The tyres on your car perform a pretty important function. They are all that separates your two tonne car from the road surface. Not only that, they dramatically affect the handling and general feel of your car when you drive. Needless to say, when you are looking for a new set of tyres, spend some time and make sure you get the right set. To help you understand a little more about tyres, let’s look at what goes into their construction.
Four major components
Tyres are made up of four major components: Beads, Cords, Steel Belts, and Tread. The beads are the two circular parts of the tyre that sit on the wheel rim. Beads are constructed of a loop of high-strength steel cable coated with rubber. Cords are layers of polyester cord that form the body of the tyre. The cords in a radial tyre run perpendicular to the tread and diagonal bias tyres have cords that run at an angle to the tread. Cords are coated with rubber bonding material that forms a solid mass. Sometimes manufacturers put fine steel wires into the cord assembly for extra toughness. Finally the tread is the outside of a tyre and is made from a mixture of many different kinds of natural and synthetic rubbers. The design of the tread pattern is determined by CAD design and much testing.
All of the major components as listed above are assembled in a tyre-building machine. A tyre-building machine is a round metal device that holds all the tyre components into the right shape. After assembly, the tyre is then heat cured in the machine. This process is called vulcanizing and it bonds all of the tyre’s components together. When cooled, the tyre is removed from the tyre machine and goes through an inspection process before it is shipped to dealers.
What the numbers mean
You have undoubtedly noted that tyres have lots of numbers on their sidewalls. Thanks to Mr. Ed of Phoenix, a local vehicle financing company in Phoenix, AZ, we’ve got the scoop. Let’s consider a tyre that is has the marking: P215/65R15 99H on the sidewall.
Tyre Type – The P designates that the tyre is a passenger vehicle tyre. You will find T for temporary tyres (spare tyres) and LT for light truck.
Tyre Width – The number 215 is the width of the tyre in millimetres, measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is taken when the tyre is mounted on the rim and properly inflated.
Aspect Ratio – The number 65 designates the aspect ratio of the tyre. The aspect ratio is calculated by measuring the height of the tyre, from the bead to the top of the tread, and dividing it by its width. The result is a percentage, as in 65% in this case. Note that the smaller the aspect ratio, the wider the tyre is in relation to its height – the fatter it looks.
Tyre Construction – The R designates that the tire was made using radial construction (the polyester cords run perpendicular to the beads. Older tyres were made using diagonal bias (D) or bias belted (B) construction. The polyester cords run at angles to each other.
Rim Diameter – 15 specifies, in inches, the wheel rim diameter the tyre is designed for.
Uniform Tyre Quality Grading (UTQG) – The 99H is a grade that this specified by the uniform tire quality grading (UTQG) system. A tyre’s UTQG rating tells you about three things: How quickly the tread wears, the traction you can expect and maximum temperature the tyre should be operated at.