The tire provides a cushion between the vehicle and the road to reduce the transmission of road shocks. It also provides friction to allow the vehicle perform its normal operations.
Modern tyres are manufactured from a range of materials. The rubber is mainly synthetic.
Two types of tire construction are common - cross-ply and radial.
Most passenger cars now use radial tires, as do most 4-wheel-drives and heavy vehicles.
Tube tires require an inner tube to seal the air inside the tire.
Tubeless tires eliminate the inner tube by making the complete wheel and tire assembly air-tight.
A special, air-tight valve assembly is needed. This can be a tight fit into the rim, or it can be held with a nut and sealing washers.
Tires can be identified by markings on the sidewalls. This typically includes the maker’s name, the rim size, the type of tire construction, aspect ratio, maximum load and speed, and, in some cases, intended use.
Regulations cover the allowable dimensions for wheels and tires on a particular vehicle. These dimensions are usually set out on the tire placard attached to the vehicle.
Incorrectly selected wheels and tires can overload wheel bearings and change steering characteristics. The tire placard lists the wheel and tire sizes approved by the manufacturer for the vehicle. Using other wheels and tires may be illegal.
List allowable tire and wheel sizes for 3 different vehicles, using the tire placard recommendations.
Radial ply tires
Radial ply tires have much more flexible sidewalls due to their construction. They use 2 or more layers of casing plies, with the cord loops running radially from bead to bead.
The sidewalls are more flexible because the casing cords do not cross over each other. However, a belt of 2 or more bracing layers must be placed under the tread.
The cords of the bracing layers may be of fabric, or of steel, and are placed at 12, to 15 degrees to the circumference line. This forms triangles where the belt cords cross over the radial cords.
The stiff bracing layer links the cord loops together to give fore and aft stability, when accelerating, or braking, and it prevents any movement of the cords during cornering.
The cord plies flex and deform only in the area above the road contact patch.
There are no heavy plies to distort, and flexing of the thin casing generates little heat, which is easily dispersed.
A radial ply tire runs cooler than a comparable cross-ply tire, and this increases tread life. Also, a radial tire has less rolling resistance as it moves over the road surface.
Radial ply tire sidewalls
The sidewalls of radial ply tires bulge where the tire meets the road, making it difficult to estimate inflation pressure visually. It needs to be checked with an accurate tire gauge.
Using correct inflation pressures extends tire life, and is vital for safety.
Sidewalls of an under-inflated tire flex too far, which pushes the center section of the tread up and away from the road surface. This causes wear at the shoulders of the tire.
In an over-inflated tire, the sidewalls are straightened, which pulls the edges of the tread away from the road, and causes wear at the center of the tread.
A tube-type tire uses an inner tube which provides an air-tight container inside the tire.
A tubeless tire is lined with a soft rubber layer to form an air-tight seal. This inner liner also seals against small penetrations, letting air escape only relatively slowly. When a tubeless tire is fitted, an air-tight valve assembly is used. It can be a tight fit into the rim, or be held with a nut and sealing washers.
A tire and wheel assembly must be balanced. As the wheel rotates, centrifugal force acts outwards. Any part heavier than the rest will vibrate vertically, with the heavy area slapping the road surface with each turn of the wheel. This is called static unbalance.
Dynamic unbalance causes the wheel assembly to turn inwards, and then outwards, with each half revolution. As speed rises, rapid side movement of the front wheels causes a sideways-vibration, or wheel wobble effect, at the front of the vehicle.
These conditions must be corrected to prevent cupping or dishing of the tread, and reduced tread life.
Tread life can also be reduced by incorrect wheel alignment. The feathered edge of this tire indicates an incorrect toe-setting. And wear on the one shoulder of this tire could be due to incorrect camber setting.
Most passenger car tires have tread-wear indicators molded into the tread pattern. They generally provide an indication when the depth of a tire groove falls to 1-and-a-half millimeters.
Control of a vehicle in any weather conditions depends finally on frictional forces generated between the tires and road surface.
On a dry road, a smooth rubber surface can provide a high coefficient of friction, sufficient to maintain a degree of control during braking, accelerating, and cornering.
In wet conditions, the coefficient of friction between a smooth tire and the road surface falls to an extremely low value.
Grooves in the tread pattern clear water away from the contact patch area. This allows a relatively “dry area” to be formed, and for road adhesion to be maintained.
