Rubber Tyre Manufacture

The wired on type quickly secured a dominating position in re spect to cycle tyres, but clincher tyres were used extensively for some years on motor cars. This beaded edge was entirely dropped for original equipment by American manufacturers of motor cars in 1923.

With the standardization of all tyres to one type, the design of rims has undergone some change. In America the straight side rim of the flat base type has been most commonly used for trucks and motor coaches. Its construction varies principally in the method of mounting the tyre and in attaching the rim to the wheel. Some of these rims are made with a loose flange of a self locking type; some have two loose flanges with a separate locking ring ; and there is also the transversely split type. They all require the use of a "flap" or band of rubber to cover the line of contact between the beads and the rim to prevent chafing of the inner tube. For passenger car tyres, a rim which embodies the principle of the Welch tyre of 189o, and which has been continuously used for cycle tyres and known as the well base or drop centre rim, has now been universally adopted for motor cars both in England and in America. These drop centre rims enable the tyres to be very readily and easily mounted and dismounted. The method is well known to all motorists and may be briefly described as de pressing the endless wired edge of the tyre into the well of the rim at one point in its circumference thus allowing the wired edge of the tyre to be lifted over the edge of the rim at the opposite point of its circumference.

Cord and Canvas Tyres.

The bodies of pneumatic tyres are now made universally of parallel, cabled cords without cross threads or with only enough light filler yarns to hold the cords in the form of a sheet to facilitate shipment to the manufacturer and handling during application of the rubber. This form of fabric reinforcement has replaced the square woven fabrics of earlier days in which warp and filling were substantially equal in size and strength. The cords are usually made of cotton, although other materials, notably rayon and steel, have recently been adopted for certain kinds of service.

High Pressure and Low Pressure (or Balloon) Tyres.—When motor tyres were being almost ex clusively made of square woven cotton fabric, the dimensions of the various sizes of tyres required for carrying certain loads had become fairly well stabilized, and equally the inflation pressures necessary for each size of tyre had also been uniformly ac cepted.

The superiority of the cord type, due partly to the cord construction and partly to its in creased sectional dimensions led to further developments in which the sectional sizes of tyres were still further increased; and, since the larger the section of the tyre employed to carry a given load the lower the inflation pressure necessary to support the load, these larger tyres could be made with considerably thinner re taining casings because of the lower inflation pressure which they had to withstand.

Such larger tyres quickly received the name of balloon tyres— a title naturally suggesting itself to an observer struck by the enlarged dimensions. The name balloon quickly became very gen erally applied to all tyres of larger sectional dimensions, em ploying lower pressures.

The balloon motor tyre has demonstrated its superior cushion ing properties to motorists generally, and to-day low pressure tyres are the standard equipment of all passenger cars. With larger sectional dimensions, smaller rim diameters have been re quired. Changes in dimensions and air pressures may be illus trated by comparing the data for 30"x3-1-" high pressure tyres with those for 6.00"x16" balloon tyres, now used for cars of approxi mately the same weight. Instead of 23" diameter rims and solb.

pressure the newer type calls for 16" rims and about 281b. pres sure.

The designation of tyre size indicates the type of tyre. For example, 32"x6" refers to a high pressure tyre of section 6" in diameter and of 32" maximum tyre diameter (i.e. 20" rim), whereas 6.00"x1 6" indicates a balloon tyre of approximately 6" cross section width and of 16" rim diameter (i.e. 28" maximum tyre diameter).