The tyre - a critical contact point

Kevin Baxter, Contributor

How much do we pay attention to the one component of our motor vehicle that interfaces with the road surface? Between your powerful hunk of mechanical muscle and the road are single points of contact not larger than a slice of bread.

Drivers are often oblivious to the fact that this component was built to specifications such as size, speed rating, maximum load and maximum inflation, which determine its optimal service. If these specifications are exceeded or under-utilised, then the intended performance will not be achieved.

Your tyre is simply an enclosed, flexible, ringed container (usually rubber) filled with compressed air. They can either be tubed or tubeless. Modern manufacturers have moved away from tubed tyres for motor cars and some light trucks; however, tubed tyres can still be found on heavy-duty vehicles.

The job of the tyre is basically to ensure that every move of the steering wheel and depressing of the brake or accelerator pedal is complete by its interface with the road surface. Your safety and that of others rests (pun intended) on the integrity of this component.

Consider driving in the rain. The water is actually forming a layer between the tyre and the road. If an oil leak had occurred previously that oil will now be floating on the water (oil being less dense than water). The layers are now road surface, water, oil then tyre. If we add the variables of speed and load, then you can imagine the possibilities. It becomes even more critical that your tyre is at an acceptable standard.

Most of our information actually lies on the sidewall (side) of the tyre. If your tyre threads are barely visible (tyre worn) and you are actively searching for a replacement, located on the sides of the tyre is an alphanumeric code that usually takes this form, for example, P155/60R14 63H M+S. At the point of your vehicle's manufacture they are factory fitted with a size tyre designed to ensure optimum tyre and vehicle's performance.

Recommended tyre size

Over the course of time, however, these tyres may get replaced to suit a variety of needs. Luckily, contained within the vehicle manual or imprinted to the chassis pillar is the recommended tyre size for your vehicle under prescribed weight conditions.

The alphanumeric code P155/60R14 63H M+S therefore means:

P - It's a passenger tyre. Another classification may be LT for light truck.

155 - Measures the tyre width (mm) from inner sidewall to outer sidewall.

60 - This is termed the tyre aspect ratio. It's simply a ratio of height to width which means the tyre height is 60 per cent of its width.

R - Radial tyre construction (most tyres are). Other constructions B (belted bias) or D (diagonal bias).

14 - Rim diameter (inches)

63 - This is the load index rating of the tyre. The load index is an industry standard that indicates the maximum service weight the tyre can support when inflated. Load indices range from 74 - 150 (350 - 3500kg) for passenger tyres, with each numeric value corresponding to a certain carrying capacity. The carrying capacity for each value can be found on a load index chart.

H - Speed rating - maximum 210km/h. Different letters designate corresponding maximum speeds. Each letter coincides to the maximum speed a tyre can sustain under its recommended load capacity. For example, for S it is 180km/h, T - 190km/h V - 240km/h and ZR - 270km/h. Even though a tyre can perform at this speed, it is not wise to try out its capabilities outside of a controlled testing environment.

M+S -Tyre is suitable for all-season driving, mud or snow. In our temperate climate, this is usually negligible.

Critical information

Other critical information that can be found on your tyre are:

Thread Wear: 100 is the standard. Greater than 100 is better, lesser than 100 is poorer. Strictly speaking, a tyre that is rated 200 is expected to wear twice as long as a tyre rated 100. This, of course, is totally dependent on the driving conditions, driving habits and road characteristics. This comparison is only valid for tyres made by the same manufacturer.

Traction: This represents your tyre's ability to stop when the vehicle is moving straight ahead on wet pavement. It does not take into consideration cornering ability. Traction is usually graded A (best), B (intermediate) and C (acceptable).

Temperature: A is best, B is intermediate and C acceptable. These grades represent a tyre's resistance to the generation of heat. Sustained high temperatures will cause tyre deterioration, which ultimately reduces your tyre's life and may lead to failure.

Another important marking that can be found on the tyre sidewall is the DOT number. The US Department of Transportation (DOT) has stipulated that tyre manufacturers imprint the date of manufacture for each tyre. Since 2000, four numbers are used for the date of manufacture. The first two numbers identify the week and the last two numbers identify the year of manufacture. This is critical information for the purchaser, as some manufacturers and safety administrators recommend a tyre shelf life of six to 10 years. The final choice, however, rests with the purchaser.

Next we will look at at the tyre and parameters to be considered when purchasing.

Kevin Baxter is acting manager of the HEART Trust/NTA Jamaican-German Automotive School.

Lanarda Morris, engineer at the Bureau of Standards, inside one of the labs where tyres are tested. - File

Nick McKoy, general manager of Chad Ad Distributors Limited, shows a bald tyre that has lost all its treading. Treading helps with traction and a completely bald tyre is extremely dangerous, especially in wet road conditions. - Contributed