For most of history wheels had very little in the way of shock absorption and journeys were very bumpy and uncomfortable. The modern tire came about in stages in the 19th century. In 1844, Charles Goodyear invented vulcanization, the process that would later be used to produce cured rubber tires.
John Boyd Dunlop, a Scottish veterinary surgeon working in Belfast, Ireland, is mainly recognized as the father of the modern tire, although he was not the first with the idea. In 1845 the first pneumatic (inflatable) tire was patented by fellow Scotsman, the engineer Robert William Thomson, born in Stonehaven, Scotland, as the Aerial Wheel. This invention consisted of a canvas inner tube surrounded by a leather outer tire. The tire gave a good ride, but there were so many manufacturing and fitting problems that the idea had to be abandoned. John Dunlop re-invented the tire for his ten year old son's tricycle in 1887 and was awarded a patent for his tire in 1888 (rescinded 1890). Dunlop's tire had a modified leather hosepipe as an inner tube and rubber treads. It wasn't long before rubber inner tubes were invented. Because neither bicycles nor automobiles had been invented when Thomson produced his tire, that tire was only applied to horse-drawn carriages. By Dunlop's time, the bicycle had been fully developed (see Rover) and it proved a far more suitable application for pneumatic tires. Pneumatic tires were first installed on aircraft in 1906.
Dunlop partnered with William Harvey du Cros to form a company which later became the Dunlop Rubber Company to produce his invention. The invention quickly caught on for bicycles and was later adapted for use on cars. Dunlop is now a subsidiary of the Goodyear Tire and Rubber Company.
The radial tire was invented by Michelin, a French company, in 1946, but did not see wide use in the United States, the largest market at that time, until the 1970s. This type of tire uses parallel carcass plies for the sidewalls and crossed belts for the crown of the tire. All modern car tires are now radial. In 2005, Michelin was reported to be attempting to develop a tire and wheel combination, the Tweel, which does not use air.Force of Friction=Coefficient of Friction * Normal Force
where Normal Force is the opposite and equal reaction to the force of gravity (mass*9.81 ms-2) on the car. The heavier the car, the higher the normal force, and the higher the force of friction created.
Additionally, the angle in which a tire hits the ground also contributes to the traction. Wheels are given an outward tilt, most commonly less than a degree, so that when the vehicle becomes loaded, the wheels are nearly vertical (Mc Graw Hill, 1984). This allows the tires to become as close to parallel as possible to the road, allowing for maximum traction.
The grooves or treads found in most tires are there to improve contact between the tire and the road in wet conditions. Without such grooves, the water on the surface of the road would be unable to escape out to the sides of the wheel as the tire presses down onto the road. This causes a thin layer of water to remain between the road and the tire's surface, which causes a severe loss of grip. At higher speeds, this can cause hydroplaning, substantially reducing traction during braking, cornering and hard acceleration. The grooves in the tread provide an escape path for the water - and it is even claimed by some tire manufacturers that their tread pattern is designed to actively pump water out from under the tire by the action of the tread flexing.
If the road is dry, tire treads actually reduce grip since they reduce the contact area between the rubber and the road. For this reason treadless or 'slick tires' are often used in motor racing when the track is known to be smooth and dry. If it should rain unexpectedly during the race these slick tires can cause a dangerous loss of grip - which is why they are seldom used on conventional road cars. Another technique to improve traction is to use a softer rubber in the treads so that they mould themselves to the shape of the road surface; decreasing tire pressure also increases road contact, but decreases energy efficiency and increases the risk of hydroplaning. Since the rubber is softer when warm, race pit crews may even keep tires in a freezer to keep new tires at the optimum temperature until they are to be driven on. Soft compound rubber would also improve traction in street vehicles, but it is seldom used because these tires wear out too quickly for normal use. The depth of the tread grooves is an important part of car safety but that depth gradually reduces due to wear throughout the lifetime of a tire. When the tread on the outer perimeter of the tire wears away, reducing the tread depth, the tire should be replaced. Many countries have laws regulating the minimum tread depth on road vehicles and most modern road tires have built-in tread wear indicators. These take the form of small blocks of rubber moulded into the bottoms of the grooves of the tread at intervals around the tire. When the tread has worn down until the tops of those blocks are level with the top of the tread - then the tire needs to be replaced. If these blocks are not present, a tire tread depth gauge should be used to measure the depth. In most vehicles, either the front or rear tires will wear faster than the others. Having mismatched tread depths can alter the handling of the car in unacceptable ways - so it is generally advisable to swap the front and rear tires as they wear down to even out the wear patterns. This is called rotating the tires. If the vehicle's suspension is somewhat out of adjustment, it is also possible for the tires to wear more on one side than the other - so it may also be beneficial to rotate the tires from one side of the car to the other - however, careful attention should be paid to the owner's manual since some vehicles require particular tire rotation patterns. Notably, some tires are designed to provide best traction only when spinning in a clockwise or counter-clockwise direction. In such cases one must not rotate the tires from one side of the car to the other because that would put a 'clockwise tire' onto a wheel that turns in a counter-clockwise direction (and vice versa). Such tires typically have an arrow moulded into the sidewall to indicate the preferred direction.
