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| 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. |
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| 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. |
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| A lower filler |
This is responsible for
transferring propulsion and braking torques from the wheel rim to the road surface contact
area. |
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| Beads |
These clamp the tyre
firmly against the wheel rim. The beads can withstand forces up to 1,800 kg. |
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| 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. |
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| 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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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. |
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| 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. |
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| 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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