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Shop Talk: Sidewalls & load ratings

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Serious 4×4 tyres offer a 3-ply sidewall, which will obviously be stronger than a 2-ply. The major benefit is the puncture-resistance added to the vulnerable sidewall and tread areas.

In a normal 2-ply radial tyre, each ply runs at the same 90-degree angle from the bead. The rubber gaps between the sidewall cords are the weakest point of a tyre.   A nail, stake or thorny bush can push against the sidewall cord, be deflected, and penetrate through the weakest point (the rubber), resulting in a puncture.

In the case of Cooper and Mickey Thompson tyres, the 3-ply sidewall forms a stronger, more powerful casing, and features a unique third ply running at an 8-degree angle. (This proprietary sidewall construction technology is labelled Armok-Tek3 and Powerply respectively.) The extra ply significantly strengthens the sidewall, making it more tear-resistant. In addition, this construction lessens the twist in the plies, resulting in greater traction and faster steering response, which is a huge benefit when driving at lower pressures on loose surfaces.

Although plies are important, an equally vital consideration is in the basic tyre construction, where rubber density and overall thickness play a major part in determining how tough a tyre will be in the real world. Take the Cooper Discoverer ST Maxx as an example. It features a 3-ply sidewall rating, but has the important phenomenally-robust sidewall construction.

How do you know if a tyre is robustly (thickly) constructed? You’ll find a Load Range Letter written on the sidewall of the tyre. This alphabetical symbol equates tyre thickness to a comparable ply rating. (Modern load/ply ratings are a hangover from early bias ply tyres, when plies making up the internal structure of a tyre were counted as a measure of load-carrying capacity. Modern tyres using steel and synthetics can get away with fewer plies to achieve greater strength.)

For example, the letter “E” written on the side of a 265/65 R17 ST Maxx tyre suggests that the comparable sidewall rating would equate to a 10-ply. Off-road tyres worth considering are either a C (6-ply), D (8-ply), or E (10-ply) rating.

Click the link below to see a cutaway of a Discoverer tyre.

DiscSTMaxx cutwy labeled v1

By Johann Viljoen

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