How is a car tyre made? Production starts with a halobutyl rubber sheet that has special air-retaining additives. This sheet forms the tyre's inner liner. A three-layer body ply comes next, in which a central layer of reinforcing fabric is sandwiched between two layers of rubber. Cotton used to be the material of choice for the reinforcing fabric but this has been replaced with Nylon, Rayon, polyester or even Kevlar. The body ply's role is to give the tyre strength and flexibility; two such plies may be used in a tyre.
The acceptable faces of the tyre, the sidewalls, include antiozonants and antioxidants to enhance their life. The sidewalls can resist chemical attack and abrasion, and the tyre's ratings and size codes are moulded into them.
Of course, the tyre's sidewalls need to form a seal on the wheel rim. Bands of high tensile steel wire, copper or brass coated for corrosion resistance, form the beads on the inner diameter of the sidewalls. The triangular rubber section that joins the beads to the sidewalls is called the apex. The bead structure is what forms an airtight seal with the wheel rim.
Now for the tread and its underlying structure. The belt package is the part of the tyre that sits inside the tread. It's made up of two layers of rubber that enclose a layer of steel cords. These cords run radially in the belt package, hence the term 'radial tyres'. The belt package is a major player in the tyre's strength. The tread pattern is visible but the rubber compound used to make it is just as important. The choice of compound is a matter of a selecting a balance, between hardness (wear) and (grip). Think 'compromise'.
These are the parts of a tyre and they must obviously be assembled and made into a tough, resilient structure. This procedure begins with wrapping the inner liner, body ply(s), sidewalls and beads around a special drum. Then, the belt package and tread are added, and the various elements are spliced together. At this stage the tyre is yet to be cured, and is known as a 'green' tyre. In this state, it is inflated and shaped.
Curing comes next, to bond the various parts of the tyre into a coherent whole. A rubber bladder is inflated inside the tyre, forcing it into a mould. Hot water, steam or an inert gas is used to inflate this bladder, and the curing process involves a great deal of heat and pressure...350 degrees Fahrenheit and 350 pounds per square inch to be precise. The curing process takes about 15 minutes.
The final stage is to test the cured tyre on a mock road surface, to seek out localized inflexibility and significant imbalances. Every tyre is given a visual inspection and sometimes, an X-ray inspection too.
So, car tyres are built tough to be tough, which they must be to take on the tough job they do. As they are all that keep your car on the road, it's reassuring to know how much effort and care go into making them.