CARCASS CONSTRUCTION

LT vs. P-metric

As noted by Cooper Tyre's John Pecoraro, manager of product marketing, an LT tyre is built more robustly than a P-metric tire, to handle heavy loads under adverse conditions.

Heavier body plies and larger bead bundles allow LT tyres to be inflated to higher pressures, thereby increasing the tyre's load capacity. It is the air inside the tyre that carries the load. Usually LT tyres have deeper tread depths than their P-metric counterparts.

Officially, LT tyres should always be replaced with LT tyres. P-metric tyres are passenger tyres that are better suited for on-road use. There are some occasions where an LT tyre may be a better replacement application than the OE P-metric, such as when better off-road durability is desired.

By the same token, a P-metric tyre may be acceptable for a light truck or SUV application, providing the tyre offers sufficient load rating (to handle vehicle weight), and providing the vehicle won’t be driven in an off-road situation.

In addition to a concern for load carrying capacity, LT tyres feature heavier construction in sidewall, shoulder and tread areas to better withstand rough-terrain exposure.

A P-metric tyre should only be considered if the vehicle will only be operated on paved roadways.

As we mentioned earlier, if a set of P-metric tyres are to be installed, replacing an original set of LT tyres, the P-metric tyres should feature a 10% increase in load carrying capacity in order to provide an acceptable margin of safety.

For example, if the vehicle weighs 2041kg, normally each tyre should be capable of carrying a minimum of 510kg. If a P-metric tyre size is chosen, the tyres should be able to carry at least 561kg.

Tyre load ratings

Tyres (and their inflation) are totally responsible for supporting vehicle weight. If the tyre's load rating is insufficient, the tyre may become overheated, resulting in the potential for tyre failure (through no fault of the tyre, if the tyre model is not correct for the specific vehicle).

For example, if a passenger car performance tyre is selected to replace an OE light truck tyre on an SUV, the new tyre must meet or exceed the requirement for the specific vehicle weight.

Consider the vehicle’s gross weight and divide this by four in order to roughly determine the load capacity for each individual tyre. If the vehicle gross weight is 2041kg pounds, each tyre should be able to safely support at least 510kg.

However, you should never select a tyre that only meets this minimum weight capability. Always select a tyre that offers a greater, or reserve load capacity, which will help the vehicle handle and respond to higher-stress emergency situations.

The tyre's load rating, or “Max Load” indicates the individual tyre's safe maximum load-carrying capacity, when inflated to its recommended pressure. Regardless of what the customer may initially request, never exceed a tyre's maximum load rating (the limit that is moulded into the tyre sidewall, or the maximum vehicle load limit shown on the vehicle tyre placard, whichever is less).

Load and inflation

The Max Load and Max inflation numbers found on the tyre sidewall indicate the maximum load that can safely be carried and the maximum allowable tyre pressure. The construction of the tyre (belts, bead, carcass, liner) dictates the tyre's ability to withstand pressure.

The stronger the reinforcements, the greater pressure the tyre can hold.

Most alpha-numeric tyres feature a load range of B, which indicates that they are restricted to the load that can be carried at a maximum inflation pressure of 2.2 bar. C, D or E tyres are capable of greater loads.

Most load range C, D and E tyres are intended for light truck applications.

Tyre load-carrying capacity of P-metric tyres is rated as either Standard or Extra Load. Standard Load tyre are limited by the load that can be supported with a maximum inflation pressure of 2.4 bar.

Extra Load-rated tyres are limited to the load that can be carried at a maximum inflation pressure of 2.8 bar Generally, a Standard Load tyre will not feature a special designation mark, while Extra Load tyres will feature an “Extra Load” marking.

Extra load P-metric tyres will be branded as “Extra Load” and may be identified by an “XL” (for example: LT245/75R15 XL).

It’s important to note that a Standard Load tyre (with a normal inflation pressure recommendation of 2.4 bar) may be marked with a maximum inflation pressure of 3.0 bar.

This does not indicate an increase of the tyre's load carrying capacity, but indicates the tyre's ability to handle higher inflation pressure in order to accommodate special performance requirements. Generally speaking, load indexes of passenger car tyres and light trucks range from 70 to 110.

A speed-rated tyre's sidewall markings will indicate size, followed by the load rating index and the speed rating. For example, a P195/60R15 87S indicates that this tyre carries a load rating of 87 and a speed rating of S (this load rating is 544kg, and the speed rating is 180km/h).