Tyres...........its Not A Black Art

Monster-Mat · August 6, 2003

How Tyres Work

Tyres is a black art - pun very much intended.

How Tyres are made

The Bead Bundle

The bead is a loop of high-strength, steel cable coated with rubber. It gives the tyre the strength it needs to stay seated on the wheel rim and to handle the forces applied by tyre-mounting machines when the tyres are installed on rims.

The Body

The body is made up of several layers of different fabrics, called plies. The most common ply fabric is polyester cord. The cords in a radial tyre run perpendicular to the tread. Some older tyres used diagonal bias tyres - in which the fabric ran at an angle to the tread. The plies are coated with rubber to help them bond with the other components and to seal in the air.

A tyres's strength is often described by the number of plies it has. Most car tyres have two body plies. By comparison, large commercial jetliners often have tyres with 30 or more plies.

The Belts

In steel-belted radial tyres, belts made from steel are used to reinforce the area under the tread. These belts provide puncture resistance and help the tyre stay flat so that it makes the best contact with the road.

Cap Plies

Some tyres have cap plies, an extra layer or two of polyster fabric to hold everything in place. These cap plies are not found on all tyres; they are mostly used on tyres with higher speed ratings to help all the components stay in place at high speeds.

The Sidewall

The sidewall provides lateral stability for the tyre, protects the body plies and helps keep the air from escaping. It may contain additional components to help increase the lateral stability.

The Tread

The tread is made from a mixture of many different kinds of natural and synthetic rubbers. The tread and the sidewalls are extruded and cut to length. The tread is just smooth rubber at this point; it does not have the tread patterns that give the tyre traction.

Assembly

All of these components are assembled in the tyre-bulding machine. This machine ensures that all of the components are in the correct location and then forms the tyre into a shape and size fairly close to its finished dimension.

At this point the tyre has all of its pieces, but it's not held together very tightly, and it doesn't have any markings or tread patterns. This is called a 'green tyre'. The next step is to run the tyre into a curing machine, which functions something like a waffle iron, moulding in all of the markings and traction patterns. The heat also bonds all of the tyre's components together. This is called vulcanising. After a few finishing and inspection procedures, the tyre is ready.

The Tubeless Tyre

It has been over a century from the time Dunlop patented his 'mummified wheel' to the modern radial tyres of today. Yet with all the improvements a tyre has undergone, one thing remained unchanged, which is only when it is inflated to the optimised level and that inflation is kept constant that it can deliver maximum comfort and performance. This is one of the basic reasons all tyre manufacturers try to focus on in the development stage of a tyre which can have the best air retention ability. Usage of a tube or an extra air container within the tyre was regarded as the best solution for many years.

It may come as a surprise to many readers that in 1903, engineer Paul Weeks Litchfield, then in his early 20s, was granted a patent for the first 'tubeless' tyres. He later rose to be the chairman of the Board of Goodyear in the year 1940. Just like many other patents, which were granted during that period, this concept was not pursued until late 1939 when the requirement for the first amphibious tyre was felt. The 120x33.5 - 66 smooth tread Marsh Buggy tyres, by far the largest tyres produced then, were used on Admiral Byrd's Snow Cruiser. This vehicle was capable of carrying very heavy loads over all sorts of terrain, even float on water. These were off-the-road tyres, flexible but inextensible pressure vessel that were pre-stressed and skin-stressed by air pressure. To produce such tyres Goodyear at Akron employed the idea of Litchfield, using nylon cords for the first time and a newly developed synthetic rubber compound called Chemigum to line the inner casing of this tyre to lighten its weight and eliminate the tube.

The Second World War highlighted the need for reliable tyres as loss of air or punctures cost precious moments or even endangered lives. Though the tubeless concept was not used during the war, subsequent development of tyres with a 'run-flat' capability by introducing tubes, which had a special construction of a sealant on the lower side, this allowed it to run without an air loss even after a penetration. The added weight of the tube made the steering wheel heavy and restricted speed. They were used on low speed trucks, which traveled on areas with puncture hazards like wrecker's equipment, dock and warehouse vehicles, and other utility trucks.

To reduce weight lifeguard tubes were introduced, having two air chambers, the outer rubber tube with a thick canvas tube inside. In case of a blowout only the outer chamber gave way, while the reserve air in the thick canvas tube would not allow the tyre to be completely deflated allowing the vehicle for a safe straight line gradual stop.

