05/31/2011

What requirements do tyres have to meet?

What is definite is that the expectations are extremely high – and that is a good thing, since everyone’s safety is in the final analysis involved. The list of requirements mirrors the assignments a tyre has to carry out and is divided up into four sections. Tyres are required, firstly, to be economic in every respect, i.e. they need to be cheap to buy and have a long useful life; their rolling resistance has to be as low as possible in order to save fuel and it also has to be possible to retread them. The second point is particularly important to most car drivers: comfort. That feeling: what could be more pleasant than having all four tyres in full contact with the road, with the tyres moving over the asphalt so quietly than nothing can be heard inside the car as the tread does its job on the road. Acoustic performance is another important factor where environmental compatibility is concerned. It would be impossible to tolerate tyres that sounded like a little less noisy version of the caterpillar tracks used on tanks.

Material consumption should be kept as low as possible in tyre production. The issues here: responsible care, protection of resources that are in short supply etc. Consideration should at the same time already be given to the time after the end of a tyre’s useful life when it is being manufactured: what is the solution when retreading is not possible any more and the tyre has to be disposed of. Recycling is particularly important with tyres. The fourth and last point is vehicle safety. Tyres need to be weather-resistant and in a position to withstand the strains and stresses encountered when driving. They are required in addition to adapt to a certain extent to any changes in the weather conditions. The chemical industry has made crucial advances here.

Tyre structure

Looked at superficially, it is possible to get the impression that car tyres are moulded from single pieces of rubber – with the exception of the rim, of course. It is not true, however! Appearances are deceptive here, as is so often the case. A tyre is in actual fact made from anything but just a single piece of rubber. Quite apart from which: when a car tyre is being designed, it is not merely important for the car to start moving efficiently. The tyre is an essential safety feature of the vehicle. The only conclusion that can be drawn from this: tyre production is a precision process.
All modern steel belted tyres are based on the same structural principle, no matter who the manufacturer is. For example, all car tyres contain about 16 different rubber blends, in addition to such synthetic materials as Kevlar, nylon, rayon and steel cord in a strong bond. The differences between the specific brands are therefore in the details at most.
As has already been mentioned, tyres are we know them consist of different layers. This means, among other things, that they are not produced like in a cake mould but are assembled instead from numerous individual parts that are then given their final form in the vulcanisation mould. Tests have been made in the past involving tyres made from a single piece of polyurethane, but they were not suitable for cars.

Bead

The bead with its wire core forms the basis of a tyre, the part that is clamped behind the rim (sometimes called the tyre foot in the industry). It is in general unusual to see the bead of a car tyre. However, anyone who has had to change a bicycle tyre after a puncture is familiar with it. And knows too how difficult it is to get the bead back onto the rim after the inner tube has been mended, put back in the tyre and pumped up to some extent. There is a reason for the high tension that is involved here: it makes sure that the tyre is secured firmly on the rim when the vehicle is moving.

Sidewall

The sidewall of the tyre is clearly visible whenever one gets into a vehicle through the driver’s or front seat passenger’s door. Driving performance and comfort can be affected by the sidewall, which is a very sensitive part of the tyre: if the car is driven hard over a curb, for example, carcass threads in the sidewall are crushed and break either immediately or later. The sidewall is also there to provide all the information about the tyre (e.g. 155/55 R 13).
The sidewall is reinforced with steel and, where the rim begins, with rubber, so that the substructure is protected from damage.

lnner liner

A look through a tyre from the inside would start with the inner liner. The inner liner replaces the tube in the tyre – this is what is meant when the phrase “tubeless tyre” is used. This rubber layer makes sure that the inflated interior formed by the tyre and the tightly sealed rim is sealed off completely.

Carcase

The next thing that the eye would see is the carcase. It is an essential element of the load-bearing tyre substructure. It is supposed to guarantee tyre stability at high air pressure. In other words: the carcase provides strength and cohesion. There are one or more layers of rubberised cord threads (they used to be made of cotton, but generally consist nowadays of synthetic fibres, essentially rayon) which extend radially, i.e. at right angles to the direction of movement (which is the reason why they are also called radial tyres, that are known as belted tyres too). The diagonal tyres that used to be common but are hardly used at all nowadays had single- or multilayer carcases with threads that ran a different angles (diagonally) from bead to bead. And diagonal tyres did not have a belt under the tread.

Belt

If the eye was a knife, it would meet with considerably more resistance in the next layer than in the other layers before: the belt, which consists in most cases of twisted steel wires (steel cord), goes around the tyre underneath the tread. Its purpose is to reduce rolling resistance and to improve performance at high speed. Although there have been and still are other belt materials made from textile or fibreglass threads and aramid fibres (Kevlar) and material combinations, steel belts now dominate the market to a large extent. In contrast to the carcase, the threads of the belt are at an acute angle to the direction of tyre movement.

Tread

If we continue our journey through the tyre and leave the belt behind us, we immediately reach the end of the little rubber universe, where the only thing stopping us from seeing the road surface is the tread. This is the part of the tyre that receives the most attention; its purpose is to apply driving or braking force to the road. It also protects the carcase from abrasion and destruction.

What the surface of a tyre, i.e. its tread, looks like depends entirely on what the tyre is being used for. For example: the racing slicks used on Formula 1 cars do not have any tread at all. They only have small holes or notches that act as wear indicators. They may well be the best choice on absolutely dry roads. If it starts raining, however, just a brief shower is all that it takes for the driver of a car with such slicks to start skidding all over the place: a liquid film forms between the surface of the tyre and the surface of the road. The tyre is said to have no traction any more, so that it is not possible to apply force any longer. The result is that the car starts to float – aquaplaning is the consequence and steering becomes impossible, so that a crash is unavoidable. Incidentally: many of the cases where the police discover tyres with inadequate tread while they are making general traffic controls suggest that the driver has been dreaming of imitating Michael Schumacher as a Formula 1 racing driver. Fortunately, however, most of the vehicles that are allowed to drive on the roads in Germany at least have enough tread, which moves water outwards from the middle of the tyre through grooves, voids and lamellae when it rains. Aquaplaning can only occur at excessively high speed or in extremely heavy rain, when the amount of water underneath the tyre has exceeded a certain limit and cannot be removed by the channels provided for this purpose in the tread.

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