Knowledge on Tyres

1.What is a Tyre?


The tyre is the only point of contact a vehicle has with the road. It bears the load, roll, and steer, transmits forces and absorbs shocks. A tyre is a composite product, i.e. a high-precision stationary assembly of materials that have widely differing properties.

The main subassemblies making up the tyre are as follows :

An innermost sheet of airtight synthetic rubber
This performs the "inner tube" function.

The carcass ply

This is made up of thin textile fibre cables, laid out in straight lines and bonded into the rubber. These cables are largely responsible for determining the strength of the tyre structure. The carcass ply of a car tyre has about 1,400 cables, each of which are capable of withstanding 15 kg.


TA lower filler
This is responsible for transferring propulsion and braking torques from the wheel rim to the road surface contact area.

Beads
These clamp the tyre firmly against the wheel rim. The beads can withstand forces up to 1,800 kg.

Simple rubber walls
These protect the tyre against impacts (with kerbs, etc) that might otherwise damage the carcass. There is also a hard rubber link between the tyre and the rim.

Crown plies

These consist of oblique overlapping layers of rubber reinforced with very thin, but very strong, metal wires. The overlap between these wires and the carcass cables forms a series of non-deformable triangles, an arrangement which lends great rigidity to the tyre structure. These piles, which cover the whole of the tyre crown, provide sufficient circumferential rigidity to prevent elongation under the effect of centrifugal force, and thus ensure a constant tyre diameter under all conditions. They also provide lateral rigidity, to resist sway effects. Finally they remain supple in the vertical direction, to "drink up" obstacles. To make up the crown piles, the metal wires must be firmly bonded onto the rubber. Perfect bonding between these two highly dissimilar materials is difficult to achieve but nonetheless essential.


The tread

This is bonded onto the crown plies then sculpted with the special tread pattern. Since this is a part of the tyre that comes into contact with the road surface. It must be able to withstand very high forces, and it must be able to grip dry and wet road surfaces. In addition, it must resist wear and abrasion, and it must not overheat. Once the tread has been fitted and sculpted, the whole assembly is vulcanised for maximum solidity.


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2.What are Tyre Markings


General

On the sidewalls of a tyre cover will be found several markings, the most important of these being the size marking and the ply rating marking.


Size and Ply Rating
Example: 9.00-20 12 ply rating

The size marking, e.g. 9.00 represents the nominal cross-sectional width of the tyre in inches. The size marking, e.g. 20 is the nominal diameter of the tyre from bead to bead in inches, and indicates the correct rim diameter size. The size marking, e.g. 12 ply rating, identifies a given size of tyre with its maximum recommended load when used in a specific service. It is an index of tyre strength and does not necessarily represent the number of actual plies of material in the tyre.

The above system of marking the size and ply rating on a tyre is now standardised for all truck and bus tyres. This uniform system serves to describe effectively a tyre and to identify it with its load rating, and hence its capacity to perform a given task, i.e., to carry a defined maximum load consistent with the air (measured by a pressure gauge) that it contains in order to obtain optimum performance (lowest tyre cost per mile). It should be noted that the use of ply rating allows the inclusion under the same size marking, of tyres having a range of load carrying capacities.


Tyre Serial Numbers
In addition to the size and ply rating markings, each tyre bears its own individual serial number - a number which is used for purposes of record and identification.


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3.Identification of Causes of tyre wear


Problem Cause
1.Shoulder Wear
Both shoulders wearing faster than the tread
Under-Inflation
Repeated high speed cornering
Improper matching of rims and tyres
Tyres have not been rotated recently
Centre Wear
The centre of the tread is wearing faster than the shoulders
Over-Inflation
Improper matching of rims and tyres
Tyres have not been rotated recently
One-sided wear
Once side of the tyre wearing unusually fast
Improper wheel alignment (especially chamber)
Tyres have not been rotated recently
Spot wear
A part (or a few parts) of the circumference of the tread is (are) wearing faster than other parts
Faulty suspension, rotating parts or brake parts
Dynamic imbalance of tyre / rim assembly
Excessive runout of tyre and rim assembly
Sudden braking and rapid starting
Under-Inflation
Diagonal Wear
A part (or few parts) of the tread are weaning diagonally faster than the other parts
Faulty suspension, rotating parts or brake parts
Dynamic imbalance of tyre / rim assembly
Improper wheel alignment
Tyres have not been rotated properly
Under-Inflation
Feather-edged wear
The blocks or ribs of the tread are wearing in a feather edge pattern
Bent axle beam
Improper wheel alignment (faulty toe-in). A toe-in is where the front edge of the wheels are closer together than the rear

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