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Shop Talk: Tyre construction basics

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Tyres look simple enough from the outside, but the mix of components inside is the result of extensive (and expensive) research and development by the tyre manufacturers.

Each tyre type and profile is designed to meet a specific and exacting set of requirements, either set down by the vehicle manufacturers (when the tyre is specified as Original Equipment) or to meet a defined range of demands that make it attractive as an aftermarket fitment.

TYRE TERMINOLOGY

Body ply (or carcass): Most tyres have one or two body plies, though there may be eight or more. These typically comprise of polyester, rayon, or nylon cords within a rubber layer. Body plies function as the structure of the tyre and provide the strength to contain the inflation pressure.

Bead: Tyre bead bundles (usually strands of wire) secure the tyre to the wheel.

Inner liner: This is a rubber compound used to retain the inflation pressure inside the tyre.

Sidewall: This rubber compound is used to cover the body plies on the sides of the tyre, and provides resistance to abrasion, scuff and weathering.

Tread: The tread rubber compound and tread pattern provide grip and abrasion-resistance, thus contributing to traction and tyre life.

All the components of a tyre work together to meet a specific set of requirements, and consumers need to know what their requirements are, and what the trade-offs might be, when they select a tyre for their 4×4.

Even with current levels of technology, one cannot escape the so-called “magic triangle” − which has wet grip, wear-resistance and rolling-resistance as its three corners. Change one internal component or compound, and the shape of the triangle shifts; in other words, the tyre’s ability in one of these three areas will be enhanced or retarded.

What this means is that changes to the tread pattern, to the internal construction, and to the materials used in the various parts of the tyre, all have a significant effect on tyre performance.

For any 4×4 application, the construction of the carcass, the sidewall and the tread pattern is of primary importance. These are all areas of the tyre that are vulnerable when tackling rocky or thorny tracks in the bush. For these conditions, a strengthened carcass and sidewall is of primary importance when choosing a tyre.

The carcass (or body) of the tyre can be manufactured as either Light Truck (LT) or P-metric (Passenger). An LT tyre is built more robustly than a P-metric tyre, enabling it to handle heavy loads under adverse conditions. The LT marking will be clearly stamped on the tyre sidewall. This is important, as, in some instances, the same tyre name and tread pattern is supplied in both LT and P-metric derivatives.

Heavier body plies and larger bead bundles allow LT tyres to be inflated to higher pressures, thereby increasing the tyre’s load capacity. It has been said before, but bears repeating: it is the air inside the tyre that carries the load. This is why load ratings are crucial in matching a tyre to your vehicle and to your driving requirements. If the load rating is too low for the load, the tyre may overheat, and can potentially fail. In addition to higher load ratings, and the ability to run cooler under load, LT tyres typically have deeper tread depths than their P-metric counterparts – which promises a longer useful life.

Officially, LT tyres should always be replaced with LT tyres. P-metric tyres, on the other hand, are passenger tyres that are better suited for on-road use. A P-metric tyre should only be considered if the vehicle will only, or primarily, be operated on paved roadways.

Note that, when better off-road durability is desired, an LT tyre is invariably a better replacement option than the Original Equipment P-metric. However, it is essential to check that the circumference of the tyre is close enough to the original so that it will not affect the vehicle, and that this replacement will not void the manufacturer’s warranty.
By Johann Viljoen

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