In order to increase stability against side sway the width of rims has been increased. To-day 6.00"x16" tyres are mounted on rims measuring 4" between flanges instead of 21-", the width of the rims for the high pressure tyres which they superseded. As already stated, all passenger car tyres are now mounted on drop centre rims. In order to facilitate mounting and demounting of the tyres and to obtain snug fit to the rims the shape of rims and beads has been changed from a horizontal seat to a bevel as shown in fig. 6.
All rims and tyre dimensions and their designations are regu lated in the United States by the Tire and Rim Association, an organization composed of representatives of tyre and rim manu facturers. This body co-operates with similar groups in other countries with a view to establishing international standards for tyre and rim dimensions.
Briefly, the construction of a pneumatic tyre can be sum marized as consisting in—first, the preparation of the cord mate rial, which is now a specialized operation on which many of the largest cotton mills in the world are exclusively employed. The cord, after passing from the cotton mills, is first treated in the tyre factory by a coating of rubber applied on a calendering ma chine to both sides of the sheets of cord fabric. These rubberized sheets are then cut into bias strips of suitable widths and brought to building machines on which the casing is built up, and the wire rings which retain the finished tyre upon the rim are enclosed within the edges of the casing. Various parts of fabric and rub ber are added in the form of chafing strips, filler strips, rubber insulation plies, and cotton breaker strips, and finally the tread and sidewall coverings are added. Early practice of building this structure over a core of approximately the finished shape of the tyre has been generally discontinued. Tyres are now built in the form of wide, flat endless bands which, before vulcanization, are subjected to a shaping process either by producing a vacuum outside the band in a forming box or by applying pressure against the inner surface of the band. The tyres thus formed are mounted on expansible rubber bags and placed in steel moulds for vulcani zation. For many years it was common practice to stack the moulds containing the tyres one above another on an hydraulic ram inside a vertical cylindrical steam pressure vessel. After the cover of the vessel was closed the ram was raised, forcing the halves of the moulds tightly together around the tyres, and heat was supplied by steam outside the moulds while the bags inside the tyres were expanded with air, steam, or hot water or corn binations of them. Hot water or steam furnished heat from the
inside in addition to that supplied by the steam surrounding the moulds. When air was used all heating was from the outside, and if there was any leakage of air to the outside of the moulds through the connections to the bags the rate of heat transfer was greatly reduced. The use of steam or water avoided the dangers of variability in vulcanization inherent to the use of air. In mod ern practice the tyres and their enclosed bags are each placed in individual, hinged steam-jacketed moulds (watch-case heaters). With these heaters air from inside the bags cannot, of course, leak into the steam cavity. This process of vulcanization under pres sure results in the moulding of the tyre into its finished form, uniting all parts firmly together, and produces within the rubber mass certain reactions between the rubber and the sulphur and other ingredients which have been previously incorporated in the rubber, resulting in a tough and durable product.
Inner Tubes are designed to hold the air inside the tyre cas ings. The early practice of vul canizing straight tubes on cylin drical mandrels and afterward splicing their ends together, has now been abandoned in favour of vulcanization in circular, steam jacketed moulds while the rubber is expanded by air against the mould surface. All tubes are, of course, fitted with non-return valves. Metal valve stems, once universally used, have been large ly supplanted by flexible rubber stems with metal inserts. Inner tubes are made 15% to 20% smaller in volume than the casings into which they fit. When inflated they expand to fill the inside of the casings.
Self puncture-sealing tubes are now common. One type con sists of a structure in which the portion of the tube liable to puncture is under compression so that the rubber presses tightly against the puncturing object. Another type contains inside the rubber wall a layer of plastic material which flows under the in ternal pressure to surround the penetrating object and to plug the hole if it is withdrawn. Neither type is intended to offer perma nent sealing but to prevent rapid deflation of the tyre on the high way. Another device consists of a double-chambered structure, the two parts connected and inflated to the same pressure. If the outer chamber is punctured the inner one will carry the load safely until the car can be stopped.