Friction from moving contact with the road causes the tread on the outer perimeter of the tire to eventually wear away. When the tire tread becomes too shallow, the tire is worn out and should be replaced. The same wheels can usually be used throughout the lifetime of the car. Uneven or accelerated tire wear can be caused by under-inflation, overload or bad wheel alignment. Greater wear on a tire facing the outside or the inside of a car is often a sign of bad wheel alignment. When the tread is worn away completely, especially when the wear on the outer rubber exposes the reinforcing threads within, the tire is said to be bald and should be replaced as soon as possible. Sometimes tires with worn tread are recapped, i. e. a new layer of rubber with grooves is bonded onto the outer perimeter of a worn tire. Since this bonding may occasionally come loose, new tires are considered superior to recapped ones. Sometimes a pneumatic tire gets a hole or a leak through which the air inside leaks out resulting in a flat tire, a condition which must be fixed before the car can be driven safely. A leak may be a slow one, such as when the seal between the rim and tire edge is not perfect. Many leaks in flat tires are caused by nails, screws, caltrops, broken glass or other sharp objects puncturing the tire. If the hole is small and not elongated, the tire can often be repaired by using plugs from a tire repair kit. A leak in a tire can often be located by submerging the pressurized tire in water to see where air bubbles emerge. If submerging a tire underwater is not possible, the leak can be searched for by covering the pressurized tire surface with a soap and water solution to see where leaking air forms soap bubbles. A puncturing object such as a nail or a screw can be pulled out using pliers. Then a plug coated with a semi-liquid form of rubber can be inserted into the hole with a special tool. The rubber covering the plug solidifies rather quickly, then the protruding ends of the plug can be cut off, and the tire can be refilled with air to the appropriate pressure, and the repaired wheel reinstalled on the vehicle. Patches covering a hole can be glued or rubber-cemented to the interior surface of a tire, particularly if a hole is too elongated for a simple plug. Tire repair with such patches requires the tire to be taken off the rim and then remounted after the patch is applied.
It should be noted that a plug-only or patch-only type repair is not an acceptable repair. Ref. Sometimes a more serious rupture of the tire material occurs resulting in a blowout. A "blowout" may also be caused by running at highway speeds while the tire is significantly under-inflated. The heat generated can melt the body cord and an explosive loss of air may occur if the driver continues to operate the vehicle. A tire thus damaged usually must be replaced. A leaking valve stem may occasionally be the cause of a leak, necessitating valve stem replacement. This replacement means the tire will have to be taken off the rim and remounted after the valve replacement. Occasionally other types of damage require replacement of a tire.
Vehicles typically carry a spare tire, already mounted on a rim, to be used in the event of flat tire or blowout. Many spare tires (sometimes called "doughnuts") for modern cars are smaller than normal tires (to save on trunk space, gas mileage, weight and cost) and should not be driven very far before replacement with a full-size tire. A few modern vehicle models may use conventional spare tires. Jacks and for emergency replacement of a flat tire with a spare tire are included with a new car. Not included, but sometimes available separately, are hand or foot pumps for filling a tire with air by the vehicle owner. Cans of pressurized "gas" can sometimes be bought separately for convenient emergency refill of a tire. Some modern cars and trucks are equipped with run flat tires that may be driven with a puncture over a distance of 80 km to 100 km. This eliminates the need for an immediate stop, and the associated expensive tow service or tire change.
Front tires, especially on front wheel drive vehicles, have a tendency to wear out more quickly than rear tires. Routine maintenance including tire rotation (exchanging the front and rear tires with each other) is often done periodically to facilitate uniform tire wear. There are simple hand-held tire-pressure gauges which can be temporarily attached to the valve stem to check a tire's interior air pressure. This measurement of tire inflation pressure should be made at least once a month. The properinflation pressure is located in the owner's manual and on the Tire Placard. Because of slow leaks or changes in weather or other conditions tire pressure may occasionally have to be corrected, usually via the valve stem with compressed air which is often available at service stations.
Some modern cars now incorporate automatic tire pressure sensing with a warning light indicating when tires have become dangerously deflated. These systems use the measurements from the wheel speed sensors at each wheel. Since a partially deflated tire has a slightly smaller diameter than a correctly inflated tire, the car ABS computer can check that all four wheels make approximately the same number of rotations when averaged over many miles of driving. If one wheel consistently makes more rotations than the others then it must be deflated, and the warning light is lit. However, vehicle operators should not wait for the low pressure warning light to illuminate before they check their tire pressures. In most cars the tire pressure sensing must be reset (typically by holding down a button) whenever the tire pressure is corrected. Tires may gradually lose pressure in all four wheels simultaneously, a situation that the pressure sensing system cannot detect. Road holding and fuel economy may be compromised by a smaller loss of pressure than the sensor is able to detect. An alternate system directly measures the inflation pressure of the tire.