In the previous chapters considerable has bees mentioned about tires and their functions. However, in this chapter the con-struction of a motor truck tire and its mounting on the felloe band of the wheel will be described. To be absolutely efficient, a commercial car must be able to carry its load whenever and wherever needed. The vehicle itself may be as nearly as possible absolutely efficient, when measured by this standard, but as a whole it can be no more efficient than its weakest part. Each part must be so designed and so co-ordinated with other parts as to perform in the most efficient manner. In this respect the tires are no exception.
The functions which the tires perform, reduced to their simplest terms, may be isted as follows: (1) To give traction to the wheels and prevent slipping, (2) to protect the mechanism of the vehicle from jars and vibrations, (3) to cushion the load.
Before the advent of the motor truck, solid rubber tires were used almost exclusively on the wheels of carriages to provide easier riding. On these vehicles the wheels were merely rolling members and performed no tractive effort, as the horses did the pulling. Such tires were held in place by means of wires embedded circumferentially in rubber, the whole unit being mounted on a steel channel shrunk on the felloe of the wheel. These tires were easily applied, but possessed certain dis-advantages such as slipping in the channel, cutting at the base and release of the robber adjacent to the wires.
This type of tire did not prove very satisfactory for heavy vehicles, for the reasons mentioned above. In order to overcome these shortcomings, a new tire was introduced, which was called the side-wire type. In general shape and appearance it was the same as the earlier type. However, it was retained in the channel by means of short cross wires embedded in the base of the tire which projected on either side. These cross wires were held in place as securely as possible by two other wires running circumferentialy around base just inside edges of the channel.
With the introduction of the commercial car, an entirely new and different function was required of the tires, that of transmitting the driving power from the rim of the wheel to the road surface, i.e., the tires became part of a tractive rather than a rolling member. Their carrying capacity was also increased because the gasoline engine could move heavier loads than the horse, while the weight of the truck itself was considerably more than the wagon, and the speed was also increased considerably. The carriage type of tire was found entirely too light to perform the work required of it. This condition brought about the invention of the solid motor truck tire, a tire vulcanized in circular endless form to fit the dimensions of the wheel. This type of tire has been brought out in different designs and types such as flange, internal wire, side wire, hard rubber and metal-base types, also the demountable.
Carriage tires were made in oval shape, in cross sections from three-fourths inch and of compound rubber which is formed through a tubing machine die, vulcanized in long moulds with many cavities, in the shape in which it had been designed.
The construction of solid truck tires that have been put on the market is very similar in a general way. The rubber is forced through a die or tubing machine, or built up from sheets of calendered stock in endless form to fit the exact dimensions of a wheel on a particular style of base or retaining body of the tire as designed by the different manufacturers, according to their ideas, which have taken various forms, such as circumferential and side retaining wires which are engaged over embedded cross wires, bases of hard rubber in various forms also semi-hard rubber which can be moulded into the tire and more recently the metal-base type.
This tire is built on the rim at the factory and cannot be separated from it. The surface of the metal rim is cut with grooves, under cut notches or in other ways, so that the hard rubber gets a firm anchorage into the rim. In manufacture this rubber base is applied in some factories in layers, just as you wrap a bandage on your finger. The base is relatively thin, perhaps not one-eighth the radial thickness of the tire. On the top of this is built the regular rubber part of the tire, of softer rubber to afford the desired resilience. This part is also in some factories built up similar to wrapping a bandage until the desired thickness is obtained, which, when done, the tire is trimmed to shape and vulcanized.