Tyre Sizes & Designations or Tyre Markings & Ratings or Specifications:
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| Fig 1: Tyre Size specification |
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| Fig 2: Tyre Dimensions or Sizes |
The tyre chart lists the wheel and tyre sizes approved by the manufacturer of the vehicle.
In radial ply tyres, tyre size is always indicates on sidewall provide by tyre manufacturer. For example, "195/55R16", which means, a tyre have 195 mm section width, 55 is the aspect ratio (h/w), R indicates for Radial ply construction while 16 is the diameter of tyre inner rim in inches as specified in fig 1, & fig 2 also for tyre size and designations.
In cross-ply tyres, size is, the bead diameter and the section width are stated in inches. For example, “7.25 X 16” X 10PR”, indicates a tyre with a section width of 7.25 inches and a bead diameter suitable for fitting to a rim which is 16 inches in diameter, across the bead seats. The load index / capacity is indicated by the ply rating 10PR.
Aspect ratio: (for tyre size) [2]
Aspect ratio, also called the profile ratio, is the ratio of the section height to the section width of a tyre. Three aspect ratios are available (in percentage), namely 60, 70, and 80. For example, in a tyre with an aspect ratio of 70, the tyre width is 70 per cent of the section height. The tyre aspect ratio is shown in figure (3).
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| Fig 3: Aspect ratio |
The aspect ratio of a tyre is the ratio of its height to its width. It is usually given as a percentage. The lower a tyre’s aspect ratio, the wider the tyre is in relation to its height. An aspect ratio of 98% means the section height of the tyre is slightly less than the section width. This is called a cushion or balloon tyre. An aspect ratio of 88% means the height is 12% less than the width, giving a lower profile. It is called a medium low profile tyre. The profile of cross-ply tyres was reduced further to between 78% and 82%, called a super low section. However the stiffness of cross-ply tyres makes them unsuitable for further reduction in profile. Radial ply tyres have been manufactured in 78% profile, but are also made with further reductions in profile, from 75%, to 45%.
The size of a tyre must satisfy some basic conditions. The bead diameter must suit the wheel rim diameter. Section width must be suitable for use on the wheel rim and large enough to have a suitable load-carrying capacity (Load Index) for the vehicle. The overall tyre size must allow sufficient clearance between the tyre and the vehicle frame.
Tyre Designation or Markings & Ratings:
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| Fig 4: Passenger Tyre Designation |
Every tyre comes with specified manufacturer marking and ratings, and they are namely; application, width, aspect ratio, ply construction, wheel rim diameter or size, load index and speed ratings. All manufacturers should also provide miscellaneous information such as inflation pressure, temperature rating, approval sign, direction of rotation arrows and date of mould/manufacture about the tyre into its sidewall.
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| Fig 5: Passenger Tyre Designation [2] |
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| Fig 6: Truck tyre specification [2] |
Tyres can be identified by markings & ratings on the sidewalls. This typically includes the maker’s name, the rim size, the type of tyre construction, aspect ratio, maximum load and speed and, in some cases, intended use. Regulations cover the allowable dimensions for wheels and tyres on a particular vehicle. These dimensions are usually set out on the tyre spec attached to the vehicle. Incorrectly selected wheels and tyres can overload wheel bearings and change steering characteristics.
Tyre Load Index and Speed Ratings: [1]
Information on sidewall marking is
now included with the type of construction, load carrying capacity (load
Index), speed rating and maximum permitted pressure.
The 'Load Index/Capacity' of a tyre
determines what payload each tyre can carry. It is vital that you check with
your manufacturer what capacity should be put on your car tyre. The following table no. 1 shows what Index specification can carry.
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| Table 1: tyre load index [1] |
The 'Speed Rating' of the tyre is given by the letter code, which indicates maximum recommended speed for that tyre. Common symbols for different car tyres include as indicates in table no.2;
| Speed Ratings | Maximum Speed (kph) | Vehicle Type |
| S | 180 | passenger sedan |
| T | 190 | sedan, minivan |
| H | 210 | sport, sedan, coupe |
| V | 240 | sport car |
| W | 270 | sport car |
| Z | 240+ | sport car |
The different tyre markings are available on sidewall of any tyre. Inflation pressure in Psi, temperature sign in alphabets, approval sign, directional of rotation arrows, date of manufacture etc.
Tyre markings may remain
traditional, e.g. P 255 45 Z-R 17 C, there is a worldwide move towards an ISO
metric standard which uses letters:
- P for passenger,
- LT for light truck,
- C for commercial,
- And T means temporary use as a spare wheel.
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| Fig 7: Commercial tyre marking [1] |
The tyre may have a load index
number, indicating the maximum load a tyre can carry at the speed indicated by
its speed symbol, which follows the number. e.g. a P-series metric size code
may read in full – P205/65 R15 92H.
- P for passenger car tyre,
- 205 is the section width in millimetres,
- With 65% aspect ratio,
- R – Radial ply construction,
- 15 inch diameter rim,
- 92 load index, for a maximum load of 630 kilograms,
- And H for a speed rating up to 210 kilometres per hour.
Further development of high-speed
tyres has expanded the speed categories to include W and Y.
Z indicates a speed over 240
kilometres per hour, but the load and speed rating is taken as the maximum load
and speed, that is,
- 89 for a maximum load of 580 kilograms,
- And W for 270 kilometres per hour.
Tyre Ratings for ‘Temperature & Traction’
One of the markings on the sidewall
of a tyre is a Uniform Tyre Quality Grading or UTQG grade. The tyre’s UTQG
rating provides information on three aspects of the tyres durability and
operational characteristics. They are: Tread wear, traction and temperature. [1]
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| Fig 8: tyre temperature ratings [1] |
The tread wear number comes from
testing the tyre in controlled conditions. The higher the number the longer the
life expectancy of the tread. Since no one vehicle will be subjected to exactly
the same surfaces and at the same speeds as the controlled conditions, the
number can only be an indicator of expected tread life in “normal conditions”. [1]
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