Tires and Wheels


Tires have two main functions: to absorb road shocks and to provide traction for steering, accelerating, and braking.

The tire bead is a high tensile steel wire covered with rubber. Its function is to prevent the tire from being thrown off the rim by spinning action of the wheel. The bead forms an airtight seal against the wheel flange.

The cord body supports the load on the tire. It is composed of strips of material that have been bonded into one unit. Commonly used materials are rayon, nylon, polyester and kevlar.

The tire may be constructed by applying the cord body material in three main ways:

"bias ply" in which two or more layers of cord body is alternated in a criss-cross pattern. This pattern is simple and strong but the tends to creates heat and "squirm" against the road.

"bias belted" in which two or more belts are added around the circumference of the tire. This helps to reduce heat and "squirm".

"radial ply" in which the cord body material is applied across the tire with two or more belts added around the circumference of the tire. This pattern produces little heat and no "squirm".

Radial ply tires provide the least amount of tread flexing or squirming as the tire rotates against the road. This causes less resistance to rolling, resulting in longer tire life and better fuel economy.

Radial tires have a more flexible construction resulting in greater traction to the road during cornering.

Tire treads are the part of the tire that are in contact with the road. The tread is laid over the cord body and bonded to it. Tread type can affect mileage, handling, ride, road noise and traction.

Many tires have tread wear indicators, which are portions of the tread groove that have been filled with rubber. When the non-filled tread wears down to the level of the filled sections the tire must be replaced.

Treads may be smooth and shallow for good mileage and low road noise, but must be deep enough to channel water out from under the tire to prevent hydroplaning.

Treads may be very rough and deep to provide better traction in soft material such as mud or snow.

Most information about a tire can be found on its sidewall. The designation "P195/75R14" provides the following information about the tire:

P: Application (passenger);

195: Tread Width (in mm);

75: Aspect Ratio (sidewall height expressed as a percentage of tread width, e.g., 75% of 195mm);

R: Tire construction (radial);

14: Rim Size (14 in.).

A tire marked "T" is for temporary use (i.e., as a spare only). It will not withstand prolonged use.


The inflation information written on the side of the tire represents the maximum pressure for the tire at its maximum-rated load. This is not the recommended pressure for everyday use. The manufacturer's recommended tire pressure, which is usually found on the owner’s manual or on the vehicle’s doorpost, is a better guide for inflation for everyday use.

Check the tire pressure when the tire is cold (i.e., has not been operated for more than one mile for at least three hours). Tire pressure can increase nearly 10 psi. when the tire is hot.

Over-inflated tires can result in a rough ride and in reduced traction. It can also cause increased wear in the centre of the tread.

Under-inflated tires can result in poor handing, high heat, belt damage, increased fuel consumption and increased wear on the edge of the tread.

Tires must be rotated to different positions on the vehicle periodically to maximise the tread life. On most rear wheel-drive vehicles the right, rear tire wears twice as fast as the right, front tire. This is because most roads are crowned, causing the vehicle to lean out slightly. This in turn causes most of the driving force to be applied to the right.


The wheels are the part of the vehicle to which the tires are mounted. Wheels are usually made by welding the following two components together:

disk, the centre section of the wheel, which contains the mounting holes for the wheel and centres the wheel on the vehicle;

rim, the outer ring around the disc, which is shaped to include a drop well to allow the installation and removal of tires and a flange to prevent the tire from coming off the rim. Modern tires do not use tubes to contain air, therefore the rim must be air tight to prevent leakage.

Safety rims have humps between the tire bead flanges and the drop well at the Centre of the wheel. Should the tire lose pressure during operation, the humps help to prevent the tire from moving into the drop well and coming off the rim.

Wheel nuts and bolts are tapered to align with the taper of the mounting hole permitting the wheel to be accurately centred on the spindle.

Wheel nuts must be torqued to correct specifications. A wheel nut that is too loose can result in movement that will wear out the mounting hole, or more seriously, can cause the loss of the wheel. A nut that is over-tightened can fracture the wheel stud or warp the brake disc, causing vibration while braking.

Three causes of wheel failure are:

road impacts, causing the wheel to be bent, wobble or fail to seal the tire;

corroded flanges which fail to seal the tire;

damaged mounting holes that are usually caused by improperly tightened or installed wheel nuts.