A tire provides a cushion between the vehicle and the road, to reduce the transmission of road shocks.
The air in the tire supports the vehicle’s mass, and the tread provides frictional contact with the road surface, so the vehicle can maneuver for normal use.
Radial ply tires are usually manufactured in stages.
The casing is initially formed by laying the rubber inner liner, and the first layer of textile ply cords, around a flat drum mold.
The rubber-covered bead wire and sidewalls are then locked into position.
The rubber sidewalls protect a finished tire from kerb damage and weathering.
At the second stage-building machine, the tire is shaped. Belts of steel wire are guided into place.
The tread is then positioned and the uncured tire is consolidated by rollers, before it is placed in the mold.
During the molding and curing stage, the tire is subjected to high temperature and pressure, and it takes on its final fixed identity, with its own distinctive tread pattern.
It is then trimmed and checked for balance and quality before it is inflated, and run under load against a rotating drum.
Two types of tire construction are common, cross-ply and radial ply.
The cross-ply tire is the older form. It is also called a bias-ply or conventional tire. It is constructed of 2 or more plies or layers of textile casing cords, positioned diagonally from bead to bead.
The rubber-encased cords run at an angle of between 30 and 38 degrees to the centerline, with each cord wrapped around the beads.
A latticed criss-crossed structure is formed, with alternate layers crossing over each other, and laid with the cord angles in opposite directions.
This provides a strong, stable casing, with relatively stiff sidewalls.
However, during cornering, stiff sidewalls can distort the tread and partially lift it off the road surface, and that reduces the friction between the road and the tire.
Stiff sidewalls can also make tires run at a high temperature. This is because, as the tire rotates, the cords in the plies flex over each other, causing friction and heat.
And a tire that overheats can wear prematurely.
Modern tires are made from a range of materials. The rubber is mostly synthetic, with carbon black added to increase strength and toughness. When used in the tread, this combination gives a long life.
Natural rubber is weaker than the synthetic version. It’s used mainly in sidewalls.
The plies are made from cords of fabric, coated with rubber.
Early tires used cords of cotton, but with increased vehicle speeds, and loads, rayon and nylon cords are now common.
Cords of synthetic fabric have high tensile strength. They resist stretching, but are flexible under load.
The cords are placed in parallel and impregnated with rubber to form sheets called plies. Plies have high strength in one direction, and are flexible in other directions.
When cotton was used as a cord, the number of plies or layers in a tire was a measure of the tire’s strength.
Newer cord materials use fewer plies. A modern steel-belted radial with a 6-ply rating may have just 2 plies in its sidewall.
Having fewer plies makes the tire more flexible.
Higher numbers of plies make a tire’s response to bumps harsher.
The bead of the tire is made of a cord of high-tensile steel, coated with rubber. The end of the ply is wrapped around the bead, which is then wrapped in rubber to stop chafing of the plies, and also to seal the bead against the rim.
The length of the wire used for the bead determines the rim diameter of the tire. Belts that reinforce the tread area of the tire are mostly made of braided, high-tensile steel wire, but they can also be made of rayon, or polyester.
The inner liner of a tubeless tire is made of soft rubber. The inner liner must be flexible, and airtight.
Hysteresis can best be described as the energy lost, usually in the form of a build up of heat, when a section of vulcanised rubber is deformed in a regular, constant manner.
The more you subject a tire to flexing and deformation the more heat will build up within the tire. As we know, excessive heat is the enemy of a tire so this build up has to be kept under control.
The size of a tire 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 for the vehicle.
The overall tire size must allow sufficient clearance between the tire and the vehicle frame.
All manufacturers mold information about the tire into its sidewall.
In cross-ply tires, the bead diameter and the section width are stated in inches. For example, six hundred by sixteen indicates a tire with a section width of 6 inches and a bead diameter suitable for fitting to a rim which is 16 inches in diameter, across the bead seats.
The load capacity is indicated by the ply rating, for example, 6PR.
The aspect ratio of a tire is the ratio of its height to its width. It is usually given as a percentage. The lower a tire’s aspect ratio, the wider the tire is in relation to its height.
An aspect ratio of 98% means the section height of the tire is slightly less than the section width. This is called a cushion or balloon tire.
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 tire.
The profile of cross-ply tires was reduced further to between 78 and 82%, called a super low section. However the stiffness of cross-ply tires makes them unsuitable for further reduction in profile.
Radial ply tires have been manufactured in 78% profile, but are also made with further reductions in profile, from 75%, to 45%.
Tire information
Information on tire aspect ratio is now included in the sidewall marking, together with the type of construction, and the speed rating.