Tire pressure monitoring systems
Maintaining proper tire pressure is essential for the safety and performance of a vehicle. It also plays a significant role in decreasing fuel consumption and extending tire life.
All tires lose inflation over time and, as many modern vehicles have extended service intervals, tires can become dangerously under-inflated without regular checking by the vehicle driver.
In addition to increased fuel consumption and tire wear, long periods of driving with low tire pressures can cause additional stress on the tire sidewalls. This results in increased operating temperatures that can lead to premature tire failure.
Tires operating with low pressures can also affect the vehicle's handling and performance. In a worst-case scenario, under-inflation can lead to a tire blowout or tread separation.
Automated Tire Pressure Monitoring Systems or TPMS provide a means of reliable and continuous monitoring of the vehicle tire pressure and are designed to increase safety, decrease fuel consumption and improve vehicle performance.
There are two types of tire pressure monitoring; direct and indirect.
The direct monitoring system uses a pressure sensor mounted inside each wheel and uses a wireless transmitter to give direct tire pressure readings.
The indirect system uses the vehicles wheel speed sensors to determine if a tire is underinflated when compared to each of the other tires.
TPMS can be fitted to all vehicle types using conventional and run-flat tires.
With a Tire Pressure Monitoring System installed on a vehicle, drivers can monitor the tire pressures and temperatures from the driver's seat to ensure that their tires are properly inflated under all operating conditions. The systems are also designed to ignore normal pressure variations caused by changes in ambient temperature.
The sensor installed inside each wheel is able to respond to as little as a 3PSI or 20Kpa drop in pressure. Real-time information is sent via wireless signal to a display in the vehicle. If a fluctuation occurs, an audible and visual warning instantly alerts the driver allowing time for the vehicle to be stopped or driven to a service station for tire repair or re-inflation. The tire is used to enclose the unit as protection from the outside environment. An on-board computer receives the radio messages from the sensors, which are coded for individual wheel identification.
The interactive display inside the vehicle shows:
- The required tire pressure
- the actual tire pressure
- the tire pressure status
- and the temperature of the tire
The driver can use the display control buttons to check the status of each tire.
In OEM installations, each time the ignition is switched on, an indicator on the instrument panel and on a system display provides information about all four tire pressures, and gives a "Pressure OK" message if all is well. An indicator on the display will “flash” whenever pressure loss is detected. In the case of minor deflation, an orange "Service" light is shown, and indicates the faulty tire. If the pressure is dangerously low, a red "Stop" light flashes, accompanied by a punctured tire icon, indicating that an immediate wheel change is needed.
The sensors are activated by a centrifugal switch and transmit only when the vehicle is in motion. When the vehicle stops the sensors return to sleep mode to extend battery life, however the driver is still able to review the latest signals received from the wheels before the vehicle was stopped.
Runflat tires
The major safety benefit of Run Flat technology or RFT is that it enables a driver to maintain control if a vehicle in motion suffers a rapid loss of tire pressure.
In addition, RFT tires enable the driver to continue the journey within specified speed and distance limits, avoiding the need to replace the wheel on the side of the road.
Tire manufactures also maintain that, as an added benefit of RFT they usually save weight by eliminating the need to carry a spare tire. However, because of their construction they are generally between two to three times heavier than their conventional counterpart, which adds additional un-sprung weight to the vehicle, affects the suspension and can increase fuel consumption.
Because of the extra materials used in construction of the tire, they are also normally more expensive to purchase. In addition, runflat tires are usually “harder riding” and noisier in operation which can be a disadvantage in some applications.
From a manufacturing perspective however, the free space created by eliminating a standard spare wheel gives the car manufacturer a range of additional design opportunities.
An “onboard” vehicle tire pressure monitoring system (TPMS) is normally mandatory for all RFT applications.
Run Flat Technology design features generally focus on two aspects of operational use - rigidity and heat resistance. This is to enable the tire to support vehicle weight when it is rotating with a total air loss.
The sidewall is constructed with reinforced rubber and is thicker than in conventional tires enabling it to carry the vehicle's weight at zero-pressure.
While the bead shape configuration of the tire itself is largely unchanged to enable compatibility with conventional rims, the bead wire is normally wider and reinforced to ensure a secure fit on the rim even at zero pressure, and a special bead filler with low heat generation is used as part of the construction.
Some runflat tires are known as “Extended Mobility Technology” or “EMT” tires.
