In the first place, the casing is an element that allows the tire to guarantee
its principal task, i.e., to support the load. When the tire is inflated, the
air is injected at a pressure in relation to the volume of the tire, which
makes it possible to support the load weighing on the tire. 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 tires 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 tires 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:
About 30 different materials such as steel and different qualities of rubber
have to be made to co-exist in the tire. Steel is also found in the bead wires:
those two elements that allow the tire to be locked onto the rim around which
the ply of the casing is fixed. In larger tires 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
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 tire, air-proof rubber, 100% butyl, guarantees the
best pressure seal.