The speed rating of the tire is given by the letter code, which indicates maximum recommended speed for that tire. Common symbols for passenger car tires include,
- S, for up to 180 kilometers per hour.
- H, up to 210 kilometers per hour.
- V, up to 240 kilometers per hour.
- and Z for over 240 kilometers per hour.
Radial ply tires have always been marked with the section width in millimeters, but with the rim diameter in inches. For example:
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- 185 is section width in millimeters.
- 70 indicates a 70% aspect ratio.
- H is the speed rating, for up to 210kilometres per hour.
- R indicates radial ply construction.
- 13 indicates the tire is suitable for fitting to a 13-inch diameter rim.
Totally metric types are also manufactured. For example:
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- 190 is the section width in millimeters.
- The aspect ratio is 65%.
- The speed rating is H, for up to 210 kilometers per hour.
- R indicates radial ply construction.
- 390 indicates the tire is suitable for fitting to a 390 mm diameter rim.
Metric-diameter rims cannot be fitted with inch-diameter tires, or vice-versa.
Although tire markings may remain traditional, say, 255,45, Z-R 17, there is a worldwide move towards an I-S-O metric standard which uses letters:
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- P for passenger
- LT for light truck
- C for commercial
- and T means temporary use as a spare wheel.
The tire may have a load index number, indicating the maximum load a tire can carry at the speed indicated by its speed symbol, which follows the number.
So a P-series metric size code may read in full - P205-slash-65, R15, 92H.
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- P for passenger car tire.
- 205 is the section width in millimeters.
- 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 kilometers per hour.
Further development of high-speed tires has expanded the speed categories to include W, and Y.
In another example:
- Z indicates a speed over 240 kilometers 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 kilometers per hour.
Tire tread designs
Tires generally fall into one of the following categories:
- Directional,
- Non-directional
- And Symmetric and Asymmetric
Directional tread patterns are designed to provide a range of attributes during particular driving conditions.
The tire can only be mounted to the wheel so that it revolves in a particular direction to correspond with the tread pattern.
An arrow on the tire sidewall indicates the designed direction of forward travel.
Non-directional tread patterns are designed in such a way that the tire can be mounted on the road wheel for any direction of rotation.
Symmetric tread patterns have the same tread pattern on both sides of the tire and can generally be classified as Non-directional tires.
Asymmetric tread patterns have a tread pattern that is different from one side of the tire to the other.
They are designed to provide good grip when traveling straight and in turns. They are generally directional tires and must be fitted to the road wheel in accordance with the fitting instructions to ensure that they perform as designed under operating conditions.
Summary
Tire treads can be directional, non-directional, symmetric and asymmetric.
Tire ratings for temperature & traction
One of the markings on the sidewall of a tire is a Uniform Tire Quality Grading or UTQG grade.
The tire's UTQG rating provides information on three aspects of the tires durability and operational characteristics.
They are:
- tread wear,
- traction, and
- temperature.
The tread wear number comes from testing the tire 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”.
The rating is based on a percentage of the projected wear life. For instance, a tire rated at 400 has a projected life twice that of a tire rated at 200.
There are many factors that influence wear, such as vehicle speed, road surface, climate, vehicle wheel alignment, and the driving characteristics of the driver. As such the rating can only be an indication of the anticipated wear characteristics of the tire in controlled conditions.
A traction rating is a letter based indicator system. The rating is based on the tire’s ability to stop a vehicle on wet concrete and asphalt in a straight-line situation. It does not indicate the tire’s cornering ability.
The Tire traction indicators are rated as AA, A, B or C. AA is the highest rating.
It is important to note that the test and relevant rating does not indicate hydroplaning resistance, dry or snow traction capacity, or cornering capability in wet, dry or snow conditions.
The Temperature rating of a tire is a letter based on a controlled step speed test. The possible ratings are A, B, and C. The rating is a measure of how well the tire dissipates heat and how well it handles the buildup of heat.
Excessive heat buildup can reduce tire life, or even lead to tire failure. However, while temperature plays a role in the speed capability of a tire, it is not the only factor.
It is important to remember that these ratings are based on standardized test conditions, and the tests do not reflect tires that are operated in overloaded, under inflated, and/or misaligned conditions.
It should also be noted that when rated one tire might be a low "A" and another a high "B", so the actual operating performance differences might be relatively small.
It is not uncommon for there to be differences in UTQG ratings within a given tire design. Sometimes a particular vehicle manufacturer will require certain properties for the tires supplied to their vehicles, which can affect the ratings, both positively and negatively.
Sometimes there are differences between small sizes and large sizes of tires in a given design. All of these aspects can affect the actual rating that is put on the sidewall.
Summary
A Uniform Tire Quality Grading or UTQG grade provides information on tread wear, traction and temperature.