EMT sidewalls can be six times thicker than traditional tires. As a result the manufactures say that EMT “Runflat” tires can be driven at speeds of 55-miles or 80-kilometers per hour for up to 200 miles or 300 kilometers in a deflated condition before being damaged.
They normally have a directional tire pattern which must be fitted in such a way that the tread rotates only in one direction.
Specialized equipment is needed to fit Runflat tires as the sidewalls are stiffer and thus not as pliable when fitting.
Some heavy-duty applications have a two–part wheel arrangement with a thick rubber expander fitted inside the tire before the two halves of the wheel are bolted together.
Whilst the tires are described as “Runflat", they are not indestructible. Major damage that slices the tire casing can still result in complete tire failure.
Summary
Run Flat technology or RFT tires enable a driver to maintain control if a vehicle in motion suffers a rapid loss of tire pressure.
Space-saver tires
Space-saver spares are designed for emergency use only.
They're designed to get you to a service center where you can have the regular tire fixed or buy a new one.
When provided with the vehicle as part of the original manufacturer’s equipment, most manufacturers warn not to exceed 50 miles or 80 kilometers per hour and 50 miles or 80 kilometers of driving on the space saver tire.
Some cars are provided with miniature or collapsible space-saver spare tires as spare wheels. These normally require a specially charged canister for inflation when being installed.
Others have small, temporary spare tires that are inflated normally with a compressed air supply but to a much higher pressure than normal road tires.
Summary
Space-saver tires are designed for emergency use only.
Tire distortion
During cornering, centrifugal force acts on a vehicle to produce a side force. This side force must be resisted by the interaction of the tire on the road surface. The greater the side force, the greater the opposing force must be.
Without this resistance, the vehicle will continue in a straight line.
The pneumatic tire provides this opposing force by being able to distort while still gripping the road.
Then since the tire’s construction makes it elastic, it exerts a force, called cornering force, which acts between the tread and the road surface. It pulls the distorted rubber back to its normal position.
The tire’s sideways distortion makes the vehicle follow a path at an angle to the direction the road wheel is pointing. This is called the slip angle. As cornering force increases, so does slip angle.
When a vehicle is being driven into a turn with decreasing radius, both slip angle and cornering force increase, until a point is reached where the tire slides, and the only resistance comes from sliding friction across the road surface.
The tire grips again only when the vehicle has slowed, or is making a turn with a larger radius. That is, when the side force is reduced to a level the tire can withstand without skidding.
Since both front and rear tires develop a slip angle in a turn, the vehicle’s path is determined by the steering of the front tires, and the slip angles of both the front and rear tires. These slip angles depend on the location of the major components.
Center of gravity
The center of gravity is the balance point of the entire vehicle. Its actual position depends on location of the major components. It is always located above the road surface, and between the tires.
When a vehicle is cornering, this is the point through which all centrifugal force is assumed to act.
Its position is determined by the load carried by the front and rear wheels, that is, by how weight is distributed. In a 40%-60%, fore-and-aft, weight distribution, 40% of the weight is carried on the front wheels, 60% on the rear, and the center of gravity is closer to the rear than the front.
A weight distribution of 60-40 has the center of gravity closer to the front than the rear.
Lateral weight distribution can be expressed in the same way.
The height of the center of gravity is determined by the height of the mass above the road surface.
Every vehicle has static weight distribution, whether it is at rest, or traveling in a straight line at a steady speed. This is changed laterally by centrifugal force when the vehicle is turning, and in a fore-and-aft direction during acceleration, or braking.
During cornering, centrifugal force puts more weight on the outside wheels.
Acceleration puts more weight on the rear wheels.
Deceleration, or braking has the opposite effect.
In a turn, centrifugal force tries to push the vehicle away from the corner. This is resisted by the cornering force of the tires.
The tires have slip angles, due to the cornering forces acting on them, and since the cornering forces at front and rear may not be equal, the slip angles at front and rear can be different too.
With the center of gravity closer to the rear, the rear tires carry more of the weight, so they operate at a greater slip angle than the front tires. Larger side forces act on the rear tires, which causes greater tire distortion.
This condition of higher slip angles at the rear causes oversteer.
More weight on the front tires means they corner with greater slip angles than the rear.
The vehicle is then said to understeer.
With equal slip angles at front and rear, the vehicle is said to have neutral steer.
Radial ply tires generate much higher cornering forces than cross-ply tires, which is why the 2 tire types should not be used on the one vehicle.
Slip is also influenced by inflation pressures, so manufacturer recommendations should always be followed. 
