Tyre Basic constructions

Radial Ply Tyres, Cross Ply Tyres and Bias Belted Tyres

The carcass of a cross ply tyre actually consists of only one part, multiply crossing plies. In practice the wires are in the rear part of the tyre at an angle of 25-45^o depending on the aspect ratio of the tyre.

Radial ply tyres always have a two-piece construction including a one- or multiply-layered carcass and a multiply-layered belt structure. The cords go radially from one bead wire to another. The belts are clearly at a small angle to each other (15-20^o). In multiple-layered constructions, the wires can be set to intersect each other a little in order to regulate the features (e.g. stability).

Bias belted tyres are in principle cross ply tyres added with belts. The tyres are marked with "B". Nokian Tyres uses this marking in mining tyres. Marking B could also be used in the logging tyres (SF).

In practice, the extra reinforcements of the cross ply tyres are called brakers. They are about at the same angle as the wires.

Many arguments can be stated why radial tyres are better than cross ply tyres.

A trend is that radial ply construction will be used more in all the product areas. The first car tyre with a radial ply construction was presented already in 1948 by Michelin. After it, the radial ply construction has become general in almost every product area.

However, there are still uses, where radial structures have not been successful, like forestry, mining, and the biggest harbour tyres. The problem with the radial structures is the endurance and sidewall stability, because the sidewalls are relatively more loose compared to cross ply tyres. There have been attempts to compensate the problem by different additional components that can easily raise the price of the tyre.

Choosing a Tyre

When choosing a tyre for a certain application, following things have to be taken into consideration:

suitability of the outer dimensions to the machine

construction (radial/cross ply)

tread pattern

sufficiency of the load capacity for different purposes

stress on the surface caused by the tyre (tyre contact pressure)

behaviour of the vehicle (without suspension).

On hard surfaces, the tyre size should be chosen so that the surface pressure is the smallest possible. The pressure on hard surfaces is a function of the tyre inflation pressure (>= pressure).

In forestry, often has to be used higher recommendations for tyre pressures, because the sidewall durability is a more important factor than the load capacity. The pressure recommendations may be even 1,5 times higher than the calculated pressures.

Following compounds are widely used in different tyre types:

Forest compounds are strongly cut resistant. Abrasion is destructive and the tyres are exposed to cutting. In tyres, chains and tracks are used much and they may gnaw the tyre. The compounds are SBR based special compounds.

Mining compounds are much like forestry compounds. The compound is in the mine under even bigger exertion than in the forest, because the conditions are very demanding (wet, sharp rock material).

Harbour compounds have a good abrasion resistance and low heat buildup. Also tear resistance is good. Typically the compounds are NR-based (NR/BR combinations). Resistance to ageing has to be taken into account.

Compounds for construction applications are like the harbour compounds. They are used a lot on the highway. Tear resistance is not required to be as high as in the case of harbour compounds.

Truck compounds resist abrasion well and produce little heat. The compounds are 100 % of NR or NR/BR combination (e.g. 80/20 % NR/BR). In on/off-road compounds SBR may be used to achieve better abrasion resistance.

Carcass Constructions

The tyre carcass is required to have extremely high strength, dimensional stability and fatigue resistance. Therefore it is important to construct the tyre so that each cord ply bears all the forces directed to it.

Cross ply construction balances well the forces between the cord plies. The reason is that under stretching the cross ply construction gives the cords more possibilities to settle. Small angle changes do not affect the cross ply construction, because the carcass tries to find its own form. Cross ply tyre constructions may have several bead wires and pockets. In radial tyres, multiwire constructions are not used, because stretching is different and it would not be possible to balance the forces between the pockets.

Tyre constructions are notified with the cord number per pocket. For a singlewire tyre, the maximum cord amount is in principle ten (6+4). In multiwire cross ply tyres the amount of pockets follow the same principle, which means that around a bead wire may be six cords at the most and around the whole bundle four cords. Also breakers can be used in tyres to increase the strength or as fortification, when the construction is given for example in the form of 6+2+2BR. The upper layers in cross ply tyres are usually cords with lower cord number (ends/10 cm) and thicker rubber than the actual carcass cords. The aim is to make it stick better on the tread and sidewall.

One type of constructions is multiply textile radial carcass. The amount of cord plies is typically 3-7. With textile carcass radial construction, it is possible to reach half of the amount of cords in cross ply construction.

The speciality among radial construction tyres is the monoply construction tyre, in which the carcass consists of one strong steel cord. With this construction, small heat build-up and long tyre life is reached.

To stabilize the tyre construction, post inflation is used. This happens immediately after curing by pressurizing the tyre (e.g. 1,3 5 standard pressure). Post inflation is used especially for Nylon carcass tyres. With post inflation, the shrinkage of the carcass is prevented when the tyre gets cool. If the tyre shrinks, it usually also stretches back to its original form in use, which may cause separations between the carcass and the surface.

Belt Constructions

The tyre belts have to stand high stress even in driving straight . The influence of the load is emphasized in driving in curves. Therefore it is very important to separate the belt ends from each other and from the carcass. The belt ends move with pumping movement under stress, and so the heat build-up of cord coating and belt edge rubber has to be low (tan d).

Belt material is either textile or steel. Textile belts are mainly of Rayon because of the good stability. Requirements for the steel cord are good stiffness and strength. Textile belts are used mainly in agricultural tyres. Steel belts are used in earthmoving tyres. Typically the number of textile belts is four, but even a bigger number can be used. The number of steel cords, on the other hand, is usually three or four (minimum 2).

Bead Area

Bead area is one of the most critical areas that affect the tyre durability, because the ply turnups often end in the flexing area the lower part of the sidewall. Therefore it is very important that the ends of the cords are staggered to each other. The cord end should not be on the apex or clinch end. By shaping the bead area it is possible to minimize the influence of flexing.

In heavy tyre constructions are often used cords of the same width, which makes the beads asymmetric: one bead with an open construction, another with a closed construction. The function of both beads has to be made certain, because their flexibility is different.