After the war a more determined effort towards elimination of the inner tube was sought as it was considered the main source of service trouble and failures while being clearly superfluous and costly. Experiments were therefore conducted both in the USA (initially by Goodrich) and in UK (by Dunlop), towards providing a near perfect seal between the tyre bead and rim, under all service conditions. This meant that the tyre had to run even at low inflation pressures or with a penetration to a safe distance without loss of vehicle control. It was in the year 1954 that the first commercially realised tubeless tyre was fitted as original equipment, by the now defunct Packard marque.

During the mid 1950s and early 1960s, India too manufactured tubeless tyres, which were not only supplied as original equipment for the cars, but also had a number of sizes meant for the replacement market. While the rest of the world accepted this new technology and by the middle of 1962, nearly all commercial vehicles, trucks and passenger cars used tubeless tyres,

Let us see where the construction difference lies. Apart from the basic construction, which remains the same with the run of the cords distinguishing the type of tyre construction, whether it is a cross-ply or a radial ply one; the main difference lies in the application of the inner liner of the carcass. Whereas in a tube-type construction the inner liner acts as a medium for reducing friction between the cord body and the tube, in a tubeless construction this is the tube itself. Thus the inner liner in a tubeless tyre is made up of a Halogenated Butyl rubber like Chlorobutyl or Bromobutyl for better air impermeability together with high heat and weather resistance.

Though compounds used in a tubeless or a tube type tyre may vary, the other major difference lies in the bead area of the tyre. While considering a radial tyre both type of tyres have a flexible yet rigid bead, where the bead bundle is very thin and the stability of the tyre is enhanced by the bead apex or bead filler controlling it, in a tubeless it also has to maintain the air pressure within. Thus the bead heel in the tubeless sits more tightly within the flange of the rim, and to ensure this tight fitting most tyre manufacturers add an extra wrapping over the bead area. This enhances high-speed performance while achieving a better cornering ability on the tubeless.

The other advantages are the absence of a tube make the tyre lighter in weight, thus has less chance of vibrations, which means that it leads to a better fuel saving. Even the rolling resistance in a tubeless radial is lower when compared to a tube type radial. This is due to the fact that the tubeless tyre sidewall is more supple as there is no internal body to create a friction. This also helps the tyre to run cooler as it eliminates heat generation caused by the internal shuffling of the tube.

The inner liner also acts as an absorbent during a nail penetration making the nail act like a plug and therefore the tyre has a slow leak and not a sudden deflation as it occurs on a tube- type tyre. This can be illustrated by a simple example. Pierce an ordinary balloon with a pin and it disintegrates, while sticking a cello tape on the balloon would enable the pin to penetrate without it bursting.

Similarly by comparing a tubeless tyre to a balloon that is not fully inflated, when squeezed this would deshape to certain extent before it bursts. Thus a tubeless can flex over an object, giving it a better impact resistance than a tube type one.

Personally, I feel that a tubeless tyre is more beneficial than a tube type tyre, but yet many people feel that since a local puncturewalla cannot attend to a tubeless it may be a bad proposition to use them. Generally it is quite simple and sometimes easier to repair a tubeless tyre than a tube type one. Of course there still remains one important criterion that the repairer must have proper tools and equipment to handle the same, which is essential for tube-type tyres as well. I still maintain that utmost care and regular checks should be carried out at regular intervals to get the best from your tyres. Secondly most tyre companies worldwide do not recommend use of tube in tyres lower than 60 aspect ratio. The other factor is the safety given by a tubeless may not be comparable with a one using a tube, as my case study will show.

But What About The Wheels

Wheels - The Money Spinner

The first Grand Prix, which was held at Magny-Cours, France, in the year 1906, presented a real challenge not only for automobile manufacturers but for tyre manufacturers as well. Since the 1200km hot dusty course caused numerous punctures, it put a strain on drivers as they had to replace tyres themselves, which was a painfully laborious process. An advertisement of the time projected it to be as easy as child's play but the truth was far from it. Thus Michelin's offer of the new technical innovation of completely detachable wheels amazed spectators, specially the performance of Ferenic Sziz who won the race at an average speed of over 101kmph on a Renault engined car. The key to Sziz's victory could be attributed to the fact that he managed to change a tyre in three minutes flat with these detachable wheels. Michelin also made history when out of the 34 cars which started only 11 finished, in which the first, second, fifth, sixth and eighth were all on these wheels.

Yet these wheels were a far cry from the wheels of today. Those days traditional carriage wheels and wire-spoke steel wheels developed for the bicycle were alternatives available for automobiles. The three types of modern wheels are of pressed steel, wire-spoke wheels and light-alloy casting wheels. The pressed wheels are light, strong, stiff and resistant to accidental damage. They require negligible maintenance and are only inferior to alloy wheels on one count; they are heavier. Over 90 per cent vehicle manufacturers use such wheels, as they are easy to produce and cheap to manufacture in large quantities. Steel wheels are made from two pressings. The inset distance and rim profile are varied to suit the car manufacturer's requirements. The flange profile, indicated by letters J, K, JJ, JK, or B in the specification, is designed to comply with the tyre bead profile.

Though I have already written about problems caused by using an incorrect flange type as well as the wrong width I repeat that it is of vital importance that correct width be maintained in relation to the tyre size as this is the factor responsible for the handling characteristics of a car. A rim too narrow in relation to the tyre width, for example, will allow the tyre to distort excessively sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not enough curvature to make them flex over road irregularities.

The earliest type of wheels were of the wire-spoke variety. They were light yet strong as they not only had to withstand the weight of the car but also forces of acceleration, braking and cornering. Normally all wheels are subjected to extreme loads and stress even in normal road use as during cornering they have to combat combined forces of braking and acceleration. Thus all loads on the wheel are transmitted from the rim to the hub by the spokes. These spokes were made of steel as they had to be stronger in tension than in compression. Spokes individually have little resistance to bending stresses, so they had to be laced in a complex crisscross pattern, ensuring that the load fed into a wheel was evenly distributed among the adequate number of spokes, thus converting the wheel to a tensile load similar to a pulling load rather than subjecting it to a pressing or bending load.

Assembling a wire-spoke wheel is a skilled operation, as each spoke has to be individually hooked at one end of the hub while its other end is pushed through a hole in the wheel rim, where a tapered nut also called as a nipple is screwed down pulling the spoke tight. If a spoke is too loose or too tight the rim that is relatively flimsy will distort. This labour intensified manufacture could be justified in the early days when the alternatives available were not so strong or light, but today such wheels are expensive because of their complicated construction. Such wheels are traditionally associated with vintage sport cars and racing cars, but strictly speaking have little justification today from an engineering point of view. Moreover the pierced rim of a wire wheel makes it impossible to fit tubeless tyres as they require airtight rims.

For steering control the wheels must be of rigid construction. With a pressed steel rim the 'spoke' portion is usually of near-conical shape for extra lateral stiffness. This proved to be of great disadvantage in the earlier designs as the disc had to be liberally perforated to allow the passage of cooling air to the brake drums, thus by piercing holes in the disc weakened it. The wheel manufacturers turned this to an advantage by using a slightly more expensive technique. The holes were swaged, which means that their edges were turned smoothly inwards, thus actually increasing the strength of the wheels. Today all wheel manufacturers use swaging technique as standard on their pressed wheels.

The third variant is light-alloy casting wheels which are generally meant to impress and essential quality and advantage is ignored. alloys have the main advantage of being lighter than the other types of rims, but with use of a combination of aluminum and magnesium alloys have a thicker flange section, which promotes stiffness and distribute stresses over a wider area. This allows wider tyres to be fitted, which improves road-holding ability especially on corners and is one of the main reasons for their use on some sport cars. Light alloys are also good conductors of heat and thus allow heat generated by brakes and tyres to disperse quicker. They react badly to salt spray and must be checked regularly for corrosion. I shall focus on alloy wheels in future but now dwell on how wheels are mounted and problems regarding the same. The most common type of wheel mounting consists of either four or five threaded studs equally spaced in a circle around the hub flange. These studs pass through holes in the wheel, which is secured by nuts screwed on to the studs. The holes through which the studs pass are not simply pierced through but the area around each hole is pressed out to form a tapered seating which ensures a corresponding tight fit. Each wheel sits on a position which is centrally located on the hub and is called the pitch circle diameter (PCD) of the wheel. The hub diameter is known as the bore diameter.