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A PRIMER ON CASINGS
Mechanical functions:
In the first place, the casing is an element that allows the tyre to guarantee its principal task, i.e., to support the load. When the tyre is inflated, the air is injected at a pressure in relation to the volume of the tyre, which makes it possible to support the load weighing on the tyre. The purpose of the casing is to contain air under pressure and consequently the load.
In addition, it ensures, vertically, the function of the suspensions. When you're behind a truck, observe how the tyres work: it is the casing that absorbs and filters most of the irregularities of the roadbed, even when the vehicle goes up onto a sidewalk!
Laterally, the casing resists drifting, resists the centrifugal force around curves and resists important deformations notable at low speeds: for proof, just look at tyres of a semi-trailer on the highway or while it maneuvers around a city square.
Finally, it's the casing that transmits the vehicle's torque stresses to the tread, which is in contact with the ground, resisting considerable longitudinal stresses to transmit the motive torque and the braking torque, which tend to separate the cables that compose it.
A composite material:
In the case of the French tyremaker, about 30 different materials such as steel and different qualities of rubber have to be made to co-exist. Steel is also found in the bead wires: those two elements that allow the tyre to be locked onto the rim around which the ply of the casing is fixed. In larger tyres the bead wires have a rectangular section that permits the perfect positioning of the bead into the housing of the rim. This design has been patented by Michelin to ensure a much longer life than that of the bead wires with a round section normally used.
Steel cables composed of 12-27 wires "coated" in rubber form the different plies making up the casing. The radial ply and the plies of the crown, arranged with different angles, form an undeformable triangular structure.
Last, the elastic cables of the protective ply situated underneath the tread improve the resistance to cuts and perforation.
Different rubber compounds:
On the crown, sidewall or in the bead, the compounds of rubber used have specific roles and require elasticity, rigidity, grip, and resistance to high temperatures. Inside the tyre, air-proof rubber, 100% butyl, guarantees the best pressure seal.
A SLIGHTLY DIFFERENT VERSION OF THIS ARTICLE ORIGINALLY APPEARED IN THE MARCH/APRIL 2002 ISSUE OF PNEURAMA, BOLOGNA, ITALY AND IS USED WITH PERMISSION.
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