Wheels And Tyres

Wheels and tyres

by Eng.Khaled Soliman

Historical Background: The pneumatic tyre was invented by R.W. Thomson, of Edinburgh, in 1845, and produced in the Macintosh Works in Manchester. It consisted of an inner tube of rubberised canvas (textile made from natural fibers) enclosed in a leather casing. This was long before rubber of adequate durability became available for use in tire production. It was John Boyde Dunlop, a Scottish veterinary surgeon living in Belfast, who introduced the first pneumatic tire for cycles. Prior to this, solid rubber tires were employed for a wide variety of vehicles. Indeed, it was not until 1912 that the first pneumatic tired truck appeared, and solid rubber tired heavy vehicles were still to be seen on the roads up to the early 1920s. Even so, it was Michelin who, in 1891, first manufactured detachable pneumatic tires for bicycles in quantity. By 1895, this company was providing tires for the rapidly expanding motor manufacturers in France. These two wheel and tyre assemblies, both invented in 1890, were the forerunners of our modern designs. The Bartlett rim,

left, was the basis of the British and US types, remaining in use for 40 years, and the Welch rim, right, is that of the continental and, in particular, the Michelin tyres.

Wheel and tyre assemblies: Design requirements for the wheel and tyre assembly are: 1. Light weight is essential but each successive peak of roughness in the road surface will lift the tire upwards clear of the surface. This will greatly reduce its efficiency in braking, acceleration and steering. 2. On the other hand the whole assembly must be extremely strong to resist both the static and dynamic forces acting upon it along all three axes, and to cope with shock loading. 3. Self-cleaning are necessary, otherwise a build-up of dirt will increase weight and could introduce out-ofbalance forces. 4. The whole assembly must be in both static and dynamic balance.

Tyre construction: Two basics of tyre construction are commonly used biasply tyres or called cross-ply tyres and radial-ply tyres.

Bias-ply tires were developed about the 1960s by American automotive industry. Over several decades radial-ply tire gradually displaced it on passenger cars, such it the standard today. In radial-ply tyre, the beads consist of parallel hoops (rings) of steel wire bound with fabric to form a tightly wrapped bundle (collection) in the form of a ring. The ends of the plies are wrapped around them. In cross-ply tyres, there are generally several plies, or layers, of fabric reinforcement in the form of cords of rayon, nylon or polyester. The cords are rubberised and set at an angle of approximately 45°. Rayon is the material most widely used for radial plies for cars and steel for heavy vehicles. Other materials include nylon, polyester, glass fibre and aramid fibre. Polyester is stronger than Rayon, but is difficult to bond to rubber and is less stable dimensionally when hot. Nylon has similar problems and has a problem when collected together in the radial ply tyres. Glass fibre is stronger but has poor fatigue resistance. Aramid fibres are very strong in relation to their weight but are costly.

Beneath the tread of a radial ply tyre there are normally two or three crossply layers of steel, or four or six of textile such as Rayon. These are termed bracing plies, In some cases the steel wires have been supplemented by nylon or polyester supporting threads. The function of bracing plies in general is to prevent the tyre from being distorted by centrifugal force at high speed, to stabilise the contact patch and to increase the resistance to punctures.

Radial-ply tyre

Tread design: The tread itself is of a special rubber mix formulated to provide good wear resistance, firm grip on the road surface and high strength to cope with the various loads applied to it in the contact patch area. Indeed, different types of rubber may be employed for the treads, side walls, beads and fillers. Tread materials and patterns are determined by the type of duty required. Racing cars, for example, have what are termed dry and wet alternatives which may be interchanged during a single race in response to weather changes.

Noise: Noise is generated by the impacts of the individual elements of the tread on the road surface, and it can be reduced in several ways. 1. The tread elements are pitched at irregular intervals, to avoid resonance of components in the vehicle structure at the frequencies of their contacts with the road surface. 2. Tread materials of high hysteresis are employed, to provide internal damping. since high hysteresis is associated with the generation of heat. 3. Patterns, shapes and sizes of the tread elements are selected that do not generate noise. 4. Block size is important too: large blocks hit the road harder than small ones. On the other hand small blocks imply high crown stresses and therefore poor wearing qualities, so we must optimize between them.

What is the hysteresis ?

the loss in as it when over in the

Size tyre marketing:

Hysteresis represents energy the rubber deforms sliding the aggregate road.

and

The data marked on the tyre wall, including those for tyre size and aspect ratio, are illustrated in next figure. The main data in this example: 235/60 ZR 18 235 represent the width of the tyre in (mms) 60 is the aspect ratio and represent height of side wall over the rim=60% of width of the tyre=96mm 18

represent the diameter of rim in (inches)

So the overall diameter of the tyre: Overall diameter=suction height + diameter of the rim (all in inches) Section height=2X96=192mm=7.6 inch

Overall diameter=7.6+18=25.5 inches

Manufacture of tyres:

A Michelin manufacturing process is illustrated diagrammatically in above Fig. First, the raw rubbers, both natural and synthetic, are removed from their bales and fed individually, according to type, into a premastication machine from which they are delivered into what is termed a batch box. Here the various compounding materials are added. The materials in the batch box are then fed into a Banbury mixing machine, whence the rubber mix is delivered to a mill for forming into sheet material. Approximately 8% powdered chamicals, 23% carbon black, 3% texile, 18% steel cord and 48% rubbers go into a modern tyre. The output from the mixing machine is cut into rectangular pieces of sizes easily handled, to form the rubber stock. This stock is reheated, and then some of it extruded to form strips of rubber for use as tread, walls and fillers. The remainder of the stock is also extruded and the strip thus produced

calendered and laminated with the rubberised cords, or rubber coated steel wires, to form the plies. Incidentally, the steel cords are wound from extremely fine wire, to obtain adequate toughness coupled with flexibility. Building up the tyre begins with the bead wires, around which the casing plies are wound. All the components of the tyre are then assembled, in turn, on to a rotary jig. Then the rubber components of the side walls and, last of all, the tread are added. Finally, the complete assembly is placed in the vulcanising mould, where it is heated while pressure is applied, both radially and axially, to mould it into its finished shape.