyre coming off the rim during cornering. This is bad news, and can lead to the vehicle rolling over, which often leads to death.

On a more mundane note, under-inflation also leads to much worse fuel consumption and excessive tread wear on the shoulders. That’s expensive.

Over-inflation is less of a problem. It means you get better fuel economy, but the grip is badly reduced both in the dry and the wet. It's best to get maximum grip in the wet by getting the inflation pressure right.

A brief guide to the different parts of a tyre

A tyre is a complex composite made up of a number of different rubber compounds reinforced with different materials, including (but not limited to) steelcord, polyester, polyamide, rayon and para-aramid such as Kevlar and Twaron.

The best geometric shape I can describe which approximates to a tyre shape is a hollow torus in which the inner part of the torus has been removed during an intersection with a co-axial cylinder. A radial cross-section yields a horseshoe shape.

The tyre is made up of a series of sub-components. Working from the outside inwards, the outer-most layer is the tread.


In a car tyre this area serves a number of often conflicting purposes. The first is to provide grip in the normal direction of motion (longitudinal grip), and more grip to resist cornering forces (lateral grip). The grip should be effective in both wet and dry conditions, but tyre engineers try to deliver more or less equal grip in both wet and dry, even if that means reducing the dry grip a little.

A second function is to provide a sacrificial wear surface. During the life of the tyre, this surface is expected to wear away, but the performance characteristics of the tyre should remain essentially unchanged during the lifetime.

A third function of the tread is to ensure that water does not build up underneath the tyre during high speed forward motion. The outer surface of the tread therefore has channels cut into it to allow water to be evacuated from underneath the contact patch. Typically, the rubber makes up 60 to 70 percent of the total area of tread, while grooves, sipes, and other shapes used to pump the water away from the contact patch make up the remaining area.

Another aspect of the tread is that the small details in the tread pattern make a very large contribution to the noise a tyre generates as it rolls along the road. The designers can play many tricks to reduce the noise, but legislation is constantly forcing permitted noise levels down, in order to reduce noise pollution.

Designers are now starting to pay more attention to the appearance of the tread pattern, as a marketing tool. The idea is that buyers typically have little data on which to base their buying decision, except possibly price and brand reputation, so they can use the appearance of the tread pattern and sidewall to make a buying decision.


This is the main body of the tyre, but is not exposed to view. It provides strength and stiffness, and transmits the forces from the tread area into the bead (see below), from where it passes into the rigid wheel structure. Most of the strength and stiffness comes from the fibres and steelcord used in the construction. Rubber compounds are used to hold the different reinforcing elements together and to ensure that the reinforcing elements can move slightly with respect to each other during normal use. The carcass can be made in two main types of construction: radial and cross-ply (bias-ply in the USA)


The sidewall is the other area of the tyre visible to the user, and as such plays an important part in the buying decision (according to the marketing people). It is also the place where information about the tyre (manufacturer, size, speed rating, load rating, place of manufacture, etc) are given.

The tyre sidewall starts off black, but over time, various chemicals (mostly anti-oxidants) migrate to the surface of the rubber and cause an unsightly brown discoloration. Tyre makers are striving to find ways to keep their tyres looking nice and black, but the best methods involve extra expense, so are not adopted (would you pay more to keep your tyres black throughout their life?).

The rubber layer on the sidewall protecting the carcass is quite thin, and if it is damaged, then the tyre really should be replaced (and not repaired).

Inner liner

This is the substitute for the inner tube in tubeless tyres. It is a skim of special rubber less than 1mm thick. It is the material on the inside surface of the tyre, (where the pressurised air touches the tyre). The material is halo-butyl rubber, which is all but impermeable to gas. That’s why people nowadays rarely check their tyre pressures. The rubber has got so good that hardly any air leaks out of the tyre.

Bead area

The beads are the stiff, metal-reinforced areas which are mounted on the rim. Bead wire is a high-tensile steel wire and the bead is extremely important to the functioning of the tyre. If this area is damaged, then any tyre professional will tell you that the tyre is scrap.

Shoulder area

This is the thick part of the tyre, where the contact patch joins the sidewall. It is the area that gets hottest in normal use and the area which is most damaged during heavy cornering. Again, damage in this area can be fatal to a tyre.