The choice of the agricultural tyre is not without a certain level of technical know-how, enabling the type of soil on which it is to be worked to be combined, the tractor to be used and the work to be carried out. At the end of the day the tyre is the point of connection between the tractor and the ground, that is to say, through the tyre all the force developed by the tractor is transmitted to the ground; a bad choice of the ground can lead to a significant loss of power, of the importance of its choice.
The main aim is to achieve maximum adhesion with a minimum compaction of the soil, which will have an impact on minimising slip losses, reducing fuel consumption, and therefore lengthening working time.
On the wheel, two distinct parts can be distinguished: a metal part formed by the disc and the rim, and another part formed by the tyre itself, which consists of a cover, and in some cases a camera.
The disc can be considered as the connection between the transmission and the tyre; it is connected by screws to the hub or flange of the wheel (the last element of the pre-wheel transmission). On the periphery of the disc we find the rim; it is a metal cylinder, which on the one hand is fixed to the disc and on the other allows the anchorage of the tyre, to facilitate the operation of Assembly and dismantling of the cover, the rim has a sunken shape in its central part.
The chamber is a closed Toric tube of flexible rubber with low porosity, in order to keep the pressure air confined inside. As previously indicated, the camera may or may not exist. Attached to it is the inflating valve. Finally and surrounding the chamber is the cover, due to its importance, in the next point its components are described.
ELEMENTS OF THE COVER
The cover consists of the following elements:
- Structure, consisting of a series of layers and flexible at the same time, of fabrics that were originally made of cotton and which are currently made of metallic threads called tarps, the resistance of these layers is expressed by the “Ply Rating” or PR and gives an idea of the loads that the tyre can bear; it usually varies between 2 and 10.
- Coating, protects the structure by means of a hard vulcanized rubber wrap.
- Tread, this is the area of contact between the tyre and the ground, where the nerves, claws or heels responsible for avoiding slipping are found, the shape of these heels is fundamental. If the claws were found parallel to the wheel’s axis, the wheel would get as much traction as possible, but after a few laps the space between the claws would be filled with dirt and the wheels would begin to skate. The other end is the case of claws in a position perpendicular to the wheel axis; the earth would be evacuated immediately, but the tyre’s traction capacity would be extremely low. For this reason a compromise solution has been reached, placing the claws between the two positions, that is to say forming approximately 45º with the wheel axis, although there are variations from one brand to another.
- Heels or eyelashes, are in charge of fixing the tire to the rim. They also serve as the beginning and end for the fixing of the tarps, for which they have inside a wire that gives them stiffness.
They are given by the nominal width B and rim diameter D, expressed in inches (1’=2.54 cm), in the form B-D. manufacturers provide tables where different characteristics are expressed.:
- Tyre name: B-D – PR
- Unloaded radius: this is the radius of the tyre swollen and unloaded.
- Radio with load: it is the one that is obtained to support the tyre its load capacity at pressure of 0,8 bar.
- Index radius: this is a standard value used for different calculations.
- Load capacity according to different maximum speeds and inflation pressures: this is the weight it can bear for a given pressure and an established weight
There are two types of tyres for agricultural tractors, depending on the arrangement of the tarpaulins of the structure, namely:
- Diagonals or conventional, consisting of layers with yarns oriented between 40 and 45 ° relative to the middle plane of the tyre. This type of cover has equal resistance throughout the support band, having a height / width ratio of more than 85 %. Have a lower support surface and inflation pressures are above 0.7 – 0.8 bar.
- Radial or low-profile, consisting of layers of steel wire arranged perpendicularly to the plane of the tyre, drawn from one heel to another. The balloon height/width ratio is between 65% and 75%. It also causes the tread to be completely rigid with great flanks flexibility, because of this feature, the resistance to advance is lower in this type of tyre, as well as the induced compaction on the ground.
Previously, in the dimensions section of a tyre, the most usual way of designating a tyre has been indicated; the following is an analysis of all the elements that characterise a tyre in accordance with international standards.
- Nominal width (B): is the width between flanks of the unbroken inflated tyre, in inches.
- Rim diameter (D): is the distance between the rim highlights where the tyre heels are supported, in inches.
- Unloaded radius( R): is half the distance between the ends of the balloon when the tyre is inflated and unloaded.
- Radio with charge (R’): the distance between the centre of the disc and the end of the ball when the tyre is inflated and loaded.
- Form relation: as already indicated, is the ratio between the height and the width of the ball, depending on this characteristic the following classification can be established: normal or standard (form ratio ” 100%), wide base or low profile (75 – 85 %), extra wide base (60 -70 %).
- Dimensional characteristics: in addition to The Shape of designation B-D in inches, other different forms can be used, let’s see them with some examples – :
– 16.9 R 34 6 PR: 16.9, width of ball in inches; R, radial; 34, the tire’s diameter in inches; 6 PR, resistance index, usually appears only on the tires, diagonal in the radial are often used by the load index.
– 520/70 R 38 (20.8/70 R 38) * 150 A8: 520, width of ball in mm; 70, a relationship of the form; R, radial; 38, the tire’s diameter in inches; (20.8/70 R 38), old form of nomination; *, operating pressure; 150, load index represents the load carrying capacity, replaces the index of resistance to PR-A8, code speed: maximum speed recommended for the tire in relation to the load that it supports (Table 3).
– MICHELIN 710/75 R 34 X M28 168 A8 165 B Tubeless Radial Þ: MICHELIN, manufacturer’s mark; 710, width of the ball in mm; 75, a relationship of the form; R, radial; 34, the tire’s diameter in inches; X, the Radial Michelin X ®; M 28, type of structure; 168 A8, index of load capacity for a speed A8; 165 B, index of load capacity for a speed B; Tubeless, tubeless tire; Radial type structure; Þ, correct sense of rotation of the tire.
– Pirelli TM 800 600/65 R 38 157 A8 154 B Tubeless Þ: Pirrelli, name of the producer; TM 800, the name of the band filming; 600, width of ball in mm; 65, a relationship of the form; R, radial; 38, the tire’s diameter in inches; 157 A8, index of load capacity for a speed A8; 154 B, index of load capacity for a speed B; Tubeless, tubeless tire; Þ, the arrow indicates the direction of rotation of the tire with the vehicle in normal